An ecological analysis of CO 2 in an ecosystem. As systems biology, systems ecology seeks a holistic view of the interactions and transactions within and between biological and ecological systems.
The Science Education Resource Center, Carleton College,
offers the following description: "Earth System science embraces
chemistry, physics, biology, mathematics and applied sciences in
transcending disciplinary boundaries to treat the Earth as an integrated
system. It seeks a deeper understanding of the physical, chemical,
biological and human interactions that determine the past, current and
future states of the Earth. Earth System science provides a physical
basis for understanding the world in which we live and upon which
humankind seeks to achieve sustainability".
Earth System science has articulated four overarching, definitive
and critically important features of the Earth System, which include:
Variability: Many of the Earth System's natural 'modes' and
variabilities across space and time are beyond human experience, because
of the stability of the recent Holocene. Much Earth System science
therefore relies on studies of the Earth's past behaviour and models to
anticipate future behaviour in response to pressures.
Life: Biological processes play a much stronger role in the
functioning and responses of the Earth System than previously thought.
It appears to be integral to every part of the Earth System.
Connectivity: Processes are connected in ways and across depths and
lateral distances that were previously unknown and inconceivable.
Non-linear: The behaviour of the Earth System is typified by strong
non-linearities. This means that abrupt change can result when
relatively small changes in a 'forcing function' push the System across a
'threshold'.
History
For millennia, humans have speculated how the physical and living
elements on the surface of the Earth combine, with gods and goddesses
frequently posited to embody specific elements. The notion that the
Earth, itself, is alive was a regular theme of Greek philosophy and
religion.
In many respects, the foundational concepts of Earth System
science can be seen in the natural philosophy 19th century geographer Alexander von Humboldt. In the 20th century, Vladimir Vernadsky (1863–1945) saw the functioning of the biosphere as a geological force generating a dynamic disequilibrium, which in turn promoted the diversity of life.
In parallel, the field of systems science was developing across numerous other scientific fields, driven in part by the increasing availability and power of computers, and leading to the development of climate models that began to allow the detailed and interacting simulations of the Earth's weather and climate. Subsequent extension of these models has led to the development of
"Earth system models" (ESMs) that include facets such as the cryosphere
and the biosphere.
In 1983 a NASA committee called the Earth System Science Committee was formed. The earliest reports of NASA's ESSC, Earth System Science: Overview (1986), and the book-length Earth System Science: A Closer View (1988), constitute a major landmark in the formal development of Earth system science. Early works discussing Earth system science, like these NASA reports,
generally emphasized the increasing human impacts on the Earth system as
a primary driver for the need of greater integration among the life and
geo-sciences, making the origins of Earth system science parallel to
the beginnings of global change studies and programs.
Climate science
Climatology and climate change have been central to Earth System
science since its inception, as evidenced by the prominent place given
to climate change in the early NASA reports discussed above. The Earth's
climate system
is a prime example of an emergent property of the whole planetary
system, that is, one which cannot be fully understood without regarding
it as a single integrated entity. It is also a system where human
impacts have been growing rapidly in recent decades, lending immense
importance to the successful development and advancement of Earth System
science research. As just one example of the centrality of climatology
to the field, the mission statement of one of the earliest centers for
Earth System science research, the Earth System Science Center at
Pennsylvania State University, reads, "the Earth System Science Center
(ESSC) maintains a mission to describe, model, and understand the
Earth's climate system".
Earth's climate system is a complex system with five interacting components: the atmosphere (air), the hydrosphere (water), the cryosphere (ice and permafrost), the lithosphere (earth's upper rocky layer) and the biosphere (living things).Climate is the statistical characterization of the climate system. It represents the average weather, typically over a period of 30 years, and is determined by a combination of processes, such as ocean currents and wind patterns. Circulation in the atmosphere and oceans transports heat from the tropical regions to regions that receive less energy from the Sun. Solar radiation is the main driving force for this circulation. The water cycle
also moves energy throughout the climate system. In addition, certain
chemical elements are constantly moving between the components of the
climate system. Two examples for these biochemical cycles are the carbon and nitrogen cycles.
The climate system can change due to internal variability and external forcings. These external forcings can be natural, such as variations in solar intensity and volcanic eruptions, or caused by humans. Accumulation of greenhouse gases in the atmosphere, mainly being emitted by people burning fossil fuels, is causing climate change. Human activity also releases cooling aerosols, but their net effect is far less than that of greenhouse gases. Changes can be amplified by feedback processes in the different climate system components.
Education
Earth System science can be studied at a postgraduate level at some universities. In general education, the American Geophysical Union, in cooperation with the Keck Geology Consortium and with support from five divisions within the National Science Foundation,
convened a workshop in 1996, "to define common educational goals among
all disciplines in the Earth sciences". In its report, participants
noted that, "The fields that make up the Earth and space sciences are
currently undergoing a major advancement that promotes understanding the
Earth as a number of interrelated systems". Recognizing the rise of
this systems approach,
the workshop report recommended that an Earth System science curriculum
be developed with support from the National Science Foundation.
In 2000, the Earth System Science Education Alliance (ESSEA) was
begun, and currently includes the participation of 40+ institutions,
with over 3,000 teachers having completed an ESSEA course as of fall
2009".
Related concepts
The concept of earth system law (still in its infancy as per 2021) is a sub-discipline of earth system governance, itself a subfield of earth system sciences analyzed from a social sciences perspective.
Secular morality is the aspect of philosophy that deals with morality outside of religious traditions. Modern examples include humanism, freethinking, and most versions of consequentialism. Additional philosophies with ancient roots include those such as skepticism and virtue ethics. Greg M. Epstein
also states that, "much of ancient Far Eastern thought is deeply
concerned with human goodness without placing much if any stock in the
importance of gods or spirits."An example is the Kural text of Valluvar, an ancient Indian theistic poet-philosopher whose work remains secular and non-denominational. Other philosophers have proposed various ideas about how to determine right and wrong actions. An example is Immanuel Kant's categorical imperative.
A variety of positions are apparent regarding the relationship between religion and morality.
Some believe that religion is necessary as a guide to a moral life.
According to some, this idea has been with us for nearly 2,000 years. Others suggest this idea goes back at least 2,600 years as exemplified in Psalm 14 of the Hebrew Bible. According to others, the idea goes back as far as 4,000 years, with the ancient Egyptians' 42 Principles of Ma'at.
Others eschew the idea that religion is required to provide a guide to right and wrong behavior. The Westminster Dictionary of Christian Ethics however states that religion and morality "are to be defined differently and have no definitional connections with each other". Some believe that religions provide poor guides to moral behavior. Various commentators, such as Richard Dawkins (The God Delusion), Sam Harris (The End of Faith) and Christopher Hitchens (God Is Not Great) are among those who have asserted this view.
"Consequentialists", as described by Peter Singer, "start not with moral rules, but with goals. They assess actions by the extent to which they further those goals." Singer also notes that utilitarianism is "the best-known, though not the only, consequentialist theory."Epicureanism
offers a pleasure-based consequential theory of ethics, and its founder
says "we think empirically concerning the actions based on the results
observed from any course of action." Consequentialism is the class of normative ethical theories holding that the consequences
of one's conduct are the ultimate basis for any judgment about the
rightness of that conduct. Thus, from a consequentialist standpoint, a
morally right act (or omission) is one that will produce a good outcome,
or consequence. In his 2010 book, The Moral Landscape, Sam Harris describes a utilitarian science of morality.
Secular humanism focuses on the way human beings can lead happy and functional lives. It posits that human beings are capable of being ethical and moral without religion or God, it neither assumes humans to be inherently evil or innately good,
nor presents humans as "above nature" or superior to it. Rather, the
humanist life stance emphasizes the unique responsibility facing
humanity and the ethical consequences of human decisions. Fundamental
to the concept of secular humanism is the strongly held viewpoint that
ideology—be it religious or political—must be thoroughly examined by
each individual and not simply accepted or rejected on faith. Along with
this, an essential part of secular humanism is a continually adapting
search for truth, primarily through science and philosophy.
According to Greg Epstein, "the idea that we can't be 'good without God'" has been with us for nearly 2,000 years. Others suggest this idea goes back further; for example in Psalm 14 of the Hebrew Bible which according to Hermann Gunkel date to the exile period of approximately 580 BCE. It states, "The fool says in his heart, 'there is no God.' They are
corrupt, they do abominable deeds, there is none who does good ... not
even one."
Friedrich Nietzsche famously declared God is Dead
but also warned "When one gives up the Christian faith, one pulls the
right to Christian morality out from under one's feet. This morality is
by no means self-evident ... Christianity is a system, a whole view of
things thought out together. By breaking one main concept out of it, the
faith in God, one breaks the whole."
This idea is still present today. "Many today ... argue that
religious beliefs are necessary to provide moral guidance and standards
of virtuous conduct in an otherwise corrupt, materialistic, and
degenerate world." For example, Christian writer and medievalistC. S. Lewis made the argument in his popular book Mere Christianity
that if a supernatural, objective standard of right and wrong does not
exist outside of the natural world, then right and wrong becomes mired
in the is-ought problem.
Thus, he wrote, preferences for one moral standard over another become
as inherently indefensible and arbitrary as preferring a certain flavor
of food over another or choosing to drive on a certain side of a road. In the same vein, Christian theologian
Ron Rhodes has remarked that "it is impossible to distinguish evil from
good unless one has an infinite reference point which is absolutely
good." Peter Singer
states that, "Traditionally, the more important link between religion
and ethics was that religion was thought to provide a reason for doing
what is right, the reason being that those who are virtuous will be
rewarded by an eternity of bliss while the rest roast in hell."
Proponents of theism argue that without a God or gods it is impossible to justify
moral behavior on metaphysical grounds and thus to make a coherent case
for abiding by moral standards. C. S. Lewis makes such an argument in Mere Christianity. Peter Robinson, a political author and commentator with Stanford's Hoover Institution,
has commented that, if an inner moral conscience is just another
adaptive or evolved feeling in the human mind like simple emotional
urges, then no inherent reason exists to consider morality as over and
above other urges. According to Thomas Dixon, "Religions certainly do provide a framework
within which people can learn the difference between right and wrong."
