Anthropocentrism (//ⓘ from Ancient Greekἄνθρωπος (ánthrōpos)'human' and κέντρον (kéntron)'center') is the belief that human beings are the central or most important entity on the planet. The term can be used interchangeably with humanocentrism, and some refer to the concept as human supremacy or human exceptionalism. From an anthropocentric perspective, humankind is seen as separate from nature and superior to it, and other entities (animals, plants, minerals, etc.) are viewed as resources for humans to use.
It is possible to distinguish between at least three types of anthropocentrism: perceptual anthropocentrism (which "characterizes paradigms informed by sense-data from human sensory organs"); descriptive anthropocentrism (which "characterizes paradigms that begin from, center upon, or are ordered around Homo sapiens / 'the human'"); and normative anthropocentrism (which "characterizes paradigms that make assumptions or assertions about the superiority of Homo sapiens, its capacities, the primacy of its values, [or] its position in the universe").
Anthropocentrism tends to interpret the world in terms of human values and experiences. It is considered to be profoundly embedded in many modern human
cultures and conscious acts. It is a major concept in the field of environmental ethics and environmental philosophy, where it is often considered to be the root cause of problems created by human action within the ecosphere. However, many proponents of anthropocentrism state that this is not
necessarily the case: they argue that a sound long-term view
acknowledges that the global environment must be made continually
suitable for humans and that the real issue is shallow anthropocentrism.
Environmental philosophy
Some
environmental philosophers have argued that anthropocentrism is a core
part of a perceived human drive to dominate or "master" the Earth.
Anthropocentrism is believed by some to be the central problematic
concept in environmental philosophy, where it is used to draw attention to claims of a systematic bias in traditional Western attitudes to the non-human world that shapes humans' sense of self and identities. Val Plumwood argued that anthropocentrism plays an analogous role in green theory to androcentrism in feminist theory and ethnocentrism in anti-racist theory. Plumwood called human-centredness "anthrocentrism" to emphasise this parallel.
One of the first extended philosophical essays addressing environmental ethics, John Passmore's Man's Responsibility for Nature, has been criticised by defenders of deep ecology because of its anthropocentrism, often claimed to be constitutive of traditional Western moral thought. Indeed, defenders of anthropocentrism concerned with the ecological crisis
contend that the maintenance of a healthy, sustainable environment is
necessary for human well-being as opposed to for its own sake. According
to William Grey, the problem with a "shallow" viewpoint is not that it
is human-centred: "What's wrong with shallow views is not their concern
about the well-being of humans, but that they do not really consider
enough in what that well-being consists. According to this view, we need
to develop an enriched, fortified anthropocentric notion of human
interest to replace the dominant short-term, sectional and
self-regarding conception." In turn, Plumwood in Environmental Culture: The Ecological Crisis of Reason argued that Grey's anthropocentrism is inadequate.
Many devoted environmentalists encompass a somewhat
anthropocentric-based philosophical view supporting the fact that they
will argue in favor of saving the environment for the sake of human
populations. Grey writes: "We should be concerned to promote a rich, diverse, and
vibrant biosphere. Human flourishing may certainly be included as a
legitimate part of such a flourishing." Such a concern for human flourishing amidst the flourishing of life as a
whole, however, is said to be indistinguishable from that of deep ecology and biocentrism, which has been proposed as both an antithesis of anthropocentrism and as a generalised form of anthropocentrism.
Cognitive psychology
In cognitive psychology, the term anthropocentric thinking has been defined as "the tendency to reason about unfamiliar biological species or processes by analogy to humans." Reasoning by analogy is an attractive thinking strategy, and it can be tempting to apply one's own experience of being human to other biological systems. For example, because death is commonly felt to be undesirable, it may be tempting to form the misconception that death at a cellular level or elsewhere in nature is similarly undesirable (whereas in reality programmed cell death is an essential physiological phenomenon, and ecosystems also rely on death). Conversely, anthropocentric thinking can also lead people to
underattribute human characteristics to other organisms. For instance,
it may be tempting to wrongly assume that an animal that is very
different from humans, such as an insect, will not share particular
biological characteristics, such as reproduction or blood circulation.
Anthropocentric thinking has predominantly been studied in young children (mostly up to the age of 10) by developmental psychologists interested in its relevance to biology education.
Children as young as 6 have been found to attribute human
characteristics to species unfamiliar to them (in Japan), such as
rabbits, grasshoppers or tulips. Although relatively little is known about its persistence at a later
age, evidence exists that this pattern of human exceptionalist thinking
can continue through young adulthood at least, even among students who
have been increasingly educated in biology.
The notion that anthropocentric thinking is an innate
human characteristic has been challenged by study of American children
raised in urban environments, among whom it appears to emerge between
the ages of 3 and 5 years as an acquired perspective. Children's recourse to anthropocentric thinking seems to vary with
their experience of nature, and cultural assumptions about the place of
humans in the natural world. For example, whereas young children who kept goldfish were found to
think of frogs as being more goldfish-like, other children tended to
think of frogs in terms of humans. More generally, children raised in rural environments appear to use
anthropocentric thinking less than their urban counterparts because of
their greater familiarity with different species of animals and plants. Studies involving children from some of the indigenous peoples of the Americas have found little use of anthropocentric thinking. Study of children among the Wichí people in South America showed a tendency to think of living organisms in terms of their perceived taxonomic similarities, ecological considerations, and animistic
traditions, resulting in a much less anthropocentric view of the
natural world than is experienced by many children in Western societies.
Abrahamic traditions
In the 1985 CBC series "A Planet For the Taking", David Suzuki explored the Old Testament roots of anthropocentrism and how it shaped human views of non-human animals. Some Christian proponents of anthropocentrism base their belief on the Bible, such as the verse 1:26 in the Book of Genesis:
And God said, Let us make man in
our image, after our likeness: and let them have dominion over the fish
of the sea, and over the fowl of the air, and over the cattle, and over
all the earth, and over every creeping thing that creepeth upon the
earth.
The use of the word "dominion" in the Genesis has been used to
justify an anthropocentric worldview, but recently some have found it
controversial, viewing it as possibly a mistranslation from the Hebrew. However an argument can be made that the Bible actually places all the
importance on God as creator, and humans as merely another part of
creation.
Jewish opposition to anthropocentrism
Moses Maimonides, a Torah
scholar who lived in the twelfth century AD, was renowned for his
staunch opposition to anthropocentrism. He referred to humans as "just a
drop in the bucket" and asserted that "humans are not the axis of the
world". He also claimed that anthropocentric thinking is what leads
humans to believe in the existence of evil things in nature. According
to Rabbi Norman Lamm, Moses Maimonides "refuted the exaggerated ideas about the importance of man and urged us to abandon these fantasies.
