The mediaeval great chain of being as a staircase, implying the possibility of progress: Ramon Lull's Ladder of Ascent and Descent of the Mind, 1305
Alternatives to evolution by natural selection, also described as non-Darwinian mechanisms of evolution, have been proposed by scholars investigating biology since classical times to explain signs of evolution and the relatedness
of different groups of living things. The alternatives in question do
not encompass religious points of view such as young or old earth creationism or intelligent design, but are limited to explanations proposed by biologists, though one was confusingly named 'theistic evolution' by Asa Gray.
Where the fact of evolutionary change was accepted but the mechanism proposed by Charles Darwin, natural selection, was denied, explanations of evolution such as Lamarckism, catastrophism, orthogenesis, vitalism, structuralism and mutationism (called saltationism before 1900) were entertained. Different factors motivated people to propose non-Darwinian
mechanisms of evolution. Natural selection, with its emphasis on death
and competition, did not appeal to some naturalists because they felt it
immoral, leaving little room for teleology
or the concept of progress in the development of life. Some who came to
accept evolution, but disliked natural selection, raised religious
objections. Others felt that evolution was an inherently progressive
process that natural selection alone was insufficient to explain. Still
others felt that nature, including the development of life, followed
orderly patterns that natural selection could not explain.
By the start of the 20th century, evolution was generally accepted by biologists but natural selection was in eclipse.
Many alternative theories were proposed, but biologists were quick to
discount theories such as orthogenesis, vitalism and Lamarckism which
offered no mechanism for evolution. Mutationism did propose a mechanism,
but it was not generally accepted. The modern synthesis
a generation later claimed to sweep away all the alternatives to
Darwinian evolution, though some have been revived as molecular
mechanisms for them have been discovered.
Unchanging forms
Aristotle did not embrace either divine creation or evolution, instead arguing in his biology that each species (eidos) was immutable, breeding true to its ideal eternal form (not the same as Plato's theory of Forms). Aristotle's suggestion in De Generatione Animalium of a fixed hierarchy in nature - a scala naturae ("ladder of nature") provided an early explanation of the continuity of living things. Aristotle saw that animals were teleological (functionally end-directed), and had parts that were homologous with those of other animals, but he did not connect these ideas into a concept of evolutionary progress.
In the Middle Ages, Scholasticism developed Aristotle's view into the idea of a great chain of being.
The image of a ladder inherently suggests the possibility of climbing,
but both the ancient Greeks and mediaeval scholastics such as Ramon Lull maintained that each species remained fixed from the moment of its creation.
By 1818, however, Étienne Geoffroy Saint-Hilaire argued in his Philosophie anatomique
that the chain was "a progressive series", where animals like molluscs
low on the chain could "rise, by addition of parts, from the simplicity
of the first formations to the complication of the creatures at the head
of the scale", given sufficient time. Accordingly, Geoffroy and later
biologists looked for explanations of such evolutionary change.
Georges Cuvier's 1812 Recherches sur les Ossements Fossiles
set out his doctrine of the correlation of parts, namely that since an
organism was a whole system, all its parts mutually corresponded,
contributing to the function of the whole. So, from a single bone the
zoologist could often tell what class or even genus the animal belonged
to. And if an animal had teeth adapted for cutting meat, the zoologist
could be sure without even looking that its sense organs would be those
of a predator and its intestines those of a carnivore. A species had an
irreducible functional complexity, and "none of its parts can change
without the others changing too".
Evolutionists expected one part to change at a time, one change to
follow another. In Cuvier's view, evolution was impossible, as any one
change would unbalance the whole delicate system.
Louis Agassiz's
1856 "Essay on Classification" exemplified German philosophical
idealism. This held that each species was complex within itself, had
complex relationships to other organisms, and fitted precisely into its
environment, as a pine tree in a forest, and could not survive outside
those circles. The argument from such ideal forms opposed evolution
without offering an actual alternative mechanism. Richard Owen held a similar view in Britain.
The Lamarckian social philosopher and evolutionist Herbert Spencer, ironically the author of the phrase "survival of the fittest" adopted by Darwin,
used an argument like Cuvier's to oppose natural selection. In 1893, he
stated that a change in any one structure of the body would require all
the other parts to adapt to fit in with the new arrangement. From this,
he argued that it was unlikely that all the changes could appear at the
right moment if each one depended on random variation; whereas in a
Lamarckian world, all the parts would naturally adapt at once, through a
changed pattern of use and disuse.
Alternative explanations of change
Where the fact of evolutionary change was accepted by biologists but natural selection was denied, including but not limited to the late 19th century eclipse of Darwinism, alternative scientific explanations such as Lamarckism, orthogenesis, structuralism, catastrophism, vitalism and theistic evolution were entertained, not necessarily separately. (Purely religious points of view such as young or old earth creationism or intelligent design
are not considered here.) Different factors motivated people to propose
non-Darwinian evolutionary mechanisms. Natural selection, with its
emphasis on death and competition, did not appeal to some naturalists
because they felt it immoral, leaving little room for teleology or the concept of progress in the development of life. Some of these scientists and philosophers, like St. George Jackson Mivart and Charles Lyell, who came to accept evolution but disliked natural selection, raised religious objections. Others, such as the biologist and philosopher Herbert Spencer, the botanist George Henslow (son of Darwin's mentor John Stevens Henslow, also a botanist), and the author Samuel Butler,
felt that evolution was an inherently progressive process that natural
selection alone was insufficient to explain. Still others, including the
American paleontologists Edward Drinker Cope and Alpheus Hyatt,
had an idealist perspective and felt that nature, including the
development of life, followed orderly patterns that natural selection
could not explain.
Some felt that natural selection would be too slow, given the estimates of the age of the earth and sun (10–100 million years) being made at the time by physicists such as Lord Kelvin,
and some felt that natural selection could not work because at the time
the models for inheritance involved blending of inherited
characteristics, an objection raised by the engineer Fleeming Jenkin in a review of Origin written shortly after its publication. Another factor at the end of the 19th century was the rise of a new faction of biologists, typified by geneticists like Hugo de Vries and Thomas Hunt Morgan, who wanted to recast biology as an experimental laboratory science. They distrusted the work of naturalists like Darwin and Alfred Russel Wallace, dependent on field observations of variation, adaptation, and biogeography, as being overly anecdotal. Instead they focused on topics like physiology and genetics
that could be investigated with controlled experiments in the
laboratory, and discounted less accessible phenomena like natural
selection and adaptation to the environment.
Vitalism holds that living organisms differ from other things in
containing something non-physical, such as a fluid or vital spirit, that
makes them live. The theory dates to ancient Egypt.
Since Early Modern times, vitalism stood in contrast to the mechanistic explanation of biological systems started by Descartes. Nineteenth century chemists set out to disprove the claim that forming organic compounds required vitalist influence. In 1828, Friedrich Wöhler showed that urea could be made entirely from inorganic chemicals. Louis Pasteur believed that fermentation required whole organisms, which he supposed carried out chemical reactions found only in living things. The embryologist Hans Driesch, experimenting on sea urchin eggs, showed that separating the first two cells led to two complete but small blastulas,
seemingly showing that cell division did not divide the egg into
sub-mechanisms, but created more cells each with the vital capability to
form a new organism. Vitalism faded out with the demonstration of more
satisfactory mechanistic explanations of each of the functions of a
living cell or organism. By 1931, biologists had "almost unanimously abandoned vitalism as an acknowledged belief."
Theistic evolution
The American botanist Asa Gray used the name "theistic evolution" for his point of view, presented in his 1876 book Essays and Reviews Pertaining to Darwinism. He argued that the deity supplies beneficial mutations to guide evolution. St George Jackson Mivart argued instead in his 1871 On the Genesis of Species
that the deity, equipped with foreknowledge, sets the direction of
evolution by specifying the (orthogenetic) laws that govern it, and
leaves species to evolve according to the conditions they experience as
time goes by. The Duke of Argyll set out similar views in his 1867 book The Reign of Law.
According to the historian Edward Larson, the theory failed as an
explanation in the minds of late 19th century biologists as it broke the
rules of methodological naturalism which they had grown to expect.