"Morality does not rely on religion"
"A man's ethical behavior should be
based effectually on sympathy, education, and social ties and needs; no
religious basis is necessary. Man would indeed be in a poor way if he
had to be restrained by fear of punishment and hopes of reward after
death."
— Albert Einstein, "Religion and Science," New York Times Magazine, 1930
Various commentators have stated that morality does not require religion as a guide. The Westminster Dictionary of Christian Ethics
states that, "it is not hard to imagine a society of people that has no
religion but has a morality, as well as a legal system, just because it
says that people cannot live together without rules against killing,
etc., and that it is not desirable for these all to be legally enforced.
There have also certainly been people who have had a morality but no
religious beliefs."Bernard Williams, an English philosopher, stated that the secular "utilitarian
outlook"—a popular ethical position wherein the morally right action is
defined as that action which effects the greatest amount of happiness
or pleasure for the greatest number of people—is "non-transcendental,
and makes no appeal outside human life, in particular not to religious
considerations."
Williams also argued that, "Either one's motives for following the
moral word of God are moral motives, or they are not. If they are, then
one is already equipped with moral motivations, and the introduction of
God adds nothing extra. But if they are not moral motives, then they
will be motives of such a kind that they cannot appropriately motivate morality
at all ... we reach the conclusion that any appeal to God in this
connection either adds to nothing at all, or it adds the wrong sort of
thing."
Socrates' "Euthyphro dilemma"
is often considered one of the earliest refutations of the idea that
morality requires religion. This line of reasoning is described by Peter
Singer:
"Some theists say that ethics
cannot do without religion because the very meaning of 'good' is nothing
other than 'what God approves'. Plato refuted a similar claim more than
two thousand years ago by arguing that if the gods approve of some
actions it must be because those actions are good, in which case it
cannot be the gods' approval that makes them good. The alternative view
makes divine approval entirely arbitrary: if the gods had happened to
approve of torture and disapprove of helping our neighbors, torture
would have been good and helping our neighbors bad. Some modern theists
have attempted to extricate themselves from this type of dilemma by
maintaining that God is good and so could not possibly approve of
torture; but these theists are caught in a trap of their own making, for
what can they possibly mean by the assertion that God is good? That God
is approved of by God?"
Greg Epstein, a Humanistchaplain at Harvard University,
dismisses the question of whether God is needed to be good "because
that question does not need to be answered—it needs to be rejected
outright," adding, "To suggest that one can't be good without belief in God is not just an opinion ... it is a prejudice. It may even be discrimination." This is in line with the Westminster Dictionary of Christian Ethics
which states that religion and morality "are to be defined differently
and have no definitional connections with each other. Conceptually and
in principle, morality and a religious value system are two distinct
kinds of value systems or action guides." Others share this view. Singer states that morality "is not something intelligible only in the context of religion". Atheistic philosopher Julian Baggini
stated that "there is nothing to stop atheists believing in morality, a
meaning for life, or human goodness. Atheism is only intrinsically
negative when it comes to belief about God. It is as capable of a
positive view of other aspects of life as any other belief."He also states that "Morality is more than possible without God, it is
entirely independent of him. That means atheists are not only more than
capable of leading moral lives, they may even be able to lead more moral
lives than religious believers who confuse divine law and punishment with right and wrong.
"I think our knowledge of right and
wrong is innate in us. Religion gets its morality from humans. We know
that we can't get along if we permit perjury, theft, murder, rape, all
societies at all times, well before the advent of monarchies and
certainly, have forbidden it... Socrates
called his daemon, it was an inner voice that stopped him when he was
trying to take advantage of someone... Why don't we just assume that we
do have some internal compass?"
Daniel
Dennett says it is a "pernicious" myth that religion or God are needed
for people to fulfill their desires to be good. However, he offers that
secular and humanist groups are still learning how to organize
effectively.
Philosopher Daniel Dennett
says that secular organizations need to learn more 'marketing' lessons
from religion—and from effective secular organizations like the TED conferences.
This is partly because Dennett says that the idea that people need God
to be morally good is an extremely harmful, yet popular myth. He
believes it is a falsehood that persists because churches are currently
much better at organizing people to do morally good work. In Dennett's words:
"What is particularly pernicious
about it [the myth] is that it exploits a wonderful human trait; people
want to be good. They want to lead good lives... So then along come
religions that say 'Well you can't be good without God' to
convince people that they have to do this. That may be the main
motivation for people to take religions seriously—to try to take
religions seriously, to try and establish an allegiance to the
church—because they want to lead good lives."
"Religion is a poor moral guide"
Popular atheist author and biologist Richard Dawkins, writing in The God Delusion,
has stated that religious people have committed a wide variety of acts
and held certain beliefs through history that are considered today to be
morally repugnant. He has stated that Adolf Hitler and the Nazis held broadly Christian religious beliefs that inspired the Holocaust on account of antisemitic Christian doctrine, that Christians have traditionally imposed unfair restrictions on the legal and civil rights of women, and that Christians have condoned slavery
of some form or description throughout most of Christianity's history.
Dawkins insists that, since Jewish and Christian interpretations of the
Bible have changed over the span of history so that what was formerly
seen as permissible is now seen as impermissible, it is intellectually
dishonest for them to believe theism provides an absolute moral
foundation apart from secular intuition. In addition, he argued that
since Christians and other religious groups do not acknowledge the
binding authority of all parts of their holy texts (e.g., the books of Exodus and Leviticus state that those who work on the Sabbath and those caught performing acts of homosexuality, respectively, were to be put to death), they are already capable of distinguishing "right" from "wrong".
The Humanist Rabbi Greg M. Epstein notes a theme similar to that in Dostoyevsky's novel The Brothers Karamazov:
Greg notes, "If you're going to do something naughty, you're going to
do it, and all the theology in the world isn't going to stop you." For
instance the apologies by Christians who have "sinned" (such as Bill Clinton and Jimmy Swaggart)
"must embolden some who take enormous risks for the thrill of a little
immoral behavior: their Lord will forgive them, if they only ask nicely
enough when—or if—they are eventually caught." In the novel, the
well-known paraphrased passage being, "If God is dead, all is
permitted." In the novel, the character Ivan's internal conflict shows that that dictum is in and of itself a paradoxical moral justification:
"[...] 'all things are lawful' and
that's the end of it! That's all very charming; but if you want to
swindle why do you want a moral sanction for doing it? But that's our
modern Russian all over. He can't bring himself to swindle without a
moral sanction. He is so in love with truth—"
Some surveys and sociological literature suggest that theists do no
better than their secular counterparts in the percentage adhering to
widely held moral standards (e.g., lying, theft and sexual infidelity).
Other views
Some non-religious nihilistic and existentialist
thinkers have affirmed the prominent theistic position that the
existence of the personal God of theism is linked to the existence of an
objective moral standard, asserting that questions of right and wrong
inherently have no meaning and, thus, any notions of morality are
nothing but an anthropogenic fantasy. Agnostic author and Absurdist philosopher Albert Camus
discussed the issue of what he saw as the universe's indifference
towards humankind and the meaninglessness of life in his prominent novel
The Stranger, in which the protagonist accepts death via execution without sadness or feelings of injustice. In his philosophical work, The Myth of Sisyphus,
Camus argues that human beings must choose to live defiantly in spite
of their longing for purpose or direction and the apparent lack of
evidence for God or moral imperatives. The atheistic existentialist philosopher Jean-Paul Sartre proposed that the individual must create his own essence and therefore must freely and independently create his own subjective moral standards by which to live.
"How the preacher of the philosophy
of unselfish material pleasure induces his followers to act morally in
the world is not easily understood. Pushed by their own selfish desires,
people may act morally for some time, but when they think it over, they
will eventually sin. They will say to themselves: 'O my brother, don't
stay away from sense pleasures. Enjoy sense pleasures as you like, as
long as others do not know of them. Why not? I do not think the
world will collapse because of them. There is no God, an all-seeing God
who gives to us the results of our actions. What have you to fear? Just
be a little careful, so no one will know. If they learn of it, then you
will lose your good reputation, and perhaps the government or bad people
will make trouble for you. If that happens neither you nor others will
be happy.' Know for certain that if the hearts of the preachers of
atheistic morality were examined, these thoughts would be found."
Evidential findings
Cases can be seen in nature of animals exhibiting behavior that might
classify as "moral" without religious directives to guide them. These
include "detailed studies of the complex systems of altruism and
cooperation that operate among social insects" and "the posting of
altruistic sentinels by some species of bird and mammal, who risk their
own lives to warn the rest of the group of imminent danger."
Greg Epstein states that "sociologists have recently begun to pay
more attention to the fact that some of the world's most secular
countries, such as those in Scandinavia, are among the least violent,
best educated, and most likely to care for the poor". He adds that, "scientists are beginning to document, though religion
may have benefits for the brain, so may secularism and Humanism."
In April 2012, the results of a study which tested their subjects' pro-social sentiments were published in the Social Psychological and Personality Science
journal in which non-religious people had higher scores showing that
they were more inclined to show generosity in random acts of kindness,
such as lending their possessions and offering a seat on a crowded bus
or train. Religious people also had lower scores when it came to seeing
how much compassion motivated participants to be charitable in other
ways, such as in giving money or food to a homeless person and to
non-believers. But, global research done by Gallup between 2006 and 2008 on people
from 145 countries give the opposite results. According to research,
adherents of all the major world religions who attended religious
services in the past week got higher rates of generosity such as
donating money, volunteering, and helping a stranger than do their
coreligionists who did not attend services (non-attenders). For the
people who were nonreligious, but said that they attended religious
services in the past week exhibited more generous behaviors than those
who did not. Another global study by Gallup showed that highly religious people are
more likely to help others in terms of donating money, volunteering, and
helping strangers despite having, on average, lower incomes than those
who are less religious or nonreligious who reported higher incomes. In the research, it is said that these helping behaviors cannot be
conclusively attributed to the direct influence of religiosity, but that
it is intuitive that religious people are more likely to engage in
helping behaviors because of values promoted by religions such as
selflessness and generosity.