Catholicism
Catholic social teaching sees the pre-eminence of human beings over the rest of creation in terms of service rather than domination. Pope Francis, in his 2015 encyclical letter Laudato si',
notes that "an obsession with denying any pre-eminence to the human
person" endangers the concern which should be shown to protecting and
upholding the welfare of all people, which he argues should rank
alongside the "care for our common home" which is the subject of his
letter. In the same text he acknowledges that "a mistaken understanding" of
Christian belief "has at times led us to justify mistreating nature, to
exercise tyranny over creation": in such actions, Christian believers
have "not [been] faithful to the treasures of wisdom which we have been
called to protect and preserve. In his follow-up exhortation, Laudate Deum
(2023) he refers to a preferable understanding of "the unique and
central value of the human being amid the marvellous concert of all
God's creatures" as a "situated anthropocentrism".
Human rights
Anthropocentrism is the grounding for some naturalistic concepts of specifically human rights
as opposed to animal rights extended to include the human species.
Apologists of anthropocentrism argue that it is the necessary
fundamental premise to defend universal human rights, since what matters morally is simply being human. For example, noted philosopher Mortimer J. Adler
wrote, "Those who oppose injurious discrimination on the moral ground
that all human beings, being equal in their humanity, should be treated
equally in all those respects that concern their common humanity, would
have no solid basis in fact to support their normative principle." Adler
is stating here that denying what is now called human exceptionalism
could lead to tyranny, writing that if humans ever came to believe that
they do not possess a unique moral status,
the intellectual foundation of their liberties collapses: "Why, then,
should not groups of superior men be able to justify their enslavement,
exploitation, or even genocide of inferior human groups on factual and
moral grounds akin to those we now rely on to justify our treatment of
the animals we harness as beasts of burden, that we butcher for food and
clothing, or that we destroy as disease-bearing pests or as dangerous
predators?"
Author and anthropocentrism apologist Wesley J. Smith from the Discovery Institute
has written that human exceptionalism is what gives rise to human
duties to each other, the natural world, and to treat animals humanely.
Writing in A Rat is a Pig is a Dog is a Boy, a critique of animal rights ideology, "Because we are
unquestionably a unique species—the only species capable of even
contemplating ethical issues and assuming responsibilities—we uniquely
are capable of apprehending the difference between right and wrong, good
and evil, proper and improper conduct toward animals. Or to put it more
succinctly, if being human isn't what requires us to treat animals
humanely, what in the world does?"
Anthropocentrism is closely related to the notion of speciesism, defined by Richard D. Ryder
as a "a prejudice or attitude of bias in favour of the interests of
members of one's own species and against those of members of other
species".
Early critiques
One of the earliest critics of anthropocentrism was Edward Payson Evans in his book Evolutional Ethics and Animal Psychology
(1897), where he challenges the idea that humans are fundamentally
distinct from other sentient beings. He argues that anthropocentric
psychology and ethics persist, treating humans as superior and denying
mental or moral connections to other species. Evans suggests these
beliefs stem from human self-importance, which overlooks the shared
qualities of sentience across species and questions the ethical
implications of such a view.
Later, J. Howard Moore, in The Universal Kinship (1906), expanded on this critique, asserting that Charles Darwin's On the Origin of Species
(1859) "sealed the doom" of anthropocentrism. Moore argued that the
doctrine of organic evolution, which established the common genesis of
all animals, fundamentally altered humanity's view of its place in the
natural world. Before the publication of The Origin of Species,
humans may have been seen as distinct from other creatures, but with the
advent of evolutionary theory, all species were recognized as sharing a
common ancestry. Moore considered this shift one of the most
significant intellectual developments, comparable to the groundbreaking
insights of Galileo and Copernicus.
Challenging human exceptionalism
While humans cognition is relatively advanced, many traits traditionally used to justify humanity exceptionalism (such as rationality, emotional complexity and social bonds) are not unique to humans. Research in ethology has shown that non-human animals, such as primates, elephants, and cetaceans, also demonstrate complex social structures, emotional depth, and problem-solving
abilities. This challenges the claim that humans possess qualities
absent in other animals, and which would justify denying moral status to
them.
Animal welfare perspectives
Animal welfare proponents attribute moral consideration to all sentient
animals, proportional to their ability to have positive or negative
mental experiences. It is notably associated with the ethical theory of utilitarianism, which aims to maximize well-being. It is notably defended by Peter Singer. According to David Pearce, "other things being equal, equally strong interests should count equally." Jeremy Bentham is also known for raising early the issue of animal
welfare, arguing that "the question is not, Can they reason? nor, Can
they talk? but, Can they suffer?". Animal welfare proponents can in theory accept animal exploitation if
the benefits outweigh the harms. But in practice, they generally
consider that intensive animal farming causes a massive amount of suffering that outweighs the relatively minor benefit that humans get from consuming animals.
Animal rights perspectives
Animal rights
proponents argue that all animals have inherent rights, similar to
human rights, and should not be used as means to human ends. Unlike
animal welfare advocates, who focus on minimizing suffering, animal
rights supporters often call for the total abolition of practices that
exploit animals, such as intensive animal farming, animal testing, and hunting. Prominent figures like Tom Regan
argue that animals are "subjects of a life" with inherent value,
deserving moral consideration regardless of the potential benefits
humans may derive from using them.
In popular culture
In fiction from all eras and societies, there is fiction depicting
the actions of humans to ride, eat, milk, and otherwise treat
(non-human) animals as inferior. There are occasional fictional
exceptions, such as talking animals as aberrations to the rule distinguishing people from animals.
In science fiction, humanocentrism is the idea that humans, as both beings and as a species, are the superior sentients. Essentially the equivalent of racial supremacy on a galactic scale, it entails intolerant discrimination against sentient non-humans,
much like race supremacists discriminate against those not of their
race. A prime example of this concept is utilized as a story element for
the Mass Effect
series. After humanity's first contact results in a brief war, many
humans in the series develop suspicious or even hostile attitudes
towards the game's various alien races. By the time of the first game,
which takes place several decades after the war, many humans still
retain such sentiments in addition to forming 'pro-human' organizations.
The Planet of the Apes franchise focuses on the analogy of apes becoming the dominant species in society and the fall of humans (see also human extinction). In the 1968 film, Taylor, a human states "take your stinking paws off me, you damn dirty ape!". In the 2001 film,
this is contrasted with Attar (a gorilla)'s quote "take your stinking
hands off me, you damn dirty human!". This links in with allusions that
in becoming the dominant species apes are becoming more like humans (anthropomorphism). In the film Battle for the Planet of the Apes, Virgil, an orangutan
states "ape has never killed ape, let alone an ape child. Aldo has
killed an ape child. The branch did not break. It was cut with a sword."
in reference to planned murder; a stereotypical human concept.