Accordingly, by around 1900, biologists no longer saw theistic
evolution as a valid theory. In Larson's view, by then it "did not even
merit a nod among scientists." Biologists might argue about mechanisms,
but they were in no doubt that a mechanistic explanation was needed. In the 20th century, theistic evolution could take other forms, such as the orthogenesis of Teilhard de Chardin.
Orthogenesis is the hypothesis that life has an innate tendency to
change, developing in a unilinear fashion in a particular direction, or
simply making some kind of definite progress. Many different versions
have been proposed, some such as that of Teilhard de Chardin openly spiritual, others such as Theodor Eimer's
apparently simply biological. These theories often combined
orthogenesis with other supposed mechanisms. For example, Eimer believed
in Lamarckian evolution, but felt that internal laws of growth
determined which characteristics would be acquired and would guide the
long-term direction of evolution.
Orthogenesis was popular among paleontologists such as Henry Fairfield Osborn.
They believed that the fossil record showed unidirectional change, but
did not necessarily accept that the mechanism driving orthogenesis was teleological (goal-directed). Osborn argued in his 1918 book Origin and Evolution of Life that trends in Titanothere horns were both orthogenetic and non-adaptive, and could be detrimental to the organism. For instance, they supposed that the large antlers of the Irish elk had caused its extinction.
Support for orthogenesis fell during the modern synthesis
in the 1940s when it became apparent that it could not explain the
complex branching patterns of evolution revealed by statistical analysis
of the fossil record.
Work in the 21st century has supported the mechanism and existence of
mutation-biased adaptation (a form of mutationism), meaning that
constrained orthogenesis is now seen as possible. Moreover, the self-organizing processes involved in certain aspects of embryonic development
often exhibit stereotypical morphological outcomes, suggesting that
evolution will proceed in preferred directions once key molecular
components are in place.
Jean-Baptiste Lamarck's 1809 evolutionary theory, transmutation of species,
was based on a progressive (orthogenetic) drive toward greater
complexity. Lamarck also shared the belief, common at the time, that characteristics acquired during an organism's life could be inherited
by the next generation, producing adaptation to the environment. Such
characteristics were caused by the use or disuse of the affected part of
the body. This minor component of Lamarck's theory became known, much
later, as Lamarckism. Darwin included Effects of the increased Use and Disuse of Parts, as controlled by Natural Selection in On the Origin of Species,
giving examples such as large ground feeding birds getting stronger
legs through exercise, and weaker wings from not flying until, like the ostrich, they could not fly at all. In the late 19th century, neo-Lamarckism was supported by the German biologist Ernst Haeckel, the American paleontologistsEdward Drinker Cope and Alpheus Hyatt, and the American entomologistAlpheus Packard. Butler and Cope believed that this allowed organisms to effectively drive their own evolution.
Packard argued that the loss of vision in the blind cave insects he
studied was best explained through a Lamarckian process of atrophy
through disuse combined with inheritance of acquired characteristics. Meanwhile, the English botanist George Henslow
studied how environmental stress affected the development of plants,
and he wrote that the variations induced by such environmental factors
could largely explain evolution; he did not see the need to demonstrate
that such variations could actually be inherited.
Critics pointed out that there was no solid evidence for the
inheritance of acquired characteristics. Instead, the experimental work
of the German biologist August Weismann
resulted in the germ plasm theory of inheritance, which Weismann said
made the inheritance of acquired characteristics impossible, since the Weismann barrier would prevent any changes that occurred to the body after birth from being inherited by the next generation.
In modern epigenetics, biologists observe that phenotypes depend on heritable changes to gene expression that do not involve changes to the DNA sequence. These changes can cross generations in plants, animals, and prokaryotes.
This is not identical to traditional Lamarckism, as the changes do not
last indefinitely and do not affect the germ line and hence the
evolution of genes.
Catastrophism is the hypothesis, argued by the French anatomist and paleontologistGeorges Cuvier in his 1812 Recherches sur les ossements fossiles de quadrupèdes, that the various extinctions and the patterns of faunal succession seen in the fossil
record were caused by large-scale natural catastrophes such as volcanic
eruptions and, for the most recent extinctions in Eurasia, the
inundation of low-lying areas by the sea. This was explained purely by natural events: he did not mention Noah's flood,
nor did he ever refer to divine creation as the mechanism for
repopulation after an extinction event, though he did not support
evolutionary theories such as those of his contemporaries Lamarck and
Geoffroy Saint-Hilaire either. Cuvier believed that the stratigraphic
record indicated that there had been several such catastrophes,
recurring natural events, separated by long periods of stability during
the history of life on earth. This led him to believe the Earth was
several million years old.
Catastrophism has found a place in modern biology with the Cretaceous–Paleogene extinction event at the end of the Cretaceous period, as proposed in a paper by
Walter and Luis Alvarez in 1980. It argued that a 10 kilometres (6.2 mi) asteroidstruck Earth 66 million years ago at the end of the Cretaceous period. The event, whatever it was, made about 70% of all species extinct, including the dinosaurs, leaving behind the Cretaceous–Paleogene boundary. In 1990, a 180 kilometres (110 mi) candidate crater marking the impact was identified at Chicxulub in the Yucatán Peninsula of Mexico.
Biological structuralism objects to an exclusively Darwinian
explanation of natural selection, arguing that other mechanisms also
guide evolution, and sometimes implying that these supersede selection
altogether. Structuralists have proposed different mechanisms that might have guided the formation of body plans. Before Darwin, Étienne Geoffroy Saint-Hilaire argued that animals shared homologous parts, and that if one was enlarged, the others would be reduced in compensation. After Darwin, D'Arcy Thompson hinted at vitalism and offered geometric explanations in his classic 1917 book On Growth and Form. Adolf Seilacher suggested mechanical inflation for "pneu" structures in Ediacaran biota fossils such as Dickinsonia. Günter P. Wagner argued for developmental bias, structural constraints on embryonic development. Stuart Kauffman favoured self-organisation, the idea that complex structure emerges holistically and spontaneously from the dynamic interaction of all parts of an organism. Michael Denton argued for laws of form by which Platonic universals or "Types" are self-organised. In 1979 Stephen J. Gould and Richard Lewontin proposed biological "spandrels", features created as a byproduct of the adaptation of nearby structures. Gerd Müller and Stuart Newman argued that the appearance in the fossil record of most of the current phyla in the Cambrian explosion was "pre-Mendelian" evolution caused by plastic responses of morphogenetic systems that were partly organized by physical mechanisms. Brian Goodwin, described by Wagner as part of "a fringe movement in evolutionary biology", denied that biological complexity can be reduced to natural selection, and argued that pattern formation is driven by morphogenetic fields. Darwinian biologists have criticised structuralism, emphasising that there is plentiful evidence from deep homology that genes have been involved in shaping organisms throughout evolutionary history. They accept that some structures such as the cell membrane self-assemble, but question the ability of self-organisation to drive large-scale evolution.
Saltationism held that new species arise as a result of large mutations.
It was seen as a much faster alternative to the Darwinian concept of a
gradual process of small random variations being acted on by natural
selection. It was popular with early geneticists such as Hugo de Vries, who along with Carl Correns helped rediscover Gregor Mendel's laws of inheritance in 1900, William Bateson, a British zoologist who switched to genetics, and early in his career, Thomas Hunt Morgan. These ideas developed into mutationism, the mutation theory of evolution.
This held that species went through periods of rapid mutation, possibly
as a result of environmental stress, that could produce multiple
mutations, and in some cases completely new species, in a single
generation, based on de Vries's experiments with the evening primrose, Oenothera,
from 1886. The primroses seemed to be constantly producing new
varieties with striking variations in form and color, some of which
appeared to be new species because plants of the new generation could
only be crossed with one another, not with their parents. However, Hermann Joseph Muller showed in 1918 that the new varieties de Vries had observed were the result of polyploid hybrids rather than rapid genetic mutation.
Initially, de Vries and Morgan believed that mutations were so
large as to create new forms such as subspecies or even species
instantly. Morgan's 1910 fruit fly experiments, in which he isolated
mutations for characteristics such as white eyes, changed his mind. He
saw that mutations represented small Mendelian
characteristics that would only spread through a population when they
were beneficial, helped by natural selection. This represented the germ
of the modern synthesis, and the beginning of the end for mutationism as an evolutionary force.