A number of studies have been conducted on the empirics of
morality in various countries, and the overall relationship between
faith and crime is unclear. A 2001 review of studies on this topic found "The existing evidence
surrounding the effect of religion on crime is varied, contested, and
inconclusive, and currently no persuasive answer exists as to the
empirical relationship between religion and crime." Phil Zuckerman's 2008 book, Society without God, notes that Denmark and Sweden,
"which are probably the least religious countries in the world, and
possibly in the history of the world", enjoy "among the lowest violent
crime rates in the world [and] the lowest levels of corruption in the
world". Dozens of studies have been conducted on this topic since the twentieth century. A 2005 study by Gregory S. Paul published in the Journal of Religion and Society
stated that, "In general, higher rates of belief in and worship of a
creator correlate with higher rates of homicide, juvenile and early
adult mortality, STD infection rates, teen pregnancy, and abortion in
the prosperous democracies," and "In all secular developing democracies a
centuries long-term trend has seen homicide rates drop to historical
lows" with the exceptions being the United States (with a high
religiosity level) and "theistic" Portugal. In a response, Gary Jensen builds on and refines Paul's study. His conclusion is that a "complex relationship" exists between religiosity and homicide "with some
dimensions of religiosity encouraging homicide and other dimensions discouraging it".
Distributive justice concerns the socially justallocation of resources,
goods, opportunity in a society. It is concerned with how to allocate
resources fairly among members of a society, taking into account factors
such as wealth, income, and social status. Often contrasted with just process and formal equal opportunity, distributive justice concentrates on outcomes (substantive equality). This subject has been given considerable attention in philosophy and the social sciences.
Theorists have developed widely different conceptions of distributive
justice. These have contributed to debates around the arrangement of
social, political and economic institutions to promote the just
distribution of benefits and burdens within a society. Most contemporary
theories of distributive justice rest on the precondition of material
scarcity. From that precondition arises the need for principles to
resolve competing interest and claims concerning a just or at least
morally preferable distribution of scarce resources.
In social psychology,
distributive justice is defined as perceived fairness of how rewards
and costs are shared by (distributed across) group members. For example, when some workers work more hours but receive the same
pay, group members may feel that distributive justice has not occurred.
To determine whether distributive justice has taken place, individuals
often turn to the behavioral expectations of their group. If rewards and costs are allocated according to the designated
distributive norms of the group, distributive justice has occurred.
Equality: Regardless of their inputs, all group members
should be given an equal share of the rewards/costs. Equality supports
that someone who contributes 20% of the group's resources should receive
as much as someone who contributes 60%.
Equity: Members' outcomes should be based upon their inputs.
Therefore, an individual who has invested a large amount of input (e.g.
time, money, energy) should receive more from the group than someone who
has contributed very little. Members of large groups prefer to base
allocations of rewards and costs on equity.
Power: Those with more authority, status, or control over the group should receive more than those in lower level positions.
Need: Those in greatest needs should be provided with
resources needed to meet those needs. These individuals should be given
more resources than those who already possess them, regardless of their
input.
Responsibility: Group members who have the most should share their resources with those who have less.
Theories of distributive justice
The listed theories below are some of the most prominent theories
within the field. With this in mind, the list is in no way to be
considered exhaustive for distributive justice theory.
Justice as fairness
In his book A Theory of Justice,John Rawls outlines his famous theory about justice as fairness. The theory consists of three core components:
the equality of people in rights and liberties;
the equality of opportunities for all; and
an arrangement of economic inequalities focused on benefit maximisation for those who are least advantaged.
The just 'basic structure'
Building a modern view on social contract theory, Rawls bases his work on an idea of justice being rooted in the basic structure, constituting the fundamental rules in society, which shape the social and economic institutions, as well as the governance. This basic structure is what shapes the citizens' life opportunities.
According to Rawls, the structure is based on principles about basic rights
and duties that any self-interested, rational individual would accept
in order to further his/her own interests in a context of social
cooperation.
Rawls presents the concept of an original position as a hypothetical idea of how to establish "a fair procedure so that any principles agreed on will be just." In his envisioning of the original position, it is created from a
judgement made through negotiations between a group of people who will
decide on what a just distribution of primary goods is (according to
Rawls, the primary goods include freedoms, opportunities, and control
over resources). These people are assumed to be guided by self-interest, while also
having a basic idea of morality and justice, and thus capable of
understanding and evaluating a moral argument. Rawls then argues that procedural justice in the process of negotiation
will be possible via a nullification of temptations for these people to
exploit circumstances so as to favor their own position in society.
This nullification of temptations is realised through a veil of ignorance,
which these people will be behind. The veil prevents the people from
knowing what particular preferences they will have by concealing their
talents, objectives, and, most importantly, where in society they
themselves will end up. The veil, on the other hand, does not conceal
general information about the society, and the people are assumed to
possess societal and economic knowledge beyond the personal level. Thereby, such veil creates an environment for negotiations where the
evaluation of the distribution of goods is based on general
considerations, regardless of place in society, rather than biased
considerations based on personal gains for specific citizen positions. By this logic, the negotiations will be sensitive to both those who are
worst off, given that a risk of being in that category yourself will
incentivize protection of these people, but also the rest of society, as
one would not wish to hinder maximal utilisation for these in case you
would end up in higher classes.
Basic principles of a just distribution
In this original position, the main concern will be to secure the
goods that are most essential for pursuing the goals of each individual,
regardless of what this specific goal might be. With this in mind, Rawls theorizes two basic principles of just distribution.
The first principle, the liberty principle, is the equal
access to basic rights and liberties for all. With this, each person
should be able to access the most extensive set of liberties that is
compatible with similar schemes of access by other citizens. Thereby, it
is not only a question of positive individual access but also of
negative restrictions so as to respect others' basic rights and
liberties.
The second principle, the difference principle, addresses
how the arrangement of social and economic inequalities, and thus the
just distribution should look. Firstly, Rawls argues that such
distribution should be based on a reasonable expectation of advantage
for all, but also to the greatest benefit of the least advantaged in
society. Secondly, the offices and positions attached to this
arrangement should be open to all.
These principles of justice are then prioritised according to two additional principles:
the principles of the priority of liberty, wherein basic
liberties only can be restricted if this is done for the sake of
protecting liberty either:
by strengthening "the total system of liberties shared by all;" or
if a less than equal liberty is acceptable to those who are subject to this same lesser liberty.
inequality of opportunity, and the priority of efficiency & welfare, can only be acceptable if:
it enhances "the opportunities of those with lesser opportunities" in society; and/or
excessive saving either balances out or lessens the gravity of hardship for those who do not traditionally benefit.
In 1789, Jeremy Bentham published his book An Introduction to the Principles of Morals and Legislation.
Centred on individual utility and welfare, utilitarianism builds on the
notion that any action which increases the overall welfare in society
is good, and any action that decreases welfare is bad. By this notion,
utilitarianism's focus lies with its outcomes and pays little attention
to how these outcomes are shaped. This idea of utilisation maximisation, while being a much broader
philosophical consideration, also translates into a theory of justice.
Conceptualising welfare
While the basic notion that utilitarianism builds on seems simple,
one major dispute within the school of utilitarianism revolved around
the conceptualisation and measurement of welfare. With disputes over this fundamental aspect, utilitarianism is evidently
a broad term embracing many different sub-theories under its umbrella,
and while much of the theoretical framework transects across these
conceptualisations, using the different conceptualisation have clear
implications for how we understand the more practical side of
utilitarianism in distributive justice.
Bentham originally conceptualised this according to the hedonistic calculus, which also became the foundation for John Stuart Mill's focus on intellectual pleasures as the most beneficial contribution to societal welfare. Another path has been painted by Aristotle, based on an attempt to create a more universal list of conditions required for human prosperity. Opposite this, another path focuses on a subjective evaluation of happiness and satisfaction in human lives.
Based on a fundamental notion of equal worth and moral status of human beings, egalitarianism is concerned with equal treatment of all citizens in
both respect and in concern, and in relation to the state as well as one
another. Egalitarianism focuses more on the process through which distribution
takes place, egalitarianism evaluates the justification for a certain
distribution based on how the society and its institutions have been
shaped, rather than what the outcome is. Attention is mainly given to ways in which unchosen person
circumstances affect and hinder individuals and their life
opportunities. As Elizabeth Anderson defines it, "the positive aim of egalitarian
justice is...to create a community in which people stand in relation of
equality to others."
The main issue with egalitarian conceptions of distributive
justice is the question concerning what kind of equality should be
pursued. This is because one kind of equality might imply or require
inequality of another kind. Strict egalitarianism, for instance, requires the equal allocation of material resources to every person of a given society. The principle of strict equality therefore holds that even if an
unequal distribution would make everyone better off, or if an unequal
distribution would make some better off but no one worse off, the
strictly egalitarian distribution should be upheld. This notion of
distributive justice can be critiqued because it can result in Pareto
suboptimal distributions. Thus, the Pareto norm suggests that principles
of distributive justice should result in allocations in which it is no
longer possible to make anyone better off without making anyone else
worse off. This illustrates a concern for the equality of welfare, which is an ex
post conception of equality as it is concerned with the equality in
outcomes. This conception has been critiqued by those in favour of ex
ante equality, that is equality in people´s prospects, which is captured
by alternative conceptions of equality such as those that demand
equality of opportunity.
While much academic work distinguishes between luck egalitarianism and social egalitarianism, Roland Pierik presents a synthesis combining the two branches. In his synthesis, he argues that instead of focusing on compensations
for unjust inequalities in society via redistribution of primary goods,
egalitarianism scholars should instead, given the fundamental notion
upon which the theory is built, strive to create institutions that
creates and promotes meaningful equal opportunities from the get-go.
Pierik thus moves egalitarianism's otherwise reactive nature by
emphasising a need for attention to the development of fundamentally
different institutions that would eradicate the need for redistribution
and instead focus on the initial equal distribution of opportunities
from which people then themselves be able to shape their lives.
Recent research has introduced probabilistic models, such as the Boltzmann Fair Division, which apply statistical and thermodynamic principles to the allocation of resources in society. These models provide a flexible and unbiased approach to distributive
justice, allowing parameters to be tuned for equality, merit, or need.
The Boltzmann fair division framework has been shown to bridge classical
theories and practical policy applications, enabling fair and efficient
distributions across diverse settings.