Additionally, in Dawn of the Planet of the Apes, Caesar states "I always think...ape better than human. I see now...how much like them we are."
In George Orwell's novel Animal Farm,
this theme of anthropocentrism is also present. Whereas originally the
animals planned for liberation from humans and animal equality, as
evident from the "seven commandments" such as "whatever goes upon two
legs is an enemy", "Whatever goes upon four legs, or has wings, is a
friend", "All animals are equal"; the pigs would later amend the
commandments with statements such as "All animals are equal, but some
animals are more equal than others", and "Four legs good, two legs
better."
The 2012 documentary The Superior Human? systematically
analyzes anthropocentrism and concludes that value is fundamentally an
opinion, and since life forms naturally value their own traits, most
humans are misled to believe that they are actually more valuable than
other species. This natural bias, according to the film, combined with a
received sense of comfort and an excuse for exploitation of non-humans
cause anthropocentrism to remain in society.
In his 2009 book Eating Animals, Jonathan Foer
describes anthropocentrism as "The conviction that humans are the
pinnacle of evolution, the appropriate yardstick by which to measure the
lives of other animals, and the rightful owners of everything that
lives."
The adaptive unconscious, first coined by social psychologistDaniel Wegner in 2002, is described as a set of mental processes that is able to affect judgement and decision-making, but is out of reach of the conscious mind. It is thought to be adaptive as it helps to keep the organism alive. Architecturally, the adaptive unconscious is said to be unreachable
because it is buried in an unknown part of the brain. This type of
thinking evolved earlier than the conscious mind, enabling the mind to transform information and think in ways that enhance an organism's survival.
It can be described as a quick sizing up of the world which interprets
information and decides how to act very quickly and outside the
conscious view. The adaptive unconscious is active in everyday
activities such as learning new material, detecting patterns, and
filtering information. It is also characterized by being unconscious,
unintentional, uncontrollable, and efficient without requiring cognitive
tools. Lacking the need for cognitive tools does not make the adaptive
unconscious any less useful than the conscious mind as the adaptive
unconscious allows for processes like memory formation, physical balancing, language, learning, and some emotional and personalities processes that includes judgement, decision making, impression formation,
evaluations, and goal pursuing. Despite being useful, the series of
processes of the adaptive unconscious will not always result in accurate
or correct decisions by the organism. The adaptive unconscious is
affected by things like emotional reaction, estimations, and experience
and is thus inclined to stereotyping and schema
which can lead to inaccuracy in decision making. The adaptive conscious
does however help decision making to eliminate cognitive biases such as
prejudice because of its lack of cognitive tools.
Overview
The
adaptive unconscious is defined as different from conscious processing
in a number of ways. It is faster, effortless, more focused on the
present, and less flexible. It is thought to be adaptive as it helps to keep us alive. Processing information without us even realising then feeding any we do need to know to our conscious brain.
In other theories of the mind, the unconscious is limited to
"low-level" activities, such as carrying out goals which have been
decided consciously. In contrast, the adaptive unconscious is now
thought to also be involved in "high-level" cognition such as
goal-setting.
The theory of the adaptive unconscious was influenced by some of Sigmund Freud and Carl Jung's
views on the unconscious mind. According to Freud, the unconscious mind
stored a lot of mental content which needs to be repressed; however,
the term adaptiveunconscious reflects the idea that much
of what the unconscious does is actually beneficial to the organism, in
closer accordance with Jung's thought. For example, its various
processes have been streamlined through evolution to quickly evaluate and respond to patterns in an organism's environment.
Intuition
Canadian journalist and author Malcolm Gladwell described intuition, not as an emotional reaction, but a very quick thinking. He said that if an individual realized that a truck is about to hit
him, there would be no time to think through all of his options and, to
survive, he must rely on this kind of decision-making apparatus, which
is capable of making very quick judgments based on little information. Gladwell also cited another example in the case of a kouros acquired by the J. Paul Getty Museum in Los Angeles. A team of scientists vouched for its authenticity but some historians, such as Thomas Hoving, instantly knew otherwise - that they felt an "intuitive repulsion" for the piece, which was eventually proved as fake.
Intuition comes from tapping into the adaptive unconscious. The
adaptive unconscious is that liminal zone between dreams and reality,
what might be called a reciprocal of experiences, memories, and dreams.
Working within the adaptive unconscious involves browsing through a
series of sense impressions
and making comparisons regarding a situation and using past experiences
to dissolve sensory boundaries which then results in intuition. There
is also a study that cited intuition as a result of the way our brain
stores, processes and uses the information of our subconscious. It becomes useful when reasoning and rationality provide no rapid answer.
The debate over the existence of introspection began in the late 19th
century with experiments involving placing people in different stimuli
contexts and them thinking about their thoughts and feelings after.
Similar experiments have continued since, all relying on asking the
participant to think about how they feel and their thoughts. These
research efforts have however been hampered by the fact it is currently
impossible to know if they are actually accessing their unconscious as
they do this, or if the information is just coming from their conscious
mind. This fundamental flaw makes experiments in this area less reliable in creating the debate over introspection.
More recent research suggests that many of our preferences,
attitudes, and ideas come from the adaptive unconscious. However,
subjects themselves do not realize this, and they are "unaware of their
own unawareness". People wrongly think they have direct insight into the origins of their
mental states. A subject is likely to give explanations for their
behavior (i.e. their preferences, attitudes, and ideas), but the subject
tends to be inaccurate in this "insight." The false explanations of
their own behavior is what psychologists call the introspection illusion.
In some experiments, subjects provide explanations that are
fabricated, distorted, or misinterpreted memories, but not lies – a
phenomenon called confabulation. This suggests that introspection is instead an indirect, unreliable process of inference. It has been argued that this "introspection illusion" underlies a
number of perceived differences between the self and other people,
because people trust these unreliable introspections when forming
attitudes about themselves but not about others.
However, this theory of the limits of introspection has been
highly controversial, and it has been difficult to test unambiguously
how much information individuals get from introspection. The difficulties in understanding the introspective method resulted in a
lack of theoretical development of the mind and more into behaviourism.
The difficulties of finding a method that worked (i.e. not
self-reporting by the patient) mean there was a halt in this area of
research until the cognitive revolution. Due to this the need to
understand the unconscious mind increased. Psychologists started to
focus on the limits of the conscious mind and more stimuli and learning
paradigm focused experiments for the unconscious mind. This helps understand the limitations of introspection or the lack of as some would argue.