Contemporary biologists accept that mutation and selection both
play roles in evolution; the mainstream view is that while mutation
supplies material for selection in the form of variation, all non-random
outcomes are caused by natural selection. Masatoshi Nei
argues instead that the production of more efficient genotypes by
mutation is fundamental for evolution, and that evolution is often
mutation-limited. The endosymbiotic theory implies rare but major events of saltational evolution by symbiogenesis. Carl Woese and colleagues suggested that the absence of RNA signature continuum between domains of bacteria, archaea, and eukarya shows that these major lineages materialized via large saltations in cellular organization.
Saltation at a variety of scales is agreed to be possible by mechanisms including polyploidy, which certainly can create new species of plant, gene duplication, lateral gene transfer, and transposable elements (jumping genes).
The neutral theory of molecular evolution, proposed by Motoo Kimura in 1968, holds that at the molecular level most evolutionary changes and most of the variation within and between species is not caused by natural selection but by genetic drift of mutantalleles that are neutral. A neutral mutation
is one that does not affect an organism's ability to survive and
reproduce. The neutral theory allows for the possibility that most
mutations are deleterious, but holds that because these are rapidly
purged by natural selection, they do not make significant contributions
to variation within and between species at the molecular level.
Mutations that are not deleterious are assumed to be mostly neutral
rather than beneficial.
The theory was controversial as it sounded like a challenge to
Darwinian evolution; controversy was intensified by a 1969 paper by Jack Lester King and Thomas H. Jukes, provocatively but misleadingly titled "Non-Darwinian Evolution". It provided a wide variety of evidence including protein sequence comparisons, studies of the Treffers mutator gene in E. coli, analysis of the genetic code, and comparative immunology, to argue that most protein evolution is due to neutral mutations and genetic drift.
According to Kimura, the theory applies only for evolution at the molecular level, while phenotypic evolution is controlled by natural selection, so the neutral theory does not constitute a true alternative.
Combined theories
Multiple
explanations have been offered since the 19th century
for how evolution
took place, given that many scientists
initially had objections to
natural selection. Many of these
theories led to some form of directed
evolution (orthogenesis),
with or without invoking divine control directly or indirectly.
in a combination of theistic evolution, Lamarckism, vitalism,
and orthogenesis, represented by the sequence of arrows on
the extreme left of the diagram.
The various alternatives to Darwinian evolution by natural selection
were not necessarily mutually exclusive. The evolutionary philosophy of
the American palaeontologist Edward Drinker Cope
is a case in point. Cope, a religious man, began his career denying the
possibility of evolution. In the 1860s, he accepted that evolution
could occur, but, influenced by Agassiz, rejected natural selection. Cope accepted instead the theory of recapitulation of evolutionary
history during the growth of the embryo - that ontogeny recapitulates phylogeny, which Agassiz believed showed a divine plan leading straight up to man, in a pattern revealed both in embryology and palaeontology.
Cope did not go so far, seeing that evolution created a branching tree
of forms, as Darwin had suggested. Each evolutionary step was however
non-random: the direction was determined in advance and had a regular
pattern (orthogenesis), and steps were not adaptive but part of a divine
plan (theistic evolution). This left unanswered the question of why
each step should occur, and Cope switched his theory to accommodate
functional adaptation for each change. Still rejecting natural selection
as the cause of adaptation, Cope turned to Lamarckism to provide the
force guiding evolution. Finally, Cope supposed that Lamarckian use and
disuse operated by causing a vitalist growth-force substance,
"bathmism", to be concentrated in the areas of the body being most
intensively used; in turn, it made these areas develop at the expense of
the rest. Cope's complex set of beliefs thus assembled five
evolutionary philosophies: recapitulationism, orthogenesis, theistic
evolution, Lamarckism, and vitalism.
Other palaeontologists and field naturalists continued to hold beliefs
combining orthogenesis and Lamarckism until the modern synthesis in the
1930s.
Rebirth of natural selection, with continuing alternatives
By the start of the 20th century, during the eclipse of Darwinism,
biologists were doubtful of natural selection, but equally were quick
to discount theories such as orthogenesis, vitalism and Lamarckism which
offered no mechanism for evolution. Mutationism did propose a
mechanism, but it was not generally accepted. The modern synthesis
a generation later, roughly between 1918 and 1932, broadly swept away
all the alternatives to Darwinism, though some including forms of
orthogenesis, epigenetic mechanisms that resemble Lamarckian inheritance of acquired characteristics, catastrophism, structuralism, and mutationism have been revived, such as through the discovery of molecular mechanisms.
Biology has become Darwinian, but belief in some form of progress
(orthogenesis) remains both in the public mind and among biologists.
Ruse argues that evolutionary biologists will probably continue to
believe in progress for three reasons. Firstly, the anthropic principle
demands people able to ask about the process that led to their own
existence, as if they were the pinnacle of such progress. Secondly,
scientists in general and evolutionists in particular believe that their
work is leading them progressively closer to a true grasp of reality, as knowledge increases, and hence (runs the argument) there is progress in nature also. Ruse notes in this regard that Richard Dawkins explicitly compares cultural progress with memes
to biological progress with genes. Thirdly, evolutionists are
self-selected; they are people, such as the entomologist and
sociobiologist E. O. Wilson, who are interested in progress to supply a meaning for life.
The evolutionary history of life on Earth traces the processes by which both living organisms and fossil
organisms evolved since life emerged on the planet, until the present.
Earth formed about 4.5 billion years (Ga) ago and evidence suggests life
emerged prior to 3.7 Ga.
Although there is some evidence to suggest that life appeared as early
as 4.1 to 4.28 Ga this evidence remains controversial due to the
non-biological mechanisms that may have formed these potential
signatures of past life. The similarities among all known species of present-day organisms indicate that they have diverged through the process of evolution from a common ancestor. It is estimated that more than 99 percent of all species, amounting to over five billion species, that ever lived on Earth are extinct. Estimates on the number of Earth's current species range from 10 million to 14 million, of which about 1.9 million are estimated to have been named and 1.6 million documented in a central database to date.
More recently, in May 2016, scientists reported that 1 trillion species
are estimated to be on Earth currently with only one-thousandth of one
percent described.
The earliest evidence of life on Earth comes from biogeniccarbon signatures and stromatolite fossils discovered in 3.7 billion-year-old metasedimentary rocks discovered in western Greenland. In 2015, "remains of biotic life" were found in 4.1 billion-year-old rocks in Western Australia. In March 2017, putative evidence of possibly the oldest forms of life on Earth was reported in the form of fossilized microorganisms discovered in hydrothermal vent precipitates in the Nuvvuagittuq Belt of Quebec, Canada, that may have lived as early as 4.28 billion years ago, not long after the oceans formed 4.4 billion years ago, and not long after the formation of the Earth 4.54 billion years ago. According to biologist Stephen Blair Hedges, "If life arose relatively quickly on Earth ... then it could be common in the universe."
Microbial mats of coexisting bacteria and archaea were the dominant form of life in the early Archean and many of the major steps in early evolution are thought to have taken place within them. The evolution of photosynthesis, around 3.5 Ga, eventually led to a buildup of its waste product, oxygen, in the atmosphere, leading to the great oxygenation event, beginning around 2.4 Ga. The earliest evidence of eukaryotes (complex cells with organelles) dates from 1.85 Ga, and while they may have been present earlier, their diversification accelerated when they started using oxygen in their metabolism. Later, around 1.7 Ga, multicellular organisms began to appear, with differentiated cells performing specialised functions. Sexual reproduction, which involves the fusion of male and female reproductive cells (gametes) to create a zygote in a process called fertilization is, in contrast to asexual reproduction, the primary method of reproduction for the vast majority of macroscopic organisms, including almost all eukaryotes (which includes animals and plants). However the origin and evolution of sexual reproduction remain a puzzle for biologists though it did evolve from a common ancestor that was a single celled eukaryotic species. Bilateria, animals with a front and a back, appeared by 555 Ma (million years ago).