Distributive justice also affects organizational performance when efficiency and productivity are involved. Improving perceptions of justice increases performance. Organizational citizenship behaviors
(OCBs) are employee actions in support of the organization that are
outside the scope of their job description. Such behaviors depend on the
degree to which an organization is perceived to be distributively just. As organizational actions and decisions are perceived as more just,
employees are more likely to engage in OCBs. Perceptions of distributive
justice are also strongly related to the withdrawal of employees from
the organization.
Distributive justice considers whether the distribution of goods
among the members of society at a given time is subjectively acceptable.
Not all advocates of consequentialist
theories are concerned with an equitable society. What unites them is
the mutual interest in achieving the best possible results or, in terms
of the example above, the best possible distribution of wealth.
Distributive justice in an environmental context is the equitable
distribution of a society's technological and environmental risks,
impacts, and benefits. These burdens include exposure to hazardous
waste, land appropriation, armed violence, and murder. Distributive justice is an essential principle of environmental justice
because there is evidence that shows that these burdens cause health
problems, negatively affect quality of life, and drive down property
value.
The potential negative social impacts of environmental degradation and regulatory policies have been at the center environmental discussions since the rise of environmental justice. Environmental burdens fall disproportionately upon the Global South, while benefits are primarily accrued to the Global North.
In politics
Distributive justice theory argues that societies have a duty to
individuals in need and that all individuals have a duty to help others
in need. Proponents of distributive justice link it to human rights.
Many governments are known for dealing with issues of distributive
justice, especially in countries with ethnic tensions and geographically
distinctive minorities. Post-apartheidSouth Africa
is an example of a country that deals with issues of re-allocating
resources with respect to the distributive justice framework.
Within the context of Western liberal democracies
in the post-WWII decades, Friedrich von Hayek was one of the most
famous opposers of the idea of distributive justice. For him, social and
distributive justice were meaningless and impossible to attain, on the
grounds of being within a system where the outcomes are not determined
deliberately by the people but contrarily spontaneity is the norm.
Therefore, distributive justice, redistribution of wealth,
and the demands for social justice in a society ruled by an impersonal
process such as the market are in this sense incompatible with that
system.
In his book The Road to Serfdom, there can be found considerations about social assistance from the
state. There, in talking about the importance of a restrictive kind of
security (the one against physical privation) in front of one that
necessarily needs to control or abolish the market, Hayek poses that
"there can be no doubt that some minimum of food, shelter, and clothing,
sufficient to preserve health and the capacity to work, can be assured
to everybody". Providing this type of security is for Hayek compatible with individual
freedom as it does not involve planning. But already in this early
work, he acknowledges the fact that this provision must keep the
incentives and the external pressure going and not select which group
enjoys security and which does not, for under these conditions "the
striving for security tends to become higher than the love of freedom". Therefore, fostering a certain kind of security (the one that for him
socialist economic policies follow) can entail growing insecurity as the
privilege increases social differences. Notwithstanding, he concludes
that "adequate security against severe privation, and the reduction of
the avoidable causes of misdirected effort and consequent
disappointment, will have to be one of the main goals of policy".
Hayek dismisses an organizational view that ascribes certain
outcomes to an intentional design, which would be contrary to his
proposed spontaneous order. For this, Hayek famously firstly regards the
term social (or distributive) justice as meaningless when it is applied
to the results of a liberal market system that should yield spontaneous
outcomes. Justice has an individual component for Hayek, is only
understood in the aggregation of individual actions which follow common
rules, social and distributive justice are the negative opposite as they
need a command economy. Secondly, following Tebble's (2009) view, the concept of social justice is for Hayek a reminiscence of an
atavistic view towards society, that has been overcome by the survival
capacity of the catallactic order and its values.
The third Hayekian critique is about the unfeasibility of
attaining distributive justice in a free market order and this is
defended on the basis of the determinate goal that all distributive
justice aims to. In a catallactic order, the individual morality should
freely determine what are distributive fairness and the values that
govern economic activity, and the fact that it is impossible to gather
all the individual information in a single pursuit for social and
distributive justice results in realizing the fact that it cannot be
pursued. Lastly, Hayek claims for the incompatibility between the free market
and social justice, for, in essence, they are different kinds of
inequalities. The former is one determined by the interaction of free
individuals and the latter by the decision of an authority. Hayek will,
on ethical grounds, choose the former.
Robert Nozick
One of the major exponents of the libertarian outlook toward distributive justice is Robert Nozick. In his book Anarchy, State, and Utopia he stresses how the term distributive justice is not a neutral one. In
fact, there is no central distributor that can be regarded as such. What
each person gets, he or she gets from the outcomes of Lockean
self-ownership (a condition that implies one's labor mixed with the
world), or others who give to him in exchange for something, or as a
gift. For him, "there is no more a distributing or distribution of
shares than there is a distribution of mates in a society in which
persons choose whom they shall marry". This means that there can be no pattern to which to conform or aim. The
market and the result of individual actions provided the conditions for
libertarian principles of just acquisition and exchange (contained in
his Entitlement Theory) will have as a result a distribution that will
be just, without the need for considerations about the specific model or
standard it should follow.
Stages in the origin of life process range from the well understood, such as the habitable Earth and the abiotic synthesis of simple molecules, to the largely unknown, like the derivation of the last universal common ancestor (LUCA) with its complex molecular functionalities.
Abiogenesis or the origin of life (sometimes called biopoesis) is the natural process by which life arises from non-living matter, such as simple organic compounds. The prevailing scientific hypothesis is that the transition from non-living to living entities on Earth was not a single event, but a process of increasing complexity involving the formation of a habitable planet, the prebiotic synthesis of organic molecules, molecular self-replication, self-assembly, autocatalysis, and the emergence of cell membranes.
The transition from non-life to life has not been observed
experimentally, but many proposals have been made for different stages
of the process.
The study of abiogenesis aims to determine how pre-life chemical reactions gave rise to life under conditions strikingly different from those on Earth today. It uses tools from biology and chemistry, attempting a synthesis of many sciences. Life functions through the chemistry of carbon and water, and builds on four chemical families: lipids for cell membranes, carbohydrates such as sugars, amino acids for protein metabolism, and the nucleic acidsDNA and RNA for heredity. A theory of abiogenesis must explain the origins and interactions of these classes of molecules.
Many approaches investigate how self-replicating molecules came into existence. Researchers think that life descends from an RNA world, although other self-replicating and self-catalyzing molecules may have preceded RNA. Other approaches ("metabolism-first" hypotheses) focus on how catalysis on the early Earth might have provided the precursor molecules for self-replication. The 1952 Miller–Urey experiment demonstrated that amino acids can be synthesized from inorganic compounds under conditions like early Earth's. Subsequently, amino acids have been found in meteorites, comets, asteroids, and star-forming regions of space.
While the last universal common ancestor
of all modern organisms (LUCA) existed millions of years after the
origin of life, its study can guide research into early universal
characteristics. A genomics approach has sought to characterize LUCA by identifying the genes shared by Archaea and Bacteria, major branches of life. It appears there are 60 proteins common to all life and 355 prokaryotic genes that trace to LUCA; their functions imply that LUCA was anaerobic with the Wood–Ljungdahl pathway, deriving energy by chemiosmosis, and used DNA, the genetic code, and ribosomes. Earlier cells might have had a leaky membrane and been powered by a naturally occurring proton gradient near a deep-sea white smoker hydrothermal vent; or, life may have originated inside the continental crust or in water at Earth's surface.
Although Earth is the only place known to harbor life, astrobiologists assume that life exists and came into being by similar processes on other planets. Geochemical and fossil evidence informs most studies. The Earth was formed at 4.54 Gya, and the earliest evidence of life on Earth dates from 3.8 Gya from Western Australia. Fossil micro-organisms may have lived in hydrothermal vent precipitates from Quebec, soon after ocean formation during the Hadean, so the process appears to have been relatively rapid in terms of geological time.
NASA's 2015 strategy for astrobiology
aimed to solve the puzzle of the origin of life – how a fully
functioning living system could emerge from non-living components –
through research on the prebiotic origin of life's chemicals, both in space and on planets, as well as the functioning of early biomolecules to catalyse reactions and support inheritance.
The challenge for origin of life researchers is to explain how such a complex and tightly interlinked
system could develop by evolutionary steps, as at first sight all its parts are necessary
to enable it to function. For example, a cell, whether the LUCA or in a
modern organism, copies its DNA with the DNA polymerase enzyme, which
is itself produced by translating the DNA polymerase gene in the DNA.
Neither the enzyme nor the DNA can be produced without the other. The evolutionary process could have started with molecular self-replication, self-assembly such as of cell membranes, and autocatalysis via RNA ribozymes in an RNA world environment. The transition of non-life to life has not been observed experimentally. Some scientists see both life and the origin of life as aspects of the same process.
The preconditions to the development of a living cell like the
LUCA are known, though disputed in detail: a habitable world is formed
with a supply of minerals and liquid water. Prebiotic synthesis creates a
range of simple organic compounds, which are assembled into polymers
such as proteins and RNA. On the other side, the process after the LUCA
is readily understood: biological evolution caused the development of a
wide range of species with varied forms and biochemical capabilities.
However, the derivation of the LUCA from simple components is far from
understood.
Although Earth remains the only place where life is known, the science of astrobiology
seeks evidence of life on other planets. The 2015 NASA strategy on the
origin of life aimed to solve the puzzle by identifying interactions,
intermediary structures and functions, energy sources, and environmental
factors that contributed to evolvable macromolecular systems, and mapping the chemical landscape of potential primordial informational polymers. The advent of such polymers was most likely a critical step in prebiotic chemical evolution. Those polymers derived, in turn, from simple organic compounds such as nucleobases, amino acids, and sugars, likely formed by reactions in the environment.A successful theory of the origin of life must explain how all these chemicals came into being.
The Miller–Urey experiment
was a synthesis of small organic molecules in a mixture of simple gases
in a thermal gradient created by heating (right) and cooling (left) the
mixture at the same time, with electrical discharges.