Implicit-explicit relationships
The
theory of introspection is highly controversial. This is due to
research showing inconsistencies between our introspective reports and
factors affecting our stimuli. This issue lead to a new way to study
introspective access by using the adaptive unconsciousness. This is done
by looking at the implicit-explicit relationship, specifically the
differences between the two. Explicit processes involve cognitive
resources and are done with awareness. On the other hand, implicit
processes require at least one of the following: lack of intention; lack
of management; reduced awareness of where the responses came from; and
finally, high efficiency of processing. This shows the differences that
occur between the two processes and the contention around the
differences as they cannot be pinned down to one specific thing. These differences between implicit and explicit factors is argued to be
able to be used as evidence for introspection existence. If implicit processes become weaker than explicit processes then it can
result in larger differences between the two. This results in
consequences for future information processing and the well-being of the
person. However, if this occurs in the right conditions it can allow
for implicit processing output to enter the conscious mind. This leads
to a small self-insight into the adaptive unconscious allowing us to
understand it more.
Arguably, this argument of the independence of introspection
existence based on the implicit-explicit relationship may actually be
more conditional than originally thought. This view coincides with the
idea that access to our unconsciousness is dependent on the competition
between processes and their surrounding contexts. These contexts provide
the association our stimuli have with certain aspects of society. For
example, if you found pleasure in running, when running your cognitive
processes either implicit or explicit would tell your unconscious you
are feeling joy without you realising this was occurring. This could
then be translated into the conscious mind.
Adaptive unconscious versus conscious thinking
Many
used to think most of our behaviours, thoughts, feelings all came from
our conscious brain. However, as our understanding has grown it is
obvious our adaptive unconscious does much more than we originally
thought. Once we thought the creation of goals and self-reflection
occurred consciously but now we realise its all in our unconscious. Our
unconscious and conscious minds do have to work together though for us
to continue efficiently functioning. We need to understand the dual
system our brain uses between our adaptive unconscious and our conscious
mind more. Analysing information, attitudes and feelings in the
unconscious mind first which then contributes and creates our conscious
versions of this. The debate is no longer whether the adaptive unconscious exists but
more which is more important in our everyday decision making? The
adaptive unconscious or the conscious mind. Some would say it is
becoming more and more apparent that our unconscious seems to be much
more important than we originally thought especially compared to our
conscious brain. The low-level processing we used to think our adaptive
unconscious did we now realise may actually be the job of our conscious
mind. Our adaptive unconscious may actually be the power house in our brain
making the important decisions and holding the important information. It
does this all without us even realising.
A comparison of RNA (left) with DNA (right), showing the helices and nucleobases each employs
The RNA world is a hypothetical stage in the evolutionary history of life on Earth in which self-replicatingRNA molecules proliferated before the evolution of DNA and proteins. The term also refers to the hypothesis that posits the existence of this stage. Alexander Rich first proposed the concept of the RNA world in 1962, and Walter Gilbert coined the term in 1986.
Among the characteristics of RNA that suggest its original prominence are that:
Like DNA, RNA can store and replicate genetic information.
Although RNA is considerably more fragile than DNA, some ancient RNAs
may have evolved the ability to methylate other RNAs to protect them. The concurrent formation of all four RNA building blocks further strengthens the hypothesis.
Enzymes made of RNA (ribozymes) can catalyze (start or accelerate) chemical reactions that are critical for life, so it is conceivable that in an RNA world, ribozymes might have preceded enzymes made of protein.
Many coenzymes that have fundamental roles in cellular life, such as acetyl-CoA, NADH, FADH, and F420, are structurally strikingly similar to RNA and so may be surviving remnants of covalently bound coenzymes in an RNA world.
One of the most critical components of cells, the ribosome, is composed primarily of RNA.
Although alternative chemical paths to life have been proposed, and RNA-based life may not have been the first life to exist, the RNA world hypothesis seems to be the most favored abiogenesis
paradigm. However, even proponents agree that there is still not
conclusive evidence to completely falsify other paradigms and
hypotheses. Regardless of its plausibility in a prebiotic scenario, the RNA world can serve as a model system for studying the origin of life.
If the RNA world existed, it was probably followed by an age characterized by the evolution of ribonucleoproteins (RNP world), which in turn ushered in the era of DNA and longer proteins. DNA has
greater stability and durability than RNA, which may explain why it
became the predominant information storage molecule. Protein enzymes may have replaced RNA-based ribozymes as biocatalysts because the greater abundance and diversity of the monomers of which they are built makes them more versatile. As some cofactors
contain both nucleotide and amino-acid characteristics, it may be that
amino acids, peptides, and finally proteins initially were cofactors for
ribozymes.
History
One of the challenges in studying abiogenesis
is that the system of reproduction and metabolism utilized by all
extant life involves three distinct types of interdependent
macromolecules (DNA, RNA, and proteins), none of which can function and reproduce without the others, the classic chicken-and-egg paradox.
This suggests that life could not have arisen in its current form,
which has led researchers to hypothesize mechanisms whereby the current
system might have arisen from a simpler precursor system. American molecular biologist Alexander Rich was the first to posit a coherent hypothesis on the origin of nucleotides as precursors of life. In a 1962 article he explained that the primitive Earth's environment
could have produced RNA molecules (polynucleotide monomers) that
eventually acquired enzymatic and self-replicating functions.
Other mentions of RNA as a primordial molecule can be found in papers by Francis Crick and Leslie Orgel, as well as in Carl Woese's 1967 book The Genetic Code. Hans Kuhn
in 1972 laid out a possible process by which the modern genetic system
might have arisen from a nucleotide-based precursor, and this led Harold
White in 1976 to observe that many of the cofactors essential for
enzymatic function are either nucleotides or could have been derived
from nucleotides. He proposed a scenario whereby the critical
electrochemistry of enzymatic reactions would have necessitated
retention of the specific nucleotide moieties
of the original RNA-based enzymes carrying out the reactions, while the
remaining structural elements of the enzymes were gradually replaced by
protein, until all that remained of the original RNAs were these
nucleotide cofactors, "fossils of nucleic acid enzymes".
Properties of RNA
The
properties of RNA make the idea of the RNA world hypothesis
conceptually plausible, though its general acceptance as an explanation
for the origin of life requires further evidence. RNA is known to form efficient catalysts, and its similarity to DNA
makes clear its ability to store information. Opinions differ, however,
as to whether RNA constituted the first autonomous self-replicating
system or was a derivative of a still-earlier system. One version of the hypothesis is that a different type of nucleic acid, termed pre-RNA,
was the first one to emerge as a self-reproducing molecule, to be
replaced by RNA only later. On the other hand, the discovery in 2009
that activated pyrimidineribonucleotides can be synthesized under plausible prebiotic conditions suggests that it is premature to dismiss the RNA-first scenarios. Suggestions for 'simple' pre-RNA nucleic acids have included peptide nucleic acid (PNA), threose nucleic acid (TNA) or glycol nucleic acid (GNA). Despite their structural simplicity and possession of properties
comparable with RNA, the chemically plausible generation of "simpler"
nucleic acids under prebiotic conditions has yet to be demonstrated.