The earliest complex land plants date back to around 850 Ma,
from carbon isotopes in Precambrian rocks, while algae-like
multicellular land plants are dated back even to about 1 billion years
ago, although evidence suggests that microorganisms formed the earliest terrestrial ecosystems, at least 2.7 Ga. Microorganisms are thought to have paved the way for the inception of land plants in the Ordovician. Land plants were so successful that they are thought to have contributed to the Late Devonian extinction event. (The long causal chain implied seems to involve the success of early tree archaeopteris (1) drew down CO2 levels, leading to global cooling and lowered sea levels, (2) roots of archeopteris fostered soil development which increased rock weathering, and the subsequent nutrient run-off may have triggered algal blooms resulting in anoxic events which caused marine-life die-offs. Marine species were the primary victims of the Late Devonian extinction.)
The oldest meteorite fragments found on Earth are about 4.54 billion years old; this, coupled primarily with the dating of ancient lead deposits, has put the estimated age of Earth at around that time. The Moon has the same composition as Earth's crust but does not contain an iron-rich core like the Earth's. Many scientists think that about 40 million years after the formation of Earth, it collided with a body the size of Mars, throwing into orbit crust material that formed the Moon. Another hypothesis is that the Earth and Moon started to coalesce at the same time but the Earth, having much stronger gravity than the early Moon, attracted almost all the iron particles in the area.
Until 2001, the oldest rocks found on Earth were about 3.8 billion years old,
leading scientists to estimate that the Earth's surface had been molten
until then. Accordingly, they named this part of Earth's history the Hadean. However, analysis of zircons
formed 4.4 Ga indicates that Earth's crust solidified about 100 million
years after the planet's formation and that the planet quickly acquired
oceans and an atmosphere, which may have been capable of supporting life.
Evidence from the Moon indicates that from 4 to 3.8 Ga it suffered a Late Heavy Bombardment by debris that was left over from the formation of the Solar System, and the Earth should have experienced an even heavier bombardment due to its stronger gravity.
While there is no direct evidence of conditions on Earth 4 to 3.8 Ga,
there is no reason to think that the Earth was not also affected by this
late heavy bombardment. This event may well have stripped away any previous atmosphere and oceans; in this case gases and water from comet impacts may have contributed to their replacement, although outgassing from volcanoes on Earth would have supplied at least half. However, if subsurface microbial life had evolved by this point, it would have survived the bombardment.
Earliest evidence for life on Earth
The earliest identified organisms were minute and relatively
featureless, and their fossils look like small rods that are very
difficult to tell apart from structures that arise through abiotic
physical processes. The oldest undisputed evidence of life on Earth,
interpreted as fossilized bacteria, dates to 3 Ga. Other finds in rocks dated to about 3.5 Ga have been interpreted as bacteria, with geochemical evidence also seeming to show the presence of life 3.8 Ga.
However, these analyses were closely scrutinized, and non-biological
processes were found which could produce all of the "signatures of life"
that had been reported.
While this does not prove that the structures found had a
non-biological origin, they cannot be taken as clear evidence for the
presence of life. Geochemical signatures from rocks deposited 3.4 Ga
have been interpreted as evidence for life, although these statements have not been thoroughly examined by critics.
Evidence for fossilized microorganisms considered to be 3,770
million to 4,280 million years old was found in the Nuvvuagittuq
Greenstone Belt in Quebec, Canada, although the evidence is disputed as inconclusive.
Biologists reason that all living organisms on Earth must share a single last universal ancestor,
because it would be virtually impossible that two or more separate
lineages could have independently developed the many complex biochemical
mechanisms common to all living organisms.
Independent emergence on Earth
Life on Earth is based on carbon
and water. Carbon provides stable frameworks for complex chemicals and
can be easily extracted from the environment, especially from carbon dioxide. There is no other chemical element whose properties are similar enough to carbon's to be called an analogue; silicon, the element directly below carbon on the periodic table,
does not form very many complex stable molecules, and because most of
its compounds are water-insoluble, it would be more difficult for
organisms to extract. The elements boron and phosphorus have more complex chemistries, but suffer from other limitations relative to carbon. Water is an excellent solvent
and has two other useful properties: the fact that ice floats enables
aquatic organisms to survive beneath it in winter; and its molecules
have electrically negative and positive ends, which enables it to form a wider range of compounds than other solvents can. Other good solvents, such as ammonia,
are liquid only at such low temperatures that chemical reactions may be
too slow to sustain life, and lack water's other advantages. Organisms based on alternative biochemistry may, however, be possible on other planets.
Research on how life might have emerged from non-living chemicals focuses on three possible starting points: self-replication,
an organism's ability to produce offspring that are very similar to
itself; metabolism, its ability to feed and repair itself; and external cell membranes, which allow food to enter and waste products to leave, but exclude unwanted substances. Research on abiogenesis still has a long way to go, since theoretical and empirical approaches are only beginning to make contact with each other.
Replication first: RNA world
Even the simplest members of the three modern domains of life use DNA to record their "recipes" and a complex array of RNA and protein
molecules to "read" these instructions and use them for growth,
maintenance and self-replication. The discovery that some RNA molecules
can catalyze both their own replication and the construction of proteins led to the hypothesis of earlier life-forms based entirely on RNA. These ribozymes could have formed an RNA world in which there were individuals but no species, as mutations and horizontal gene transfers would have meant that the offspring in each generation were quite likely to have different genomes from those that their parents started with.[76]
RNA would later have been replaced by DNA, which is more stable and
therefore can build longer genomes, expanding the range of capabilities a
single organism can have. Ribozymes remain as the main components of ribosomes, modern cells' "protein factories." Evidence suggests the first RNA molecules formed on Earth prior to 4.17 Ga.
Although short self-replicating RNA molecules have been artificially produced in laboratories, doubts have been raised about where natural non-biological synthesis of RNA is possible. The earliest "ribozymes" may have been formed of simpler nucleic acids such as PNA, TNA or GNA, which would have been replaced later by RNA.
In 2003, it was proposed that porous metal sulfide precipitates would assist RNA synthesis at about 100 °C (212 °F) and ocean-bottom pressures near hydrothermal vents. Under this hypothesis, lipid membranes would be the last major cell components to appear and, until then, the protocells would be confined to the pores.
Metabolism first: Iron–sulfur world
A series of experiments starting in 1997 showed that early stages in
the formation of proteins from inorganic materials including carbon monoxide and hydrogen sulfide could be achieved by using iron sulfide and nickel sulfide as catalysts.
Most of the steps required temperatures of about 100 °C (212 °F) and
moderate pressures, although one stage required 250 °C (482 °F) and a
pressure equivalent to that found under 7 kilometres (4.3 mi) of rock.
Hence it was suggested that self-sustaining synthesis of proteins could
have occurred near hydrothermal vents.
It has been suggested that double-walled "bubbles" of lipids like
those that form the external membranes of cells may have been an
essential first step.
Experiments that simulated the conditions of the early Earth have
reported the formation of lipids, and these can spontaneously form liposomes, double-walled "bubbles," and then reproduce themselves. Although they are not intrinsically information-carriers as nucleic acids are, they would be subject to natural selection
for longevity and reproduction. Nucleic acids such as RNA might then
have formed more easily within the liposomes than they would have
outside.
The clay hypothesis
RNA is complex and there are doubts about whether it can be produced non-biologically in the wild. Some clays, notably montmorillonite,
have properties that make them plausible accelerators for the emergence
of an RNA world: they grow by self-replication of their crystalline
pattern; they are subject to an analog of natural selection, as the
clay "species" that grows fastest in a particular environment rapidly
becomes dominant; and they can catalyze the formation of RNA molecules. Although this idea has not become the scientific consensus, it still has active supporters.
Research in 2003 reported that montmorillonite could also accelerate the conversion of fatty acids
into "bubbles," and that the "bubbles" could encapsulate RNA attached
to the clay. These "bubbles" can then grow by absorbing additional
lipids and then divide. The formation of the earliest cells may have
been aided by similar processes.
A similar hypothesis presents self-replicating iron-rich clays as the progenitors of nucleotides, lipids and amino acids.
There are three main versions of the "seeded from elsewhere"
hypothesis: from elsewhere in our Solar System via fragments knocked
into space by a large meteor impact, in which case the most credible sources are Mars and Venus; by alien visitors, possibly as a result of accidental contamination by microorganisms that they brought with them; and from outside the Solar System but by natural means.
Experiments in low Earth orbit, such as EXOSTACK, demonstrated that some microorganism spores
can survive the shock of being catapulted into space and some can
survive exposure to outer space radiation for at least 5.7 years. Scientists are divided over the likelihood of life arising independently on Mars, or on other planets in our galaxy.