One ancient view of the origin of life, from Aristotle until the 19th century, was of spontaneous generation. This held that "lower" animals such as insects were generated by decaying organic substances, and that life arose by chance. This was questioned from the 17th century, in works like Thomas Browne's Pseudodoxia Epidemica. In 1665, Robert Hooke published the first drawings of a microorganism. In 1676, Antonie van Leeuwenhoek drew and described microorganisms, probably protozoa and bacteria. Van Leeuwenhoek disagreed with spontaneous generation, and by the 1680s
convinced himself, using experiments ranging from sealed and open meat
incubation and the close study of insect reproduction, that the theory
was incorrect. In 1668 Francesco Redi showed that no maggots appeared in meat when flies were prevented from laying eggs. By the middle of the 19th century, spontaneous generation was considered disproven.
Dating back to Anaxagoras in the 5th century BC, panspermia is the idea that life originated elsewhere in the universe and came to Earth. The modern version of panspermia holds that life may have been distributed to Earth by meteoroids, asteroids, comets or planetoids. This shifts the origin of life to another heavenly body. The advantage
is that life is not required to have formed on each planet it occurs on,
but in a more limited set of locations, and then spread about the galaxy to other star systems. There is some interest in the possibility that life originated on Mars and later transferred to Earth.
The idea that life originated from non-living matter in slow stages appeared in Herbert Spencer's 1864–1867 book Principles of Biology, and in William Turner Thiselton-Dyer's 1879 paper "On spontaneous generation and evolution". On 1 February 1871 Charles Darwin wrote about these publications to Joseph Hooker, and set out his own speculation that the original spark of life may have been in a "warm little pond, with all sorts of ammonia and phosphoric salts,—light,
heat, electricity &c present, that a protein compound was
chemically formed". Darwin explained that "at the present day such
matter would be instantly devoured or absorbed, which would not have
been the case before living creatures were formed."
Alexander Oparin in 1924 and J. B. S. Haldane in 1929 proposed that the earliest cells slowly self-organized from a primordial soup, the Oparin–Haldane hypothesis.
Haldane suggested that the Earth's prebiotic oceans consisted of a "hot
dilute soup" in which organic compounds could have formed. J. D. Bernal showed that such mechanisms could form most of the necessary molecules for life from inorganic precursors. In 1967, he suggested three "stages": the origin of biological monomers; the origin of biological polymers; and the evolution from molecules to cells.
In 1952, Stanley Miller and Harold Urey
carried out a chemical experiment to demonstrate how organic molecules
could have formed spontaneously from inorganic precursors under prebiotic conditions like those posited by the Oparin–Haldane hypothesis. It used a highly reducing (lacking oxygen) mixture of gases—methane, ammonia, and hydrogen, with water vapor—to form organic monomers such as amino acids.
Bernal said of the Miller–Urey experiment that "it is not enough
to explain the formation of such molecules, what is necessary, is a
physical-chemical explanation of the origins of these molecules that
suggests the presence of suitable sources and sinks for free energy." However, current scientific consensus describes the primitive atmosphere as weakly reducing or neutral, diminishing the amount and variety of amino acids that could be produced. The addition of iron and carbonate minerals, present in early oceans, produces a diverse array of amino acids. Later work has focused on two other potential reducing environments: outer space and deep-sea hydrothermal vents.
Soon after the Big Bang, roughly 14 Gya, the only chemical elements present in the universe were hydrogen, helium, and lithium,
the three lightest atoms in the periodic table. These elements
gradually accreted and began orbiting in disks of gas and dust.
Gravitational accretion of material at the hot and dense centers of
these protoplanetary disks formed stars by the fusion of hydrogen. Early stars were massive and short-lived, producing all the heavier elements by stellar nucleosynthesis. Such element formation proceeds to its most stable element Iron-56. Heavier elements were formed during supernovas at the end of a star's lifecycle.
Carbon, currently the fourth most abundant element in the universe, was formed mainly in white dwarf stars. As these stars reached the end of their lifecycles,
they ejected heavier elements, including carbon and oxygen, throughout
the universe. These allowed for the formation of rocky planets.
The age of the Earth is 4.54 Gya as found by radiometric dating of calcium-aluminium-rich inclusions in carbonaceous chrondrite meteorites, the oldest material in the Solar System. Earth, during the Hadean
eon (from its formation until 4.031 Gya,) was at first inhospitable to
life. During its formation, the Earth lost much of its initial mass, and
so lacked the gravity to hold molecular hydrogen and the bulk of the original inert gases. Soon after initial accretion of Earth at 4.48 Gya, its collision with Theia, a hypothesised impactor, is thought to have created the ejected debris that eventually formed the Moon. This impact removed the Earth's primary atmosphere, leaving behind
clouds of viscous silicates and carbon dioxide. This unstable atmosphere
was short-lived, soon condensing to form the bulk silicate Earth,
leaving behind an atmosphere largely consisting of water vapor, nitrogen, and carbon dioxide, with smaller amounts of carbon monoxide, hydrogen, and sulfur compounds. The solution of carbon dioxide in water is thought to have made the seas slightly acidic, with a pH of about 5.5.
Condensation to form liquid oceans is theorised to have occurred as early as the Moon-forming impact. This scenario is supported by the dating of 4.404 Gya zircon crystals with high δ18O values from metamorphosed quartzite of Mount Narryer in Western Australia. The Hadean atmosphere has been characterized as a "gigantic, productive
outdoor chemical laboratory," similar to volcanic gases today which
still support some abiotic chemistry. Despite the likely increased
volcanism from early plate tectonics, the Earth may have been a
predominantly water world between 4.4 and 4.3 Gya. It is debated whether
crust was exposed above this ocean. Immediately after the Moon-forming impact, Earth likely had little if any continental crust, a turbulent atmosphere, and a hydrosphere subject to intense ultraviolet light from a T Tauri stage Sun. It was also affected by cosmic radiation, and continued asteroid and comet impacts.
The Late Heavy Bombardment hypothesis posits that a period of intense impact occurred at 4.1 to 3.8 Gya during the Hadean and early Archean eons. Originally it was thought that the Late Heavy Bombardment was a single
cataclysmic impact event occurring at 3.9 Gya; this would have had the
potential to sterilize Earth by volatilizing liquid oceans and blocking
sunlight needed for photosynthesis, delaying the earliest possible
emergence of life. More recent research questioned the intensity of the Late Heavy
Bombardment and its potential for sterilisation. If it was not one giant
impact but a period of raised impact rate, it would have had much less
destructive power. The 3.9 Gya date arose from dating of Apollo mission sample returns collected mostly near the Imbrium Basin, biasing the age of recorded impacts. Impact modelling of the lunar surface reveals that rather than a
cataclysmic event at 3.9 Gya, multiple small-scale, short-lived periods
of bombardment likely occurred. Terrestrial data backs this idea by showing multiple periods of ejecta
in the rock record both before and after the 3.9 Gya marker, suggesting
that the early Earth was subject to continuous impacts with less impact
on extinction.
If life evolved in the ocean at depths of more than ten meters,
it would have been shielded both from late impacts and the then high
levels of ultraviolet radiation from the sun. The available energy is
maximized at 100–150 °C, the temperatures at which hyperthermophilic bacteria and thermoacidophilicarchaea live.
Based on evidence from the geologic record,
life most likely emerged on Earth between 4.32 and 3.48 Gya. In 2017,
the earliest physical evidence of life was reported to consist of microbialites in the Nuvvuagittuq Greenstone Belt of Northern Quebec, in banded iron formation
rocks at least 3.77 and possibly as old as 4.32 Gya. The
micro-organisms could have lived within hydrothermal vent precipitates,
soon after the 4.4 Gya formation of oceans
during the Hadean. The microbes resemble modern hydrothermal vent
bacteria, supporting the view that abiogenesis began in such an
environment. Later research disputed this interpretation of the data, stating that
the observations may be better explained by abiotic processes in
silica-rich waters, "chemical gardens," circulating hydrothermal fluids, or volcanic ejecta.
Biogenic graphite has been found in 3.7 Gya metasedimentary rocks from southwestern Greenland and in microbial mat fossils from 3.49 Gya cherts in the Pilbara region of Western Australia. Evidence of early life in rocks from Akilia Island, near the Isua supracrustal belt in southwestern Greenland, dating to 3.7 Gya, have shown biogenic carbon isotopes. In other parts of the Isua supracrustal belt, graphite inclusions trapped within garnet crystals are connected to the other elements of life: oxygen, nitrogen, and possibly phosphorus in the form of phosphate, providing further evidence for life 3.7 ;Gya. In the Pilbara region of Western Australia, compelling evidence of early life was found in pyrite-bearing
sandstone in a fossilized beach, with rounded tubular cells that
oxidized sulfur by photosynthesis in the absence of oxygen. Carbon isotope ratios on graphite inclusions from the Jack Hills
zircons suggest that life could have existed on Earth from 4.1 Gya. A 2024 study inferred LUCA's age as around 4.2 Gya (4.09–4.33 Gya) by
analysing pre-LUCA gene duplicates, with calibration from fossil
micro-organisms, much sooner after the origin of life than previously
thought.
The Pilbara region of Western Australia contains the Dresser Formation with rocks 3.48 Gya, including layered structures called stromatolites. Their modern counterparts are created by photosynthetic micro-organisms including cyanobacteria. These lie within undeformed hydrothermal-sedimentary strata; their
texture indicates a biogenic origin. Parts of the Dresser formation
preserve hot springs on land, but other regions seem to have been shallow seas. A molecular clock analysis suggests the LUCA emerged prior to 3.9 Gya.
All chemical elements derive from stellar nucleosynthesis except for hydrogen and some helium and lithium. Basic chemical ingredients of life – the carbon-hydrogen molecule (CH), the carbon-hydrogen positive ion (CH+) and the carbon ion (C+) – can be produced by ultraviolet light from stars. Complex molecules, including organic molecules, form naturally both in space and on planets. Organic molecules on the early Earth could have had either terrestrial
origins, with organic molecule synthesis driven by impact shocks or by
other energy sources, such as ultraviolet light, redox coupling, or electrical discharges; or extraterrestrial origins (pseudo-panspermia), with organic molecules formed in interstellar dust clouds raining down on to the planet.