In the 1980s, RNA structures capable of self-processing were discovered, with the RNA moiety of ribonuclease P acting as its catalytic subunit. These catalytic RNAs – referred to as RNA enzymes, or ribozymes – are found in today's DNA-based life and could be examples of living fossils. Ribozymes play vital roles, such as that of the ribosome. The large subunit of the ribosome includes an rRNA responsible for the peptide bond-forming peptidyl transferase activity of protein synthesis. Many other ribozyme activities exist; for example, the hammerhead ribozyme performs self-cleavage and an RNA polymerase ribozyme can synthesize a short RNA strand from a primed RNA template.
Among the enzymatic properties important for the beginning of life are:
Self-replication
The ability to self-replicate
or synthesize other RNA molecules; relatively short RNA molecules that
can synthesize others have been artificially produced in the lab. The
shortest was 165 bases long, though it has been estimated that only part
of the molecule was crucial for this function.
One version, 189 bases long, had an error rate of just 1.1% per
nucleotide when synthesizing an 11-nucleotide long RNA strand from
primed template strands. This 189-base pair ribozyme could polymerize a template of at most 14
nucleotides in length, which is too short for self-replication, but is a
potential lead for further investigation. The longest primer extension performed by a ribozyme polymerase was 20 bases.
In 2016, researchers reported the use of in vitro evolution to
improve dramatically the activity and generality of an RNA polymerase
ribozyme by selecting variants that can synthesize functional RNA
molecules from an RNA template. Each RNA polymerase ribozyme was engineered to remain linked to its
new, synthesized RNA strand; this allowed the team to isolate successful
polymerases. The isolated RNA polymerases were again used for another
round of evolution. After several rounds of evolution, they obtained one
RNA polymerase ribozyme called 24-3 that was able to copy almost any
other RNA, from small catalysts to long RNA-based enzymes. Particular
RNAs were amplified up to 10,000 times, a first RNA version of the polymerase chain reaction (PCR).
Life is thought to have emerged from inanimate matter more than 3.5 billion years ago when a rudimentary abiogenesis process gradually evolved into an autocatalytic process capable of template-based replication. It was proposed on the basis of experimentally feasible RNA reactions catalyzed by a ribozyme, that the emergence of life was likely a gradual process involving the evolutionary properties of variation, heredity and reproduction, ultimately allowing for Darwinian evolution.
Recent efforts have been directed at trying to demonstrate RNA replication under conditions that assume the presence during early evolution of plausible nucleotide
intermediates and plausible environmental conditions that could favor
strand replication alternating with strand separation. One such effort
was the demonstration of high fidelity RNA copying using 2',3'-cyclic
phosphate ligation to allow polynucleotide synthesis under conditions also compatible with strand separation. In another study, it was shown that in a model oscillating Hadean
environment likely to have been abundant during early evolution, that
ribozyme-mediated RNA synthesis and replication can occur.
Catalysis
The ability to catalyze
simple chemical reactions—which would enhance creation of molecules
that are building blocks of RNA molecules (i.e., a strand of RNA that
would make creating more strands of RNA easier). Relatively short RNA
molecules with such abilities have been artificially formed in the lab.A recent study showed that almost any nucleic acid can evolve into a
catalytic sequence under appropriate selection. For instance, an
arbitrarily chosen 50-nucleotide DNA fragment encoding for the Bos taurus (cattle) albumin mRNA was subjected to test-tube evolution to derive a catalytic DNA (a deoxyribozyme,
also called a DNAzyme) with RNA-cleavage activity. After only a few
weeks, a DNAzyme with significant catalytic activity had evolved. In general, DNA is much more chemically inert than RNA and hence much
more resistant to obtaining catalytic properties. If in vitro evolution
works for DNA it will happen much more easily with RNA. In 2022, Nick
Lane and coauthors showed in a computational simulation that short RNA
sequences could have been capable of catalyzing CO2 fixation which supported protocell replication and growth.
Amino acid-RNA ligation
The ability to conjugate an amino acid to the 3'-end of an RNA in order to use its chemical groups or provide a long-branched aliphatic
sidechain. It has been suggested that amino acids may have initially
been involved with RNA molecules as cofactors enhancing or diversifying
their enzymatic capabilities, before evolving into more complex
peptides. In today's world, this is most commonly seen in the form of aminoacyl-tRNA.
Peptide bond formation
The ability to catalyse the formation of peptide bonds between amino acids to produce short peptides or longer proteins. This is done in modern cells by ribosomes, a complex of several RNA molecules known as rRNA
together with many proteins. The rRNA molecules are thought responsible
for its enzymatic activity, as no amino-acid residues lie within 18Å of the enzyme's active site, and, when the majority of the amino-acid residues in the ribosome were
stringently removed, the resulting ribosome retained its full peptidyl transferase activity, fully able to catalyze the formation of peptide bonds between amino acids.
A pseudo 2 fold symmetry of the region surrounding the peptidyl
transferase center (PTC) led to the hypothesis of the Proto-Ribosome,
that a vestige of an ancient dimeric molecule from the RNA world is
functioning within the ribosome. An RNA molecule derived from the 23S ribosomal RNA sequence for this
region has been synthesized in the lab in 2022 to test the
proto-ribosome hypothesis. It was able to dimerize and to form peptide
bonds.
A much shorter RNA molecule has been synthesized in the laboratory in 1999 with the ability to form peptide bonds, and it has been suggested that rRNA has evolved from a similar molecule.
tRNA
is suggested to have also evolved from RNA molecules that began to
catalyze amino acid transfer (also see the discussion of amino acid-RNA
ligation above). The current core of the ribosome, the PTC, may also have evolved from the concatenation of five proto-tRNAs.
A RNP world-type
hypothesis is that the tRNA acceptor stem and the catalytic domain of
the aaRS came earlier than the genetic code and the PTC.
Cofactors
Protein
enzymes catalyze various chemical reactions, but over half of them
incorporate cofactors to facilitate and diversify their catalytic
activities. Cofactors are essential in biology, as they are based largely on
nucleotides rather than amino acids. Ribozymes use nucleotide cofactors
to create metabolism, with two basic choices: non-covalent binding or
covalent attachment. Both approaches have been demonstrated using
directed evolution to reinvent RNA dupes of protein-catalyzed processes.
Lorsch and Szostak investigated ribozymes that could phosphorylate themselves and use ATP-γS
as a substrate. However, only one of the seven classes of selected
ribozymes had detectable ATP affinity, indicating that the ability to
bind ATP was compromised. NAD+- dependent redox ribozymes were also evaluated. The select ribozyme had a rate of enhancement of more than 107 fold and was proven to catalyze the reverse reaction - benzaldehyde reduction by NADH. Since the usage of adenosine as a cofactor is prevalent in current
metabolism and is likely to have been common in the RNA world, these
discoveries are essential for the evolution of metabolism in the RNA
world.