Environmental and evolutionary impact of microbial mats
Microbial mats are multi-layered, multi-species colonies of bacteria
and other organisms that are generally only a few millimeters thick, but
still contain a wide range of chemical environments, each of which
favors a different set of microorganisms. To some extent each mat forms its own food chain, as the by-products of each group of microorganisms generally serve as "food" for adjacent groups.
Stromatolites
are stubby pillars built as microorganisms in mats slowly migrate
upwards to avoid being smothered by sediment deposited on them by water. There has been vigorous debate about the validity of alleged fossils from before 3 Ga, with critics arguing that so-called stromatolites could have been formed by non-biological processes. In 2006, another find of stromatolites was reported from the same part of Australia as previous ones, in rocks dated to 3.5 Ga.
In modern underwater mats the top layer often consists of photosynthesizing cyanobacteria
which create an oxygen-rich environment, while the bottom layer is
oxygen-free and often dominated by hydrogen sulfide emitted by the
organisms living there. It is estimated that the appearance of oxygenic photosynthesis by bacteria in mats increased biological productivity by a factor of between 100 and 1,000. The reducing agent
used by oxygenic photosynthesis is water, which is much more plentiful
than the geologically produced reducing agents required by the earlier
non-oxygenic photosynthesis. From this point onwards life itself produced significantly more of the resources it needed than did geochemical processes. Oxygen is toxic to organisms that are not adapted to it, but greatly increases the metabolic efficiency of oxygen-adapted organisms. Oxygen became a significant component of Earth's atmosphere about 2.4 Ga. Although eukaryotes may have been present much earlier,
the oxygenation of the atmosphere was a prerequisite for the evolution
of the most complex eukaryotic cells, from which all multicellular
organisms are built.
The boundary between oxygen-rich and oxygen-free layers in microbial
mats would have moved upwards when photosynthesis shut down overnight,
and then downwards as it resumed on the next day. This would have
created selection pressure for organisms in this intermediate zone to acquire the ability to tolerate and then to use oxygen, possibly via endosymbiosis, where one organism lives inside another and both of them benefit from their association.
Cyanobacteria have the most complete biochemical "toolkits" of
all the mat-forming organisms. Hence they are the most self-sufficient
of the mat organisms and were well-adapted to strike out on their own
both as floating mats and as the first of the phytoplankton, providing the basis of most marine food chains.
Diversification of eukaryotes
Chromatin, nucleus, endomembrane system, and mitochondria
Eukaryotes may have been present long before the oxygenation of the atmosphere, but most modern eukaryotes require oxygen, which their mitochondria use to fuel the production of ATP, the internal energy supply of all known cells.
In the 1970s it was proposed and, after much debate, widely accepted
that eukaryotes emerged as a result of a sequence of endosymbiosis
between "prokaryotes." For example: a predatory microorganism invaded a large prokaryote, probably an archaean,
but the attack was neutralized, and the attacker took up residence and
evolved into the first of the mitochondria; one of these chimeras
later tried to swallow a photosynthesizing cyanobacterium, but the
victim survived inside the attacker and the new combination became the
ancestor of plants;
and so on. After each endosymbiosis began, the partners would have
eliminated unproductive duplication of genetic functions by re-arranging
their genomes, a process which sometimes involved transfer of genes
between them. Another hypothesis proposes that mitochondria were originally sulfur- or hydrogen-metabolising endosymbionts, and became oxygen-consumers later. On the other hand, mitochondria might have been part of eukaryotes' original equipment.
There is a debate about when eukaryotes first appeared: the presence of steranes in Australian shales may indicate that eukaryotes were present 2.7 Ga;
however, an analysis in 2008 concluded that these chemicals infiltrated
the rocks less than 2.2 Ga and prove nothing about the origins of
eukaryotes. Fossils of the algaeGrypania have been reported in 1.85 billion-year-old rocks (originally dated to 2.1 Ga but later revised), and indicates that eukaryotes with organelles had already evolved.[122] A diverse collection of fossil algae were found in rocks dated between 1.5 and 1.4 Ga. The earliest known fossils of fungi date from 1.43 Ga.
The defining characteristics of sexual reproduction in eukaryotes are meiosis and fertilization. There is much genetic recombination in this kind of reproduction, in which offspring receive 50% of their genes from each parent, in contrast with asexual reproduction, in which there is no recombination. Bacteria also exchange DNA by bacterial conjugation, the benefits of which include resistance to antibiotics and other toxins, and the ability to utilize new metabolites.
However, conjugation is not a means of reproduction, and is not limited
to members of the same species – there are cases where bacteria
transfer DNA to plants and animals.
On the other hand, bacterial transformation is clearly an
adaptation for transfer of DNA between bacteria of the same species.
Bacterial transformation is a complex process involving the products of
numerous bacterial genes and can be regarded as a bacterial form of sex. This process occurs naturally in at least 67 prokaryotic species (in seven different phyla). Sexual reproduction in eukaryotes may have evolved from bacterial transformation.
The disadvantages of sexual reproduction are well-known: the
genetic reshuffle of recombination may break up favorable combinations
of genes; and since males do not directly increase the number of
offspring in the next generation, an asexual population can out-breed
and displace in as little as 50 generations a sexual population that is
equal in every other respect. Nevertheless, the great majority of animals, plants, fungi and protists
reproduce sexually. There is strong evidence that sexual reproduction
arose early in the history of eukaryotes and that the genes controlling
it have changed very little since then. How sexual reproduction evolved and survived is an unsolved puzzle.
Horodyskia may have been an early metazoan, or a colonialforaminiferan. It apparently re-arranged itself into fewer but larger main masses as the sediment grew deeper round its base.
The Red Queen hypothesis suggests that sexual reproduction provides protection against parasites, because it is easier for parasites to evolve means of overcoming the defenses of genetically identical clones
than those of sexual species that present moving targets, and there is
some experimental evidence for this. However, there is still doubt about
whether it would explain the survival of sexual species if multiple
similar clone species were present, as one of the clones may survive the
attacks of parasites for long enough to out-breed the sexual species.
Furthermore, contrary to the expectations of the Red Queen hypothesis,
Kathryn A. Hanley et al. found that the prevalence, abundance and mean
intensity of mites was significantly higher in sexual geckos than in
asexuals sharing the same habitat.
In addition, biologist Matthew Parker, after reviewing numerous genetic
studies on plant disease resistance, failed to find a single example
consistent with the concept that pathogens are the primary selective
agent responsible for sexual reproduction in the host.
Alexey Kondrashov's deterministic mutation hypothesis
(DMH) assumes that each organism has more than one harmful mutation and
the combined effects of these mutations are more harmful than the sum
of the harm done by each individual mutation. If so, sexual
recombination of genes will reduce the harm that bad mutations do to
offspring and at the same time eliminate some bad mutations from the gene pool
by isolating them in individuals that perish quickly because they have
an above-average number of bad mutations. However, the evidence suggests
that the DMH's assumptions are shaky, because many species have on
average less than one harmful mutation per individual and no species
that has been investigated shows evidence of synergy between harmful mutations.
The random nature of recombination causes the relative abundance
of alternative traits to vary from one generation to another. This genetic drift
is insufficient on its own to make sexual reproduction advantageous,
but a combination of genetic drift and natural selection may be
sufficient. When chance produces combinations of good traits, natural
selection gives a large advantage to lineages in which these traits
become genetically linked. On the other hand, the benefits of good
traits are neutralized if they appear along with bad traits. Sexual
recombination gives good traits the opportunities to become linked with
other good traits, and mathematical models suggest this may be more than
enough to offset the disadvantages of sexual reproduction. Other combinations of hypotheses that are inadequate on their own are also being examined.
The adaptive function of sex today remains a major unresolved
issue in biology. The competing models to explain the adaptive function
of sex were reviewed by John A. Birdsell and Christopher Wills.
The hypotheses discussed above all depend on possible beneficial
effects of random genetic variation produced by genetic recombination.
An alternative view is that sex arose, and is maintained, as a process
for repairing DNA damage, and that the genetic variation produced is an
occasionally beneficial byproduct.