An organic compound is a chemical whose molecules contain carbon. Carbon is abundant in the Sun, stars, comets, and in the atmospheres of most planets of the Solar System. Organic compounds are relatively common in space, formed by "factories
of complex molecular synthesis" which occur in molecular clouds and circumstellar envelopes, and chemically evolve after reactions are initiated mostly by ionizing radiation. Purine and pyrimidine nucleobases including guanine, adenine, cytosine, uracil, and thymine, as well as sugars, have been found in meteorites. These could have provided the materials for DNA and RNA to form on the early Earth. The amino acid glycine was found in material ejected from comet Wild 2; it had earlier been detected in meteorites. Comets are encrusted with dark material, thought to be a tar-like
organic substance formed from simple carbon compounds under ionizing
radiation. A rain of material from comets could have brought such
complex organic molecules to Earth. During the Late Heavy Bombardment, meteorites may have delivered up to five million tons of organic prebiotic elements to Earth per year. Currently 40,000 tons of cosmic dust falls to Earth each year.
Polycyclic aromatic hydrocarbons (PAH) are the most common and abundant polyatomic molecules in the observable universe, and are a major store of carbon. They seem to have formed shortly after the Big Bang, and are associated with new stars and exoplanets. They are a likely constituent of Earth's primordial sea. PAHs have been detected in nebulae, and in the interstellar medium, in comets, and in meteorites.
A star, HH 46-IR, resembling the sun early in its life, is
surrounded by a disk of material which contains molecules including
cyanide compounds, hydrocarbons, and carbon monoxide. PAHs in the interstellar medium can be transformed through hydrogenation, oxygenation, and hydroxylation to more complex organic compounds used in living cells.
Organic compounds introduced on Earth by interstellar dust particles can help to form complex molecules, thanks to their peculiar surface-catalytic activities.The RNA component uracil and related molecules, including xanthine, in the Murchison meteorite were likely formed extraterrestrially, as suggested by studies of 12C/13C isotopic ratios. NASA studies of meteorites suggest that all four DNA nucleobases
(adenine, guanine and related organic molecules) have been formed in
outer space. The cosmic dust permeating the universe contains complex organics ("amorphous organic solids with a mixed aromatic–aliphatic structure") that could be created rapidly by stars. Glycolaldehyde, a sugar molecule and RNA precursor, has been detected in regions of space including around protostars and on meteorites.
Laboratory synthesis
As early as the 1860s, experiments demonstrated that biologically
relevant molecules can be produced from interaction of simple carbon
sources with abundant inorganic catalysts. The spontaneous formation of
complex polymers from abiotically generated monomers under the
conditions posited by the "soup" theory is not straightforward. Besides
the necessary basic organic monomers, compounds that would have
prohibited the formation of polymers were also formed in high
concentration during the Miller–Urey experiment and Joan Oró experiments. Biology uses essentially 20 amino acids for its coded protein enzymes,
representing a very small subset of the structurally possible products.
Since life tends to use whatever is available, an explanation is needed
for why the set used is so small. Formamide is attractive as a medium that potentially provided a source
of amino acid derivatives from simple aldehyde and nitrile feedstocks.
The Breslow catalytic cycle for formaldehyde dimerization and C2-C6 sugar formation
Alexander Butlerov showed in 1861 that the formose reaction created sugars including tetroses, pentoses, and hexoses when formaldehyde
is heated under basic conditions with divalent metal ions like calcium.
R. Breslow proposed that the reaction was autocatalytic in 1959.
Nucleobases
Nucleobases, such as guanine and adenine, can be synthesized from simple carbon and nitrogen sources, such as hydrogen cyanide (HCN) and ammonia. On early Earth, HCN has been shown in modelling experiments to have
likely been supplied via photochemical production in transient, highly
reducing atmospheres (see Prebiotic atmosphere) following major impacts. Formamide,
produced by the reaction of water and HCN, is ubiquitous and produces
all four ribonucleotides when warmed with terrestrial minerals. It can
be concentrated by the evaporation of water. HCN is poisonous only to aerobic organisms,
which did not exist during the earliest phases of life's origin. It can
contribute to chemical processes such as the synthesis of the amino
acid glycine.
DNA and RNA components including uracil, cytosine and thymine can
be synthesized under outer space conditions, using starting chemicals
such as pyrimidine found in meteorites. Pyrimidine may have been formed
in red giant stars, in interstellar dust and gas clouds, or may have been synthesized on Earth via precursors such as cyanoacetylene and other intermediates made available following early asteroid impacts.All four RNA-bases may be synthesized from formamide in high-energy density events like extraterrestrial impacts. Several ribonucleotides for RNA formation have been synthesized in a laboratory environment which replicates prebiotic conditions via autocatalytic formose reaction.
Other pathways for synthesizing bases from inorganic materials have been reported. Freezing temperatures assist the synthesis of purines, by concentrating key precursors such as HCN. However, while adenine and guanine require freezing conditions, cytosine and uracil may require boiling temperatures. Seven amino acids and eleven types of nucleobases formed in ice when ammonia and cyanide were left in a freezer for 25 years. S-triazines
(alternative nucleobases), pyrimidines including cytosine and uracil,
and adenine can be synthesized by subjecting a urea solution to
freeze-thaw cycles under a reductive atmosphere with spark discharges. The unusual speed of these low-temperature reactions is due to eutectic freezing, which crowds impurities in microscopic pockets of liquid within the ice.
Peptides
Prebiotic peptide synthesis could have occurred by several routes.
Some center on high temperature/concentration conditions in which
condensation becomes energetically favorable, while others use plausible
prebiotic condensing agents.
Experimental evidence for the formation of peptides in uniquely
concentrated environments is bolstered by work suggesting that wet-dry
cycles and the presence of specific salts can greatly increase
spontaneous condensation of glycine into poly-glycine chains. Other work suggests that while mineral surfaces, such as those of
pyrite, calcite, and rutile catalyze peptide condensation, they also
catalyze their hydrolysis. The authors suggest that additional chemical
activation or coupling would be necessary to produce peptides at
sufficient concentrations. Thus, mineral surface catalysis, while
important, is not sufficient alone for peptide synthesis.
Many prebiotically plausible condensing/activating agents have
been identified, including the following: cyanamide, dicyanamide,
dicyandiamide, diaminomaleonitrile, urea, trimetaphosphate, NaCl, CuCl2, (Ni,Fe)S, CO, carbonyl sulfide (COS), carbon disulfide (CS2), SO2, and diammonium phosphate (DAP).
A 2024 experiment used a sapphire substrate with a web of thin cracks under a heat flow, mimicking deep-ocean vents,
to concentrate prebiotically-relevant building blocks from a dilute
mixture by up to three orders of magnitude. This could help to create
biopolymers such as peptides. A similar role has been suggested for clays, though this speculation has not been supported through experimental evidence.
The prebiotic synthesis of peptides from simpler molecules such as CO, NH3 and C, skipping the step of amino acid formation, is also very efficient.
The largest unanswered question in evolution is how simple protocells first arose and differed in reproductive contribution to the following generation, thus initiating evolution. The lipid world theory postulates that the first self-replicating object was lipid-like. Phospholipids form lipid bilayers
(as in cell membranes) in water while under agitation. These molecules
were not present on early Earth, but other membrane-forming amphiphilic long-chain molecules were. These bodies may expand by insertion of additional lipids, and may spontaneously split into two offspring
of similar size and composition. Lipid bodies may have provided
sheltering envelopes for information storage, allowing the evolution of
information-storing polymers like RNA. Only one or two types of
vesicle-forming amphiphiles have been studied. There is an enormous number of possible arrangements of lipid bilayer
membranes, and those with the best reproductive characteristics would
have converged toward a hypercycle reaction, a positive feedback
composed of two mutual catalysts represented by a membrane site and a
specific compound trapped in the vesicle. Such site/compound pairs are
transmissible to the daughter vesicles, leading to the emergence of
distinct lineages of vesicles, subject to natural selection.
A protocell is a self-organized, self-ordered, spherical collection of lipids proposed as a stepping-stone to life. A functional protocell has (as of 2014) not yet been achieved in a laboratory setting. Self-assembled vesicles are essential components of primitive cells. The theory of classical irreversible thermodynamics treats
self-assembly under a generalized chemical potential within the
framework of dissipative systems. The second law of thermodynamics requires that overall entropy increases, yet life is distinguished by its great degree of organization. Therefore, a boundary is needed to separate ordered life processes from chaotic non-living matter.
Irene Chen and Jack W. Szostak
suggest that elementary protocells can give rise to cellular behaviors
including primitive forms of differential reproduction, competition, and
energy storage. Competition for membrane molecules would favor stabilized membranes,
suggesting a selective advantage for cross-linked fatty acids and even
modern phospholipids. Such micro-encapsulation
would allow for metabolism within the membrane and the exchange of
small molecules, while retaining large biomolecules inside. Such a
membrane is needed for a cell to create its own electrochemical gradient. Fatty acid vesicles in alkaline hydrothermal vent conditions can be
stabilized by isoprenoids, synthesized by the formose reaction; the
advantages and disadvantages of isoprenoids within the lipid bilayer in
different microenvironments might have led to the divergence of the
membranes of archaea and bacteria.
Vesicles can undergo an evolutionary process under pressure cycling conditions. Simulating the systemic environment in tectonic fault zones within the Earth's crust, pressure cycling forms vesicles periodically, as well as random peptide
chains which are selected for ability to integrate into the vesicle
membrane. Further selection of vesicles for stability could lead to
functional peptide structures, increasing vesicle survival rate.
Life requires a loss of entropy, or disorder, as molecules organize
themselves into living matter. At the same time, the emergence of life
is associated with the formation of structures beyond a certain
threshold of complexity. The emergence of life with increasing order and complexity does not
contradict the second law of thermodynamics, which states that overall
entropy never decreases, since a living organism creates order in some
places (e.g. its living body) at the expense of an increase of entropy
elsewhere (e.g. heat and waste production).