RNA in information storage
RNA
is a very similar molecule to DNA, with only two significant chemical
differences (the backbone of RNA uses ribose instead of deoxyribose and
its nucleobases include uracil instead of thymine).
The overall structure of RNA and DNA are immensely similar—one strand
of DNA and one of RNA can bind to form a double helical structure. This
makes the storage of information in RNA possible in a very similar way
to the storage of information in DNA. However, RNA is less stable, being
more prone to hydrolysis due to the presence of a hydroxyl group at the
ribose 2' position.
The major difference between RNA and DNA is the presence of a hydroxyl group at the 2'-position.
The major difference between RNA and DNA is the presence of a hydroxyl group at the 2'-position of the ribose sugar in RNA (illustration, right). This group makes the molecule less stable because, when not constrained
in a double helix, the 2' hydroxyl can chemically attack the adjacent phosphodiester bond to cleave the phosphodiester backbone. The hydroxyl group also forces the ribose into the C3'-endo sugar conformation unlike the C2'-endo conformation of the deoxyribose sugar in DNA. This forces an RNA double helix to change from a B-DNA structure to one more closely resembling A-DNA.
RNA also uses a different set of bases than DNA—adenine, guanine, cytosine and uracil, instead of adenine, guanine, cytosine and thymine. Chemically, uracil is similar to thymine, differing only by a methyl group, and its production requires less energy. In terms of base pairing, this has no effect. Adenine readily binds uracil or thymine. Uracil is, however, one product of damage to cytosine that makes RNA particularly susceptible to mutations that can replace a GC base pair with a GU (wobble) or AUbase pair.
RNA is thought to have preceded DNA, because of their ordering in the biosynthetic pathways. The deoxyribonucleotides used to make DNA are made from
ribonucleotides, the building blocks of RNA, by removing the 2'-hydroxyl
group. As a consequence, a cell must have the ability to make RNA
before it can make DNA.
Limitations of information storage in RNA
The chemical properties of RNA make large RNA molecules inherently fragile, and they can easily be broken down into their constituent nucleotides through hydrolysis. These limitations do not make use of RNA as an information storage
system impossible, simply energy intensive (to repair or replace
damaged RNA molecules) and prone to mutation. While this makes it
unsuitable for current 'DNA optimised' life, it may have been acceptable
for more primitive life.
Riboswitches have been found to act as regulators of gene expression, particularly in bacteria, but also in plants and archaea. Riboswitches alter their secondary structure in response to the binding of a metabolite. Riboswitch classes have highly conserved aptamer
domains, even among diverse organisms. When a target metabolite is
bound to this aptamer, conformational changes occur, modulating the
expression of genes carried by mRNA. These changes occur in an
expression platform, located downstream from the aptamer. This change in structure can result in the formation or disruption of a terminator, truncating or permitting transcription respectively. Alternatively, riboswitches may bind or occlude the Shine–Dalgarno sequence, affecting translation. It has been suggested that these originated in an RNA-based world. In addition, RNA thermometers regulate gene expression in response to temperature changes.
Support and difficulties
The RNA world hypothesis is supported by RNA's ability to do all three of to store, to transmit, and to duplicate genetic information, as DNA
does, and to perform enzymatic reactions, like protein-based enzymes.
Because it can carry out the types of tasks now performed by proteins
and DNA, RNA is believed to have once been capable of supporting
independent life on its own. Some viruses use RNA as their genetic material, rather than DNA. Further, while nucleotides were not found in experiments based on Miller-Urey experiment, their formation in prebiotically plausible conditions was reported in 2009; a purine base, adenine, is merely a pentamer of hydrogen cyanide, and it happens that this particular base is used as omnipresent energy vehicle in the cell: adenosine triphosphate is used everywhere in preference to guanosine triphosphate, cytidine triphosphate, uridine triphosphate or even deoxythymidine triphosphate,
which could serve just as well but are practically never used except as
building blocks for nucleic acid chains. Experiments with basic
ribozymes, like Bacteriophage Qβ
RNA, have shown that simple self-replicating RNA structures can
withstand even strong selective pressures (e.g., opposite-chirality
chain terminators).
Since there were no known chemical pathways for the abiogenic synthesis of nucleotides from pyrimidine nucleobases cytosine and uracil under prebiotic conditions, it is thought by some that nucleic acids did not contain these nucleobases seen in life's nucleic acids. The nucleoside cytosine has a half-life in isolation of 19 days at
100 °C (212 °F) and 17,000 years in freezing water, which some argue is
too short on the geologic time scale for accumulation. Others have questioned whether ribose and other backbone sugars could be stable enough to be found in the original genetic material, and have raised the issue that all ribose molecules would have had to be the same enantiomer, as any nucleotide of the wrong chirality acts as a chain terminator.
Pyrimidine ribonucleosides and their respective nucleotides have
been prebiotically synthesised by a sequence of reactions that by-pass
free sugars and assemble in a stepwise fashion by including nitrogenous
and oxygenous chemistries. In a series of publications, John Sutherland and his team at the School of Chemistry, University of Manchester, have demonstrated high yielding routes to cytidine and uridine ribonucleotides built from small 2- and 3-carbon fragments such as glycolaldehyde, glyceraldehyde or glyceraldehyde-3-phosphate, cyanamide, and cyanoacetylene. One of the steps in this sequence allows the isolation of enantiopureribose aminooxazoline if the enantiomeric excess of glyceraldehyde is 60% or greater, of possible interest toward biological homochirality. This can be viewed as a prebiotic purification step, where the said
compound spontaneously crystallised out from a mixture of the other
pentose aminooxazolines. Aminooxazolines can react with cyanoacetylene
in a mild and highly efficient manner, controlled by inorganic
phosphate, to give the cytidine ribonucleotides. Photoanomerization with
UV light
allows for inversion about the 1' anomeric centre to give the correct
beta stereochemistry; one problem with this chemistry is the selective
phosphorylation of alpha-cytidine at the 2' position. However, in 2009, they showed that the same simple building blocks
allow access, via phosphate controlled nucleobase elaboration, to
2',3'-cyclic pyrimidine nucleotides directly, which are known to be able
to polymerise into RNA. Organic chemist Donna Blackmond described this finding as "strong evidence" in favour of the RNA world. However, John Sutherland said that while his team's work suggests that
nucleic acids played an early and central role in the origin of life, it
did not necessarily support the RNA world hypothesis in the strict
sense, which he described as a "restrictive, hypothetical arrangement".
The Sutherland group's 2009 paper also highlighted the
possibility for the photo-sanitization of the pyrimidine-2',3'-cyclic
phosphates. A potential weakness of these routes is the generation of
enantioenriched glyceraldehyde, or its 3-phosphate derivative
(glyceraldehyde prefers to exist as its keto tautomer dihydroxyacetone).