Multicellularity
The simplest definitions of "multicellular," for example "having multiple cells," could include colonial cyanobacteria like Nostoc. Even a technical definition such as "having the same genome but different types of cell" would still include some genera of the green algae Volvox, which have cells that specialize in reproduction. Multicellularity evolved independently in organisms as diverse as sponges and other animals, fungi, plants, brown algae, cyanobacteria, slime molds and myxobacteria. For the sake of brevity, this article focuses on the organisms that
show the greatest specialization of cells and variety of cell types,
although this approach to the evolution of biological complexity could be regarded as "rather anthropocentric."
A slime mold
solves a maze. The mold (yellow) explored and filled the maze (left).
When the researchers placed sugar (red) at two separate points, the mold
concentrated most of its mass there and left only the most efficient
connection between the two points (right).
The initial advantages of multicellularity may have included: more
efficient sharing of nutrients that are digested outside the cell,
increased resistance to predators, many of which attacked by engulfing;
the ability to resist currents by attaching to a firm surface; the
ability to reach upwards to filter-feed or to obtain sunlight for
photosynthesis; the ability to create an internal environment that gives protection against the external one; and even the opportunity for a group of cells to behave "intelligently" by sharing information.
These features would also have provided opportunities for other
organisms to diversify, by creating more varied environments than flat
microbial mats could.
Multicellularity with differentiated cells is beneficial to the
organism as a whole but disadvantageous from the point of view of
individual cells, most of which lose the opportunity to reproduce
themselves. In an asexual multicellular organism, rogue cells which
retain the ability to reproduce may take over and reduce the organism to
a mass of undifferentiated cells. Sexual reproduction eliminates such
rogue cells from the next generation and therefore appears to be a
prerequisite for complex multicellularity.
The available evidence indicates that eukaryotes evolved much
earlier but remained inconspicuous until a rapid diversification around 1
Ga. The only respect in which eukaryotes clearly surpass bacteria and
archaea is their capacity for variety of forms, and sexual reproduction
enabled eukaryotes to exploit that advantage by producing organisms with
multiple cells that differed in form and function.
By comparing the composition of transcription factor families and
regulatory network motifs between unicellular organisms and
multicellular organisms, scientists found there are many novel
transcription factor families and three novel types of regulatory
network motifs in multicellular organisms, and novel family
transcription factors are preferentially wired into these novel network
motifs which are essential for multicullular development. These results
propose a plausible mechanism for the contribution of novel-family
transcription factors and novel network motifs to the origin of
multicellular organisms at transcriptional regulatory level.
Fossil evidence
The Francevillian biota fossils, dated to 2.1 Ga, are the earliest known fossil organisms that are clearly multicellular. They may have had differentiated cells. Another early multicellular fossil, Qingshania, dated to 1.7 Ga, appears to consist of virtually identical cells. The red algae called Bangiomorpha,
dated at 1.2 Ga, is the earliest known organism that certainly has
differentiated, specialized cells, and is also the oldest known sexually
reproducing organism. The 1.43 billion-year-old fossils interpreted as fungi appear to have been multicellular with differentiated cells. The "string of beads" organism Horodyskia, found in rocks dated from 1.5 Ga to 900 Ma, may have been an early metazoan; however, it has also been interpreted as a colonial foraminiferan.
Emergence of animals
Animals are multicellular eukaryotes, and are distinguished from plants, algae, and fungi by lacking cell walls. All animals are motile, if only at certain life stages. All animals except sponges have bodies differentiated into separate tissues, including muscles, which move parts of the animal by contracting, and nerve tissue, which transmits and processes signals.
The earliest widely accepted animal fossils are the rather modern-looking cnidarians (the group that includes jellyfish, sea anemones and Hydra), possibly from around 580 Ma, although fossils from the Doushantuo Formation can only be dated approximately. Their presence implies that the cnidarian and bilaterian lineages had already diverged.
The Ediacara biota, which flourished for the last 40 million years before the start of the Cambrian,
were the first animals more than a very few centimetres long. Many were
flat and had a "quilted" appearance, and seemed so strange that there
was a proposal to classify them as a separate kingdom, Vendozoa. Others, however, have been interpreted as early molluscs (Kimberella), echinoderms (Arkarua), and arthropods (Spriggina, Parvancorina).
There is still debate about the classification of these specimens,
mainly because the diagnostic features which allow taxonomists to
classify more recent organisms, such as similarities to living
organisms, are generally absent in the Ediacarans. However, there seems
little doubt that Kimberella was at least a triploblastic bilaterian animal, in other words, an animal significantly more complex than the cnidarians.
The small shelly fauna are a very mixed collection of fossils found between the Late Ediacaran and Middle Cambrian periods. The earliest, Cloudina, shows signs of successful defense against predation and may indicate the start of an evolutionary arms race. Some tiny Early Cambrian shells almost certainly belonged to molluscs, while the owners of some "armor plates," Halkieria and Microdictyon, were eventually identified when more complete specimens were found in Cambrian lagerstätten that preserved soft-bodied animals.
Opabinia made the largest single contribution to modern interest in the Cambrian explosion.
In the 1970s there was already a debate about whether the emergence
of the modern phyla was "explosive" or gradual but hidden by the
shortage of Precambrian animal fossils. A re-analysis of fossils from the Burgess Shale lagerstätte increased interest in the issue when it revealed animals, such as Opabinia, which did not fit into any known phylum.
At the time these were interpreted as evidence that the modern phyla
had evolved very rapidly in the Cambrian explosion and that the Burgess
Shale's "weird wonders" showed that the Early Cambrian was a uniquely
experimental period of animal evolution.
Later discoveries of similar animals and the development of new
theoretical approaches led to the conclusion that many of the "weird
wonders" were evolutionary "aunts" or "cousins" of modern groups—for example that Opabinia was a member of the lobopods, a group which includes the ancestors of the arthropods, and that it may have been closely related to the modern tardigrades.
Nevertheless, there is still much debate about whether the Cambrian
explosion was really explosive and, if so, how and why it happened and
why it appears unique in the history of animals.
Most of the animals at the heart of the Cambrian explosion debate are protostomes, one of the two main groups of complex animals. The other major group, the deuterostomes, contains invertebrates such as starfish and sea urchins (echinoderms), as well as chordates. Many echinoderms have hard calcite "shells," which are fairly common from the Early Cambrian small shelly fauna onwards. Other deuterostome groups are soft-bodied, and most of the significant Cambrian deuterostome fossils come from the Chengjiang fauna, a lagerstätte in China.
The chordates are another major deuterostome group: animals with a
distinct dorsal nerve cord. Chordates include soft-bodied invertebrates
such as tunicates as well as vertebrates—animals with a backbone. While tunicate fossils predate the Cambrian explosion, the Chengjiang fossils Haikouichthys and Myllokunmingia appear to be true vertebrates, and Haikouichthys had distinct vertebrae, which may have been slightly mineralized. Vertebrates with jaws, such as the acanthodians, first appeared in the Late Ordovician.
Colonization of land
Adaptation to life on land is a major challenge: all land organisms
need to avoid drying-out and all those above microscopic size must
create special structures to withstand gravity; respiration and gas exchange systems have to change; reproductive systems cannot depend on water to carry eggs and sperm towards each other. Although the earliest good evidence of land plants and animals dates back to the Ordovician period (488 to 444 Ma), and a number of microorganism lineages made it onto land much earlier, modern land ecosystems only appeared in the Late Devonian, about 385 to 359 Ma. In May 2017, evidence of the earliest known life on land may have been found in 3.48-billion-year-old geyserite and other related mineral deposits (often found around hot springs and geysers) uncovered in the Pilbara Craton of Western Australia.
Evolution of terrestrial antioxidants
Oxygen is a potent oxidant whose accumulation in terrestrial atmosphere resulted from the development of photosynthesis
over 3 Ga, in cyanobacteria (blue-green algae), which were the most
primitive oxygenic photosynthetic organisms. Brown algae accumulate
inorganic mineral antioxidants such as rubidium, vanadium, zinc, iron, copper, molybdenum, selenium and iodine
which is concentrated more than 30,000 times the concentration of this
element in seawater. Protective endogenous antioxidant enzymes and
exogenous dietary antioxidants helped to prevent oxidative damage. Most
marine mineral antioxidants act in the cells as essential trace elements in redox and antioxidant metalloenzymes.