Multiple sources of energy were available for chemical reactions on the early Earth. Heat from geothermal processes is a standard energy source for chemistry. Other examples include sunlight, lightning, atmospheric entries of micro-meteorites, and implosion of bubbles in sea and ocean waves. This has been confirmed by experiments and simulations. Unfavorable reactions can be driven by highly favorable ones, as in the
case of iron-sulfur chemistry. For example, this was probably important
for carbon fixation. Carbon fixation by reaction of CO2 with H2S
via iron-sulfur chemistry is favorable, and occurs at neutral pH and
100 °C. Iron-sulfur surfaces, which are abundant near hydrothermal
vents, can drive the production of small amounts of amino acids and
other biomolecules.
In 1961, Peter Mitchell proposed chemiosmosis
as a cell's primary system of energy conversion. The mechanism, now
ubiquitous in living cells, powers energy conversion in micro-organisms
and in the mitochondria of eukaryotes, making it a likely candidate for early life. Mitochondria produce adenosine triphosphate
(ATP), the energy currency of the cell used to drive cellular processes
such as chemical syntheses. The mechanism of ATP synthesis involves a
closed membrane in which the ATP synthase enzyme is embedded. The energy required to release strongly bound ATP has its origin in protons that move across the membrane. In modern cells, those proton movements are caused by the pumping of
ions across the membrane, maintaining an electrochemical gradient. In
the first organisms, the gradient could have been provided by the
difference in chemical composition between the flow from a hydrothermal
vent and the surrounding seawater, or perhaps meteoric quinones that were conducive to the development of
chemiosmotic energy across lipid membranes if at a terrestrial origin.
The RNA world hypothesis proposes that undirected polymerisation led to the emergence of ribozymes, and in turn to an RNA replicase.
The RNA world hypothesis describes an early Earth with self-replicating and catalytic RNA but no DNA or proteins. It was proposed in 1962 by Alexander Rich; the term was coined by Walter Gilbert in 1986. Many researchers concur that an RNA world must have preceded modern DNA-based life.However, it may not have been the first to exist. There may have been over 30 chemical events between pre-RNA world to near-LUCA, just involving RNA.
RNA is central to the translation process. Small RNAs can
catalyze all the chemical groups and information transfers required for
life. RNA both expresses and maintains genetic information in modern
organisms; its components are easily synthesized under early Earth
conditions. The structure of the ribosome has been called the "smoking gun", with a central core of RNA and no amino acid side chains within 18 Å of the active site that catalyzes peptide bond formation.
RNA replicase
can both code and catalyse further RNA replication, i.e. it is
autocatalytic. Some catalytic RNAs can link smaller RNA sequences
together, enabling self-replication. Natural selection would then favor the proliferation of such autocatalytic sets. Self-assembly of RNA may occur spontaneously in hydrothermal vents. A preliminary form of tRNA could have assembled into a replicator molecule. When this began to replicate, it may have had all three mechanisms of Darwinian selection: heritability, variation, and differential reproduction. Its fitness would have depended on its ability to adapt, determined by its nucleotide sequence, and resource availability.
A 2026 study synthesized a 45-nucleotide polymerase ribozyme,
discovered from random sequence pools, that catalyzes general
RNA-templated RNA synthesis. It can synthesize both its complementary
strand and a copy of itself with fair accuracy. The authors speculate
that polymerase ribozymes are more abundant in RNA sequence space than
previously thought.
From RNA to directed protein synthesis
In line with the RNA world hypothesis, much of modern biology's
templated protein biosynthesis is done by RNA molecules—namely tRNAs and
the ribosome (consisting of both protein and rRNA). The most central
reaction of peptide bond synthesis is carried out by base catalysis by
the 23S rRNA domain V. Di- and tripeptides can be synthesized with a system consisting of only aminoacyl phosphate adaptors and RNA guides. Aminoacylation ribozymes that can charge tRNAs with their cognate amino acids have been selected in in vitro experimentation.
Early functional peptides
The first proteins had to arise without a fully-fledged system of
protein biosynthesis. Random sequence peptides would not have had
biological function. Thus, significant study has gone into exploring how
early functional proteins could have arisen from random sequences.
Evidence on hydrolysis rates shows that abiotically plausible peptides
likely contained significant "nearest-neighbor" biases. This could have had some effect on early protein sequence diversity. A search found that approximately 1 in 1011 random sequences had ATP binding function.
Starting with the work of Carl Woese from 1977, genomics
studies have placed the last universal common ancestor (LUCA) of all
modern life-forms between Bacteria and a clade formed by Archaea and Eukaryota in the phylogenetic tree of life. It lived over 4 Gya. A minority of studies have placed the LUCA in Bacteria, proposing that
Archaea and Eukaryota are evolutionarily derived from within Eubacteria; Thomas Cavalier-Smith suggested in 2006 that the phenotypically diverse bacterial phylum Chloroflexota contained the LUCA.
In 2016, a set of 355 genes likely present in the LUCA was
identified. A total of 6.1 million prokaryotic genes from Bacteria and
Archaea were sequenced, identifying 355 protein clusters from among
286,514 protein clusters that were probably common to the LUCA. The
results suggest that the LUCA was anaerobic with a Wood–Ljungdahl (reductive Acetyl-CoA) pathway, nitrogen- and carbon-fixing, thermophilic. Its cofactors suggest dependence upon an environment rich in hydrogen, carbon dioxide, iron, and transition metals.
Its genetic material was probably DNA, requiring the 4-nucleotide
genetic code, messenger RNA, transfer RNA, and ribosomes to translate
the code into proteins such as enzymes. LUCA likely inhabited an
anaerobic hydrothermal vent setting in a geochemically active
environment. It was evidently already a complex organism, and must have
had precursors; it was not the first living thing. The physiology of LUCA has been in dispute. Previous research identified 60 proteins common to all life. Metabolic reactions inferred in LUCA are the incomplete reverse Krebs cycle, gluconeogenesis, the pentose phosphate pathway, glycolysis, reductive amination, and transamination.
A variety of geologic and environmental settings have been proposed
for an origin of life. These theories are often in competition with one
another as there are many views of prebiotic compound availability,
geophysical setting, and early life characteristics. The first organism
on Earth likely differed from LUCA.
Between the first appearance of life and where all modern phylogenies
began branching, an unknown amount of time passed, with unknown gene
transfers, extinctions, and adaptation to environmental niches. Modern phylogenies provide more genetic evidence about LUCA than about its precursors.
Early micro-fossils may have come from a hot world of gases such as methane, ammonia, carbon dioxide, and hydrogen sulfide, toxic to much current life. Analysis of the tree of life
places thermophilic and hyperthermophilic bacteria and archaea closest
to the root, suggesting that life may have evolved in a hot environment. The deep sea or alkaline hydrothermal vent theory posits that life began at submarine hydrothermal vents. William Martin and Michael Russell have suggested that this could have been in metal-sulphide-walled compartments acting as precursors for cell walls.
These form where hydrogen-rich fluids emerge from below the sea floor, as a result of serpentinization of ultra-maficolivine
with seawater and a pH interface with carbon dioxide-rich ocean water.
The vents form a sustained chemical energy source derived from redox
reactions, in which electron donors (molecular hydrogen) react with
electron acceptors (carbon dioxide); see iron–sulfur world theory. These are exothermic reactions.
Proposed
model of an early cell powered by external proton gradient near a
deep-sea hydrothermal vent. As long as the membrane (or passive ion
channels within it) is permeable to protons, the mechanism can function
without ion pumps.
Russell demonstrated that alkaline vents create an abiogenic proton motive force chemiosmotic gradient,[42]
ideal for abiogenesis. Their microscopic compartments "provide a
natural means of concentrating organic molecules," composed of
iron-sulfur minerals such as mackinawite, endowed these mineral cells with the catalytic properties envisaged by Günter Wächtershäuser. This movement of ions across the membrane depends on two factors:
Diffusion force caused by concentration gradient—all particles including ions diffuse from higher concentration to lower.
Electrostatic force caused by electrical potential gradient—cations like protons H+ diffuse down the electrical potential, anions in the opposite direction.
These two gradients together can be expressed as an electrochemical
gradient, providing energy for abiogenic synthesis. The proton motive
force measures the potential energy stored as proton and voltage
gradients across a membrane (differences in proton concentration and
electrical potential).
The surfaces of mineral particles inside deep-ocean hydrothermal
vents have catalytic properties similar to those of enzymes, and can
create simple organic molecules, such as methanol (CH3OH) and formic, acetic, and pyruvic acids out of the dissolved CO2 in the water, if driven by an applied voltage or by reaction with H2 or H2S.
Starting in 1981, researchers proposed that life might have started at hydrothermal vents,that spontaneous chemistry in the Earth's crust driven by rock–water
interactions at disequilibrium thermodynamically underpinned life's
origin, and that the founding lineages of the archaea and bacteria were H2-dependent autotrophs that used CO2 as their terminal acceptor in energy metabolism. In 2016, Martin suggested that the LUCA "may have depended heavily on the geothermal energy of the vent to survive". That same year, RNA was produced in synthetic alkaline hydrothermal chimneys simulating deep-sea vents. Researchers were able to generate RNA oligomers of up to 4 units in
length. This RNA was synthesized using activated ribonucleotides.
Additionally, these RNA oligomers could only be synthesized under
certain conditions.
Pores at deep sea hydrothermal vents are suggested to have been
occupied by membrane-bound compartments which promoted biochemical
reactions. Metabolic intermediates in the Krebs cycle, gluconeogenesis, amino acid
bio-synthetic pathways, glycolysis, the pentose phosphate pathway, and
including sugars like ribose, and lipid precursors can occur
non-enzymatically at conditions relevant to deep-sea alkaline
hydrothermal vents.
If the deep marine hydrothermal setting was the site, then life could have arisen as early as 4.0–4.2 Gya.
If life evolved in the ocean at depths of more than ten meters, it
would have been shielded both from impacts and the then high levels of
solar ultraviolet radiation. The available energy in hydrothermal vents
is maximized at 100–150 °C, the temperatures at which hyperthermophilic bacteria and thermoacidophilicarchaea live.
Arguments against a vent setting
Arguments against a hydrothermal origin of life state that hyperthermophily was a result of convergent evolution in bacteria and archaea, and that a mesophilic environment is more likely.