On August 8, 2011, a report, based on NASA studies with meteorites found on Earth, was published suggesting building blocks of RNA (adenine, guanine, and related organic molecules) may have been formed in outer space. In 2017, research using a numerical model
suggested that a RNA world may have emerged in warm ponds on the early
Earth, and that meteorites were a plausible and probable source of the
RNA building blocks (ribose and nucleic acids) to these environments. On August 29, 2012, astronomers at Copenhagen University reported the detection of a specific sugar molecule, glycolaldehyde, in a distant star system. The molecule was found around the protostellar binary IRAS 16293-2422, which is located 400 light years from Earth. Because glycolaldehyde is needed to form RNA, this finding suggests
that complex organic molecules may form in stellar systems prior to the
formation of planets, eventually arriving on young planets early in
their formation. Nitriles,
key molecular precursors of the RNA World scenario, are among the most
abundant chemical families in the universe and have been found in
molecular clouds in the center of the Milky Way, protostars of different
masses, meteorites and comets, and also in the atmosphere of Titan, the
largest moon of Saturn.
A study in 2001 shows that nicotinic acid and its precursor, quinolinic acid can be "produced in yields as high as 7% in a six-step nonenzymatic sequence from aspartic acid and dihydroxyacetone phosphate
(DHAP). The biosynthesis of ribose phosphate could have produced DHAP
and other three carbon compounds. Aspartic acid could have been
available from prebiotic synthesis or from the ribozyme synthesis of
pyrimidines." This supports that NAD could have originated in the RNA
world. RNA sequences at lengths of 30 nucleotides, 60 nucleotides, 100
nucleotides, and 140 nucleotides, were capable of catalysis of "the
synthesis of three common coenzymes, CoA, NAD, and FAD, from their
precursors, 4'-phosphopantetheine, NMN, and FMN, respectively".
Prebiotic RNA synthesis
The RNA world hypothesis proposes that spontaneous polymerization of ribonucleotides led to the emergence of ribozymes and including an RNA replicase.
Nucleotides are the fundamental molecules that combine in series to
form RNA. They consist of a nitrogenous base attached to a
sugar-phosphate backbone. RNA is made of long stretches of specific
nucleotides arranged so that their sequence of bases carries
information. The RNA world hypothesis holds that in the primordial soup (or sandwich),
there existed free-floating nucleotides. These nucleotides regularly
formed bonds with one another, which often broke because the change in
energy was so low. However, certain sequences of base pairs have
catalytic properties that lower the energy of their chain being created,
enabling them to stay together for longer periods of time. As each
chain grew longer, it attracted more matching nucleotides faster,
causing chains to now form faster than they were breaking down.
These chains have been proposed by some as the first, primitive
forms of life. In an RNA world, different sets of RNA strands would have
had different replication outputs, which would have increased or
decreased their frequency in the population, i.e., natural selection.
As the fittest sets of RNA molecules expanded their numbers, novel
catalytic properties added by mutation, which benefitted their
persistence and expansion, could accumulate in the population. Such an autocatalytic set of ribozymes, capable of self-replication in about an hour, has been identified. It was produced by molecular competition (in vitro evolution) of candidate enzyme mixtures.
Competition between RNA may have favored the emergence of
cooperation between different RNA chains, opening the way for the
formation of the first protocell. Eventually, RNA chains developed with catalytic properties that help amino acids bind together (a process called peptide-bonding).
These amino acids could then assist with RNA synthesis, giving those
RNA chains that could serve as ribozymes the selective advantage. The
ability to catalyze one step in protein synthesis, aminoacylation of RNA, has been demonstrated in a short (five-nucleotide) segment of RNA.
In 2018, researchers at Georgia Institute of Technology identified three molecular candidates for the bases that might have formed an earliest version of proto-RNA: barbituric acid, melamine, and 2,4,6-triaminopyrimidine
(TAP). These three molecules are simpler versions of the four bases in
current RNA, which could have been present in larger amounts and could
still be forward-compatible with them but may have been discarded by evolution in exchange for more optimal base pairs. Specifically, TAP can form nucleotides with a large range of sugars. Both TAP and melamine base pair with barbituric acid. All three spontaneously form nucleotides with ribose.
Evolution of DNA
One
of the challenges posed by the RNA world hypothesis is to discover the
pathway by which an RNA-based system transitioned to one based on DNA.
Geoffrey Diemer and Ken Stedman, at Portland State University in Oregon,
may have found a solution. While conducting a survey of viruses in a
hot acidic lake in Lassen Volcanic National Park, California, they
uncovered evidence that a simple DNA virus had acquired a gene from a completely unrelated RNA-based virus.
Virologist Luis Villareal of the University of California Irvine also
suggests that viruses capable of converting an RNA-based gene into DNA
and then incorporating it into a more complex DNA-based genome might
have been common in the virus world during the RNA to DNA transition
some 4 billion years ago. This finding bolsters the argument for the transfer of information from
the RNA world to the emerging DNA world before the emergence of the last universal common ancestor. From the research, the diversity of this virus world is still with us.
Additional evidence supporting the concept of an RNA world has resulted from research on viroids, the first representatives of a novel domain of "subviral pathogens". Viroids infect plants, where most are pathogens, and consist of short
stretches of highly complementary, circular, single-stranded and
non-coding RNA without a protein coat. They are extremely small, ranging
from 246 to 467 nucleobases, compared to the smallest known viruses
capable of causing an infection, with genomes about 2,000 nucleobases in
length.
Based on their characteristic properties, in 1989 plant biologist Theodor Diener argued that viroids are more plausible living relics of the RNA world than introns and other RNAs considered candidates at the time. Diener's hypothesis would be expanded by the research group of Ricardo Flores, and gained a broader audience when in 2014, a New York Times science writer published a popularized version of the proposal.
The characteristics of viroids highlighted as consistent with an
RNA world were their small size, high guanine and cytosine content,
circular structure, structural periodicity, the lack of protein-coding
ability and, in some cases, ribozyme-mediated replication. One aspect critics of the hypothesis have focused on is that the exclusive hosts of all known viroids, angiosperms,
did not evolve until billions of years after the RNA world was
replaced, making viroids more likely to have arisen through later
evolutionary mechanisms unrelated to the RNA world than to have survived
via a cryptic host over that extended period. Whether they are relics of that world or of more recent origin, their
function as autonomous naked RNA is seen as analogous to that envisioned
for an RNA world.
Eigen et al. and Woese proposed that the genomes of early protocells
were composed of single-stranded RNA, and that individual genes
corresponded to separate RNA segments, rather than being linked
end-to-end as in present-day DNA genomes. A protocell that was haploid
(one copy of each RNA gene) would be vulnerable to damage, since a
single lesion in any RNA segment would be potentially lethal to the
protocell (e.g., by blocking replication or inhibiting the function of
an essential gene).