When plants and animals began to enter rivers and land about 500
Ma, environmental deficiency of these marine mineral antioxidants was a
challenge to the evolution of terrestrial life. Terrestrial plants slowly optimized the production of “new” endogenous antioxidants such as ascorbic acid, polyphenols, flavonoids, tocopherols, etc. A few of these appeared more recently, in last 200–50 Ma, in fruits and flowers of angiosperm plants.
In fact, angiosperms (the dominant type of plant today) and most of their antioxidant pigments evolved during the Late Jurassic period. Plants employ antioxidants to defend their structures against reactive oxygen species
produced during photosynthesis. Animals are exposed to the same
oxidants, and they have evolved endogenous enzymatic antioxidant
systems. Iodine
is the most primitive and abundant electron-rich essential element in
the diet of marine and terrestrial organisms, and as iodide acts as an electron donor
and has this ancestral antioxidant function in all iodide-concentrating
cells from primitive marine algae to more recent terrestrial
vertebrates.
Evolution of soil
Before the colonization of land, soil, a combination of mineral particles and decomposed organic matter, did not exist. Land surfaces would have been either bare rock or unstable sand produced by weathering. Water and any nutrients in it would have drained away very quickly. In the Sub-Cambrian peneplain in Sweden for example maximum depth of kaolinitization by Neoproterozoicweathering is about 5 m, in contrast nearby kaolin deposits developed in the Mesozoic are much thicker. It has been argued that in the late Neoproterozoic sheet wash was a dominant process of erosion of surface material due to the lack of plants on land.
Films of cyanobacteria, which are not plants but use the same
photosynthesis mechanisms, have been found in modern deserts, and only
in areas that are unsuitable for vascular plants.
This suggests that microbial mats may have been the first organisms to
colonize dry land, possibly in the Precambrian. Mat-forming
cyanobacteria could have gradually evolved resistance to desiccation as
they spread from the seas to intertidal zones and then to land. Lichens, which are symbiotic combinations of a fungus (almost always an ascomycete) and one or more photosynthesizers (green algae or cyanobacteria), are also important colonizers of lifeless environments, and their ability to break down rocks contributes to soil formation in situations where plants cannot survive. The earliest known ascomycete fossils date from 423 to 419 Ma in the Silurian.
Soil formation would have been very slow until the appearance of
burrowing animals, which mix the mineral and organic components of soil
and whose feces are a major source of the organic components. Burrows have been found in Ordovician sediments, and are attributed to annelids ("worms") or arthropods.
In aquatic algae, almost all cells are capable of photosynthesis and
are nearly independent. Life on land required plants to become
internally more complex and specialized: photosynthesis was most
efficient at the top; roots were required in order to extract water from
the ground; the parts in between became supports and transport systems
for water and nutrients.
Spores of land plants, possibly rather like liverworts, have been found in Middle Ordovician rocks dated to about 476 Ma. In Middle Silurian rocks 430 Ma, there are fossils of actual plants including clubmosses such as Baragwanathia; most were under 10 centimetres (3.9 in) high, and some appear closely related to vascular plants, the group that includes trees.
By the Late Devonian 370 Ma, trees such as Archaeopteris were so abundant that they changed river systems from mostly braided to mostly meandering, because their roots bound the soil firmly. In fact, they caused the "Late Devonian wood crisis" because:
They removed more carbon dioxide from the atmosphere, reducing the greenhouse effect and thus causing an ice age in the Carboniferous period.
In later ecosystems the carbon dioxide "locked up" in wood is returned
to the atmosphere by decomposition of dead wood. However, the earliest
fossil evidence of fungi that can decompose wood also comes from the
Late Devonian.
The increasing depth of plants' roots led to more washing of nutrients into rivers and seas by rain. This caused algal blooms whose high consumption of oxygen caused anoxic events in deeper waters, increasing the extinction rate among deep-water animals.
Land invertebrates
Animals had to change their feeding and excretory systems, and most land animals developed internal fertilization of their eggs. The difference in refractive index
between water and air required changes in their eyes. On the other
hand, in some ways movement and breathing became easier, and the better
transmission of high-frequency sounds in air encouraged the development
of hearing.
The relative number of species contributed to the total by each phylum of animals. Nematoda is the phylum with the most individual organisms while arthropod has the most species.
The oldest known air-breathing animal is Pneumodesmus, an archipolypodanmillipede from the Middle Silurian, about 428 Ma. Its air-breathing, terrestrial nature is evidenced by the presence of spiracles, the openings to tracheal systems. However, some earlier trace fossils from the Cambrian-Ordovician boundary about 490 Ma are interpreted as the tracks of large amphibious arthropods on coastal sand dunes, and may have been made by euthycarcinoids, which are thought to be evolutionary "aunts" of myriapods. Other trace fossils from the Late Ordovician a little over 445 Ma probably represent land invertebrates, and there is clear evidence of numerous arthropods on coasts and alluvial plains shortly before the Silurian-Devonian boundary, about 415 Ma, including signs that some arthropods ate plants. Arthropods were well pre-adapted
to colonise land, because their existing jointed exoskeletons provided
protection against desiccation, support against gravity and a means of
locomotion that was not dependent on water.
The fossil record of other major invertebrate groups on land is poor: none at all for non-parasiticflatworms, nematodes or nemerteans; some parasitic nematodes have been fossilized in amber; annelid worm fossils are known from the Carboniferous, but they may still have been aquatic animals; the earliest fossils of gastropods on land date from the Late Carboniferous, and this group may have had to wait until leaf litter became abundant enough to provide the moist conditions they need.
The earliest confirmed fossils of flying insects
date from the Late Carboniferous, but it is thought that insects
developed the ability to fly in the Early Carboniferous or even Late
Devonian. This gave them a wider range of ecological niches for feeding and breeding, and a means of escape from predators and from unfavorable changes in the environment. About 99% of modern insect species fly or are descendants of flying species.
Tetrapods, vertebrates with four limbs, evolved from other rhipidistian fish over a relatively short timespan during the Late Devonian (370 to 360 Ma). The early groups are grouped together as Labyrinthodontia. They retained aquatic, fry-like tadpoles, a system still seen in modern amphibians.
Amphibian Metamorphosis
Iodine and T4/T3 stimulate the amphibian metamorphosis and the evolution of nervous systems
transforming the aquatic, vegetarian tadpole into a “more evoluted”
terrestrial, carnivorous frog with better neurological, visuospatial,
olfactory and cognitive abilities for hunting.
The new hormonal action of T3 was made possible by the formation of
T3-receptors in the cells of vertebrates. Firstly, about 600-500 million
years ago, in primitive Chordata appeared the alpha T3-receptors with a
metamorphosing action and then, about 250-150 million years ago, in the
Birds and Mammalia appeared the beta T3-receptors with metabolic and
thermogenetic actions.
From the 1950s to the early 1980s it was thought that tetrapods
evolved from fish that had already acquired the ability to crawl on
land, possibly in order to go from a pool that was drying out to one
that was deeper. However, in 1987, nearly complete fossils of Acanthostega from about 363 Ma showed that this Late Devonian transitional animal had legs and both lungs and gills,
but could never have survived on land: its limbs and its wrist and
ankle joints were too weak to bear its weight; its ribs were too short
to prevent its lungs from being squeezed flat by its weight; its
fish-like tail fin would have been damaged by dragging on the ground.
The current hypothesis is that Acanthostega, which was about 1
metre (3.3 ft) long, was a wholly aquatic predator that hunted in
shallow water. Its skeleton differed from that of most fish, in ways
that enabled it to raise its head to breathe air while its body remained
submerged, including: its jaws show modifications that would have
enabled it to gulp air; the bones at the back of its skull are locked
together, providing strong attachment points for muscles that raised its
head; the head is not joined to the shoulder girdle and it has a distinct neck.
The Devonian proliferation of land plants may help to explain why
air breathing would have been an advantage: leaves falling into streams
and rivers would have encouraged the growth of aquatic vegetation; this
would have attracted grazing invertebrates and small fish that preyed
on them; they would have been attractive prey but the environment was
unsuitable for the big marine predatory fish; air-breathing would have
been necessary because these waters would have been short of oxygen,
since warm water holds less dissolved oxygen than cooler marine water and since the decomposition of vegetation would have used some of the oxygen.