Production of prebiotic organic compounds at hydrothermal vents is estimated to be 108 kg/yr. While a large amount of key prebiotic compounds, such as methane, are
found at vents, they are in far lower concentrations than in a
Miller-Urey Experiment environment. Additionally, some organic compounds
originally thought to have been formed at vents are now understood to
have been formed by other geological processes and later inherited by
vents. Methane at alkaline vents, for example, was once thought to have
been synthesized from catalytic synthesis after serpentinization, but is
now understood to more likely come from leached fluid inclusions formed
deeper in oceanic crust from magmatic carbon. The concentrations of methane the rate is 2–4 orders of magnitude lower than those in Miller-Urey experiments.
Other counter-arguments include the inability to concentrate
prebiotic materials, due to strong dilution by seawater. This open
system cycles compounds through vent minerals, leaving little residence
time to accumulate. All modern cells rely on phosphates and potassium for nucleotide
backbone and protein formation respectively, making it likely that the
first life forms shared these functions. These elements were not
available in high quantities in the Archaean oceans, as both primarily
come from the weathering of continental rocks on land, far from vents,
and phosphate is lost into relatively insoluble apatite (calcium
phosphate). However, phosphate can be concentrated in lakes, and modern
analogs exist, such as the most phosphate-rich natural body of water in
the world, Last Chance Lake, Canada. Submarine hydrothermal vents are not conducive to condensation reactions needed for polymerisation of macromolecules.
An older argument was that key polymers were encapsulated in
vesicles after condensation, which supposedly would not happen in
saltwater. However, while salinity inhibits vesicle formation from
low-diversity mixtures of fatty acids, vesicle formation from a broader, more realistic mix of fatty-acid and 1-alkanol species is more resilient.
Importantly, no studies to date have been able to experimentally
demonstrate synthesis of de novo sugars, amino acids, nucleases,
nucleosides, nucleotides, or membrane-forming fatty acids under
plausible vent conditions.
Surface bodies of water provide environments that dry out and rewet. Wet-dry cycles concentrate prebiotic compounds and enable condensation reactions to polymerise macromolecules. Moreover, lakes and ponds receive detrital input from weathering of continental apatite-containing
rocks, the most common source of phosphates. The amount of exposed
continental crust in the Hadean is unknown, but models of early ocean
depths and rates of ocean island and continental crust growth make it
plausible that there was exposed land. Another line of evidence for a surface start to life is the requirement for Ultraviolet radiation (UV) for organism function. UV is necessary for the formation of the U+C nucleotide base pair by partial hydrolysis and nucleobase loss. Simultaneously, UV can be harmful and sterilising to life, especially
for simple early lifeforms with little ability to repair radiation
damage. Radiation levels from a young Sun were likely greater, and, with
no ozone layer,
harmful shortwave UV rays would reach the surface of Earth. For life to
begin, a shielded environment with influx from UV-exposed sources is
necessary to both benefit and protect from UV. Shielding under ice,
liquid water, mineral surfaces (e.g. clay) or regolith is possible in a
range of surface water settings.
Most branching phylogenies are thermophilic or hyperthermophilic,
making it possible that LUCA and preceding lifeforms were similarly
thermophilic. Hot springs are formed from the heating of groundwater by
geothermal activity. This intersection allows for influxes of material
from deep penetrating waters and from surface runoff that transports
eroded continental sediments. Interconnected groundwater systems create a
mechanism for distribution of life to wider area.
Mulkidjanian and co-authors argue that marine environments did
not provide the ionic balance and composition universally found in
cells, or the ions required by essential proteins and ribozymes,
especially with respect to high K+/Na+ ratio, Mn2+, Zn2+ and phosphate concentrations. They argue that the only environments that do this are hot springs similar to ones at Kamchatka. Mineral deposits in these environments under an anoxic atmosphere would
have suitable pH, contain precipitates of photocatalytic sulfide
minerals that absorb harmful ultraviolet radiation, and have wet-dry
cycles that concentrate substrate solutions enough for spontaneous
formation of biopolymers created both by chemical reactions in the hydrothermal environment, and by exposure to UV light during transport from vents to adjacent pools. The hypothesized pre-biotic environments are similar to hydrothermal
vents, with additional components that help explain peculiarities of the
LUCA.
A phylogenomic and geochemical analysis of proteins plausibly
traced to the LUCA shows that the ionic composition of its intracellular
fluid is identical to that of hot springs. The LUCA likely was
dependent upon synthesized organic matter for its growth. Experiments show that RNA-like polymers can be synthesized in wet-dry
cycling and UV light exposure. These polymers were encapsulated in
vesicles after condensation. Potential sources of organics at hot springs might have been
transported by interplanetary dust particles, extraterrestrial
projectiles, or atmospheric or geochemical synthesis. Hot springs could
have been abundant in volcanic landmasses during the Hadean.
Temperate surface bodies of water
A mesophilic start in surface bodies of waters hypothesis has evolved from Darwin's concept of a 'warm little pond' and the Oparin-Haldane hypothesis.
Freshwater bodies under temperate climates can accumulate prebiotic
materials while providing suitable environmental conditions conducive to
simple life forms. The Archaean climate is uncertain. Atmospheric
reconstructions from geochemical proxies and models suggest that
sufficient greenhouse gases were present to maintain surface
temperatures between 0–40 °C. If so, the temperature was suitable for
life to begin.
Evidence for mesophily from biomolecular studies includes Galtier's G+C
nucleotide thermometer. G+C are more abundant in thermophiles due to
the added stability of an additional hydrogen bond not present between
A+T nucleotides. rRNA sequencing of modern lifeforms shows that LUCA's reconstructed G+C content was likely representative of moderate temperatures.
The diversity of thermophiles today could be a product of
convergent evolution and horizontal gene transfer rather than an
inherited trait from LUCA. The reverse gyrasetopoisomerase is found exclusively in thermophiles and hyperthermophiles, as it allows for coiling of DNA.[269] This enzyme requires the complex molecule ATP
to function. If an origin of life is hypothesised to involve a simple
organism that had not yet evolved a membrane, let alone ATP, this would
make the existence of reverse gyrase improbable. Moreover, phylogenetic
studies show that reverse gyrase originated in archaea, and transferred
to bacteria by horizontal gene transfer, implying it was not present in
the LUCA.
Icy surface bodies of water
Cold-start theories presuppose large ice-covered regions. Stellar
evolution models predict that the Sun's luminosity was ≈25% weaker than
it is today. Fuelner states that although this significant decrease in
solar energy would have formed an icy planet, there is strong evidence
for the presence of liquid water, possibly driven by a greenhouse
effect. This would mean an early Earth with both liquid oceans and icy
poles.
Ice melts that form from ice sheets or glacier melts create
freshwater pools, another niche capable of wet-dry cycles. While surface
pools would be exposed to intense UV radiation, bodies of water within
and under ice would be shielded, while remaining connected to exposed
areas through ice cracks. Impact melting would allow freshwater and
meteoritic input, creating prebiotic components. Near-seawater levels of sodium chloride destabilize fatty acid membrane
self-assembly, making freshwater settings appealing for early
membranous life.
Icy environments would trade the faster reaction rates that occur
in warm environments for increased stability and accumulation of larger
polymers. Experiments simulating Europa-like conditions of ≈20 °C have
synthesised amino acids and adenine, showing that Miller-Urey type
syntheses can occur at low temperatures. In an RNA world,
the ribozyme would have had even more functions than in a later
DNA-RNA-protein-world. For RNA to function, it must be able to fold, a
process hindered by temperatures above 30 °C. While RNA folding in psychrophilic
organisms is slower, so is hydrolysis, so folding is more successful.
Shorter nucleotides would not suffer from higher temperatures.
Inside the continental crust
An alternative geological environment has been proposed by the
geologist Ulrich Schreiber and the physical chemist Christian Mayer: the
continental crust. Tectonic fault
zones could present a stable and well-protected environment for
long-term prebiotic evolution. Inside these systems of cracks and
cavities, water and carbon dioxide present the bulk solvents. Their
phase state could vary between liquid, gaseous and supercritical,
depending on pressure and temperature. When forming two separate phases
(e.g. liquid water and supercritical carbon dioxide in depths of little
more than 1 km), the system provides optimal conditions for phase transfer reactions.
Concurrently, the contents of the tectonic fault zones are being
supplied by a multitude of inorganic educts (e.g. carbon monoxide,
hydrogen, ammonia, hydrogen cyanide, nitrogen, and even phosphate from
dissolved apatite) and simple organic molecules formed by hydrothermal
chemistry (e.g. amino acids, long-chain amines, fatty acids, long-chain
aldehydes).
Part of the tectonic fault zones is at a depth of around 1000 m.
For the carbon dioxide part of the bulk solvent, it provides temperature
and pressure conditions near the phase transition point between the supercritical and the gaseous state. This allows lipophilic organic molecules that dissolve well in supercritical CO2 to accumulate, but not in its gaseous state, leading to their local precipitation. Periodic pressure variations such as caused by geysers or tidal influences result in periodic phase transitions, keeping the local reaction environment in a constant non-equilibrium state. In presence of amphiphilic compounds (such as the long chain amines and fatty acids), subsequent generations of vesicles are formed that are constantly selected for their stability.
Many biomolecules, such as L-glutamic acid, are asymmetric, and occur in living systems in only one of the two possible forms, in the case of amino acids the left-handed form. Prebiotic chemistry would produce both forms, creating a puzzle for abiogenesis researchers.
Homochirality is the uniformity of materials composed of chiral (non-mirror-symmetric) units. Living organisms use molecules with the same chirality: with almost no exceptions, amino acids are left-handed while nucleotides and sugars
are right-handed. Chiral molecules can be synthesized, but in the
absence of a chiral source or a chiral catalyst, they are formed in a
50/50 (racemic) mixture of both forms. Non-racemic mixtures can arise from racemic materials by asymmetric physical laws such as the electroweak interaction or asymmetric environments such as circularly polarized light.
Once established, chirality would be selected for. A small bias in the population can be amplified by asymmetric autocatalysis, as in the Soai reaction, where a chiral molecule catalyzes its own production.
![LUCA systems and environment included the Wood–Ljungdahl pathway.[10]](https://en.wikipedia.org/wiki/File:LUCA_systems_and_environment.svg)