Vulnerability to damage could be reduced by maintaining two or
more copies of each RNA segment in each protocell, i.e., by maintaining
diploidy or polyploidy. Genome redundancy would allow a damaged RNA
segment to be replaced by an additional replication of its homolog.
However, for such a simple organism, the proportion of available
resources tied up in the genetic material would be a large fraction of
the total resource budget. Under limited resource conditions, the
protocell reproductive rate would likely be inversely related to ploidy
number. The protocell's fitness would be reduced by the costs of
redundancy. Consequently, coping with damaged RNA genes while minimizing
the costs of redundancy would likely have been a fundamental problem
for early protocells.
A cost-benefit analysis was carried out in which the costs of
maintaining redundancy were balanced against the costs of genome damage. This analysis led to the conclusion that, under a wide range of
circumstances, the selected strategy would be for each protocell to be
haploid, but to periodically fuse with another haploid protocell to form
a transient diploid. The retention of the haploid state maximizes the
growth rate. The periodic fusions permit mutual reactivation of
otherwise lethally damaged protocells. If at least one damage-free copy
of each RNA gene is present in the transient diploid, viable progeny can
be formed. For two, rather than one, viable daughter cells to be
produced would require an extra replication of the intact RNA gene
homologous to any RNA gene that had been damaged prior to the division
of the fused protocell. The cycle of haploid reproduction, with
occasional fusion to a transient diploid state, followed by splitting to
the haploid state, can be considered to be the sexual cycle in its most
primitive form. In the absence of this sexual cycle, haploid protocells with damage in an essential RNA gene would simply die.
This model for the early sexual cycle is hypothetical, but it is
very similar to the known sexual behavior of the segmented RNA viruses,
which are among the simplest organisms known. Influenza virus, whose genome consists of 8 physically separated single-stranded RNA segments, is an example of this type of virus. In segmented RNA viruses, "mating"
can occur when a host cell is infected by at least two virus particles.
If these viruses each contain an RNA segment with a lethal damage,
multiple infection can lead to reactivation providing that at least one
undamaged copy of each virus gene is present in the infected cell. This
phenomenon is known as "multiplicity reactivation". Multiplicity
reactivation has been reported to occur in influenza virus infections
after induction of RNA damage by UV-irradiation, and ionizing radiation.
Patrick Forterre has been working on a novel hypothesis, called "three viruses, three domains": that viruses were instrumental in the transition from RNA to DNA and the evolution of Bacteria, Archaea, and Eukaryota. He believes the last universal common ancestor was RNA-based and evolved RNA viruses. Some of the viruses evolved into
DNA viruses to protect their genes from attack. Through the process of
viral infection into hosts the three domains of life evolved.
Another interesting proposal is the idea that RNA synthesis might have been driven by temperature gradients, in the process of thermosynthesis. Single nucleotides have been shown to catalyze organic reactions.
Steven Benner has argued that chemical conditions on the planet Mars, such as the presence of boron, molybdenum, and oxygen, may have been better for initially producing RNA molecules than those on Earth. If so, life-suitable molecules, originating on Mars, may have later migrated to Earth via mechanisms of panspermia or similar process.
The hypothesized existence of an RNA world does not exclude a
"Pre-RNA world", where a metabolic system based on a different nucleic
acid is proposed to pre-date RNA. A candidate nucleic acid is peptide
nucleic acid (PNA), which uses simple peptide bonds to link nucleobases.
Some of the difficulties of producing the precursors on earth are
bypassed by another alternative or complementary theory for their
origin, panspermia.
It discusses the possibility that the earliest life on this planet was
carried here from somewhere else in the galaxy, possibly on meteorites
similar to the Murchison meteorite. Sugar molecules, including ribose, have been found in meteorites.
Another proposal is that the dual-molecule system we see today, where
a nucleotide-based molecule is needed to synthesize protein, and a
peptide-based (protein) molecule is needed to make nucleic acid
polymers, represents the original form of life. This theory is called RNA-peptide coevolution, or the Peptide-RNA world, and offers a possible explanation for the
rapid evolution of high-quality replication in RNA (since proteins are
catalysts), with the disadvantage of having to postulate the coincident
formation of two complex molecules, an enzyme (from peptides) and a RNA
(from nucleotides). In this Peptide-RNA World scenario, RNA would have
contained the instructions for life, while peptides (simple protein
enzymes) would have accelerated key chemical reactions to carry out
those instructions. The study leaves open the question of exactly how those primitive
systems managed to replicate themselves — something neither the RNA
World hypothesis nor the Peptide-RNA World theory can yet explain,
unless polymerases (enzymes that rapidly assemble the RNA molecule) played a role.
A research project completed in March 2015 by the Sutherland
group found that a network of reactions beginning with hydrogen cyanide
and hydrogen sulfide,
in streams of water irradiated by UV light, could produce the chemical
components of proteins and lipids, alongside those of RNA. The researchers used the term "cyanosulfidic" to describe this network of reactions. In November 2017, a team at the Scripps Research Institute identified reactions involving the compound diamidophosphate
which could have linked the chemical components into short peptide and
lipid chains as well as short RNA-like chains of nucleotides.
Implications
The RNA world hypothesis, if true, has important implications for the definition of life and the origin of life. For most of the time that followed Franklin, Watson and Crick's
elucidation of DNA structure in 1953, life was largely defined in terms
of DNA and proteins: DNA and proteins seemed the dominant
macromolecules in the living cell, with RNA only aiding in creating
proteins from the DNA blueprint.
The RNA world hypothesis places RNA at center-stage when life
originated. The RNA world hypothesis is supported by the observations
that ribosomes are ribozymes: the catalytic site is composed of RNA, and proteins hold no major
structural role and are of peripheral functional importance. This was
confirmed with the deciphering of the 3-dimensional structure of the
ribosome in 2001. Specifically, peptide bond formation, the reaction
that binds amino acids together into proteins, is now known to be catalyzed by an adenine residue in the rRNA.
RNAs are known to play roles in other cellular catalytic
processes, specifically in the targeting of enzymes to specific RNA
sequences. In eukaryotes, the processing of pre-mRNA and RNA editing take place at sites determined by the base pairing between the target RNA and RNA constituents of small nuclear ribonucleoproteins (snRNPs). Such enzyme targeting is also responsible for gene down regulation through RNA interference
(RNAi), where an enzyme-associated guide RNA targets specific mRNA for
selective destruction. Likewise, in eukaryotes the maintenance of telomeres involves copying of an RNA template that is a constituent part of the telomerase ribonucleoprotein enzyme. Another cellular organelle, the vault, includes a ribonucleoprotein component, although the function of this organelle remains to be elucidated.