Later discoveries revealed earlier transitional forms between Acanthostega and completely fish-like animals.[204] Unfortunately, there is then a gap (Romer's gap)
of about 30 Ma between the fossils of ancestral tetrapods and Middle
Carboniferous fossils of vertebrates that look well-adapted for life on
land. Some of these look like early relatives of modern amphibians, most
of which need to keep their skins moist and to lay their eggs in water,
while others are accepted as early relatives of the amniotes, whose waterproof skin and egg membranes enable them to live and breed far from water.
Dinosaurs, birds and mammals
Amniotes, whose eggs can survive in dry environments, probably evolved in the Late Carboniferous period (330 to 298.9 Ma). The earliest fossils of the two surviving amniote groups, synapsids and sauropsids, date from around 313 Ma. The synapsid pelycosaurs and their descendants the therapsids are the most common land vertebrates in the best-known Permian (298.9 to 251.902 Ma) fossil beds. However, at the time these were all in temperate zones at middle latitudes, and there is evidence that hotter, drier environments nearer the Equator were dominated by sauropsids and amphibians.
The Permian–Triassic extinction event wiped out almost all land vertebrates, as well as the great majority of other life. During the slow recovery from this catastrophe, estimated to have taken 30 million years, a previously obscure sauropsid group became the most abundant and diverse terrestrial vertebrates: a few fossils of archosauriformes ("ruling lizard forms") have been found in Late Permian rocks, but, by the Middle Triassic,
archosaurs were the dominant land vertebrates. Dinosaurs distinguished
themselves from other archosaurs in the Late Triassic, and became the
dominant land vertebrates of the Jurassic and Cretaceous periods (201.3 to 66 Ma).
During the Late Jurassic, birds evolved from small, predatory theropod dinosaurs. The first birds inherited teeth and long, bony tails from their dinosaur ancestors, but some had developed horny, toothless beaks by the very Late Jurassic and short pygostyle tails by the Early Cretaceous.
While the archosaurs and dinosaurs were becoming more dominant in the Triassic, the mammaliaform successors of the therapsids evolved into small, mainly nocturnal insectivores. This ecological role may have promoted the evolution of mammals, for example nocturnal life may have accelerated the development of endothermy ("warm-bloodedness") and hair or fur. By 195 Ma in the Early Jurassic there were animals that were very like today's mammals in a number of respects. Unfortunately, there is a gap in the fossil record throughout the Middle Jurassic. However, fossil teeth discovered in Madagascar indicate that the split between the lineage leading to monotremes and the one leading to other living mammals had occurred by 167 Ma.
After dominating land vertebrate niches for about 150 Ma, the non-avian
dinosaurs perished in the Cretaceous–Paleogene extinction event (66 Ma) along with many other groups of organisms. Mammals throughout the time of the dinosaurs had been restricted to a narrow range of taxa, sizes and shapes, but increased rapidly in size and diversity after the extinction, with bats taking to the air within 13 million years, and cetaceans to the sea within 15 million years.
Flowering plants
The first flowering plants appeared around 130 Ma.
The 250,000 to 400,000 species of flowering plants outnumber all other
ground plants combined, and are the dominant vegetation in most
terrestrial ecosystems. There is fossil evidence that flowering plants
diversified rapidly in the Early Cretaceous, from 130 to 90 Ma, and that their rise was associated with that of pollinating insects. Among modern flowering plants Magnolia are thought to be close to the common ancestor of the group. However, paleontologists have not succeeded in identifying the earliest stages in the evolution of flowering plants.
The social insects are remarkable because the great majority of
individuals in each colony are sterile. This appears contrary to basic
concepts of evolution such as natural selection and the selfish gene. In fact, there are very few eusocial insect species: only 15 out of approximately 2,600 living families
of insects contain eusocial species, and it seems that eusociality has
evolved independently only 12 times among arthropods, although some
eusocial lineages have diversified into several families. Nevertheless,
social insects have been spectacularly successful; for example although ants and termites
account for only about 2% of known insect species, they form over 50%
of the total mass of insects. Their ability to control a territory
appears to be the foundation of their success.
The sacrifice of breeding opportunities by most individuals has long been explained as a consequence of these species' unusual haplodiploid method of sex determination,
which has the paradoxical consequence that two sterile worker daughters
of the same queen share more genes with each other than they would with
their offspring if they could breed. However, E. O. Wilson and Bert Hölldobler argue that this explanation is faulty: for example, it is based on kin selection, but there is no evidence of nepotism
in colonies that have multiple queens. Instead, they write, eusociality
evolves only in species that are under strong pressure from predators
and competitors, but in environments where it is possible to build
"fortresses"; after colonies have established this security, they gain
other advantages through co-operative foraging. In support of this explanation they cite the appearance of eusociality in bathyergid mole rats, which are not haplodiploid.
The earliest fossils of insects have been found in Early Devonian rocks from about 400 Ma, which preserve only a few varieties of flightless insect. The Mazon Creek lagerstätten from the Late Carboniferous, about 300 Ma,
include about 200 species, some gigantic by modern standards, and
indicate that insects had occupied their main modern ecological niches
as herbivores, detritivores
and insectivores. Social termites and ants first appear in the Early
Cretaceous, and advanced social bees have been found in Late Cretaceous
rocks but did not become abundant until the Middle Cenozoic.
The idea that, along with other life forms, modern-day humans evolved from an ancient, common ancestor was proposed by Robert Chambers in 1844 and taken up by Charles Darwin in 1871. Modern humans evolved from a lineage of upright-walking apes that has been traced back over 6 Ma to Sahelanthropus. The first known stone tools were made about 2.5 Ma, apparently by Australopithecus garhi, and were found near animal bones that bear scratches made by these tools. The earliest hominines had chimpanzee-sized
brains, but there has been a fourfold increase in the last 3 Ma; a
statistical analysis suggests that hominine brain sizes depend almost
completely on the date of the fossils, while the species to which they
are assigned has only slight influence. There is a long-running debate about whether modern humans evolved all over the world simultaneously from existing advanced hominines or are descendants of a single small population in Africa, which then migrated all over the world less than 200,000 years ago and replaced previous hominine species. There is also debate about whether anatomically modern humans had an intellectual, cultural and technological "Great Leap Forward" under 100,000 years ago and, if so, whether this was due to neurological changes that are not visible in fossils.
Apparent extinction intensity, i.e. the fraction of genera going extinct at any given time, as reconstructed from the fossil record. (Graph not meant to include the recent, ongoing Holocene extinction event).
Life on Earth has suffered occasional mass extinctions at least since 542 Ma.
Although they were disasters at the time, mass extinctions have
sometimes accelerated the evolution of life on Earth. When dominance of
particular ecological niches passes from one group of organisms to
another, it is rarely because the new dominant group is "superior" to
the old and usually because an extinction event eliminates the old
dominant group and makes way for the new one.
Phanerozoic biodiversity as shown by the fossil record
The fossil record appears to show that the gaps between mass
extinctions are becoming longer and the average and background rates of
extinction are decreasing. Both of these phenomena could be explained in
one or more ways:
The oceans may have become more hospitable to life over the last
500 Ma and less vulnerable to mass extinctions: dissolved oxygen became
more widespread and penetrated to greater depths; the development of
life on land reduced the run-off of nutrients and hence the risk of eutrophication and anoxic events; and marine ecosystems became more diversified so that food chains were less likely to be disrupted.
Reasonably complete fossils are very rare, most extinct organisms
are represented only by partial fossils, and complete fossils are rarest
in the oldest rocks. So paleontologists have mistakenly assigned parts
of the same organism to different genera, which were often defined
solely to accommodate these finds—the story of Anomalocaris
is an example of this. The risk of this mistake is higher for older
fossils because these are often both unlike parts of any living organism
and poorly conserved. Many of the "superfluous" genera are represented
by fragments which are not found again and the "superfluous" genera
appear to become extinct very quickly.
Biodiversity
in the fossil record, which is "...the number of distinct genera alive
at any given time; that is, those whose first occurrence predates and
whose last occurrence postdates that time" shows a different trend: a fairly swift rise from 542 to 400 Ma; a slight decline from 400 to 200 Ma, in which the devastating Permian–Triassic extinction event is an important factor; and a swift rise from 200 Ma to the present.
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