Ediacaran sea floor, Namibia, Southwest Africa, 543 million years ago.
Credit: Museum of Natural History, University of Oxford / Mighty Fossils.
New study suggests animals may have evolved vastly earlier than fossil evidence suggests | Oxford University
One of the strengths of science is that its conclusions remain open to revision. An assumption that once appeared reasonable can be tested against new evidence, found wanting, and replaced with something better. For creationists, who frequently portray such revisions as evidence that science cannot be trusted, this must seem a peculiar way of acquiring knowledge. Yet it is precisely how science improves its understanding: by allowing the evidence to correct the explanation, rather than requiring the evidence to conform to a predetermined answer.
A new study published in Science Advances provides another example. Researchers have challenged an assumption used in some recent estimates of when animals originated. Their analyses support an origin approximately 700–800 million years ago, potentially extending animal evolutionary history by around 200 million years beyond estimates based on a younger maximum-age constraint.
The issue is a deceptively simple one: if a fossil deposit preserves microscopic organisms in exquisite detail, but contains no definite animals, does that mean animals had not yet evolved? Some researchers have used the approximately 590-million-year-old Weng’an Biota of China in this way, reasoning that animals would have been preserved there had they existed. Its lack of unequivocal animal fossils has therefore helped set a proposed upper age limit for animal origins.
The new research exposes a difficulty with that reasoning. The younger Kheseen Biota of Mongolia also preserves exceptionally detailed microfossils, yet none can confidently be identified as animals. In this case, however, animals are known from other deposits of comparable age. Their absence from Kheseen therefore cannot demonstrate their absence from the world. Particular environments and conditions of fossilisation can leave animals out of the record, even when other organisms are beautifully preserved.
This matters because molecular clocks — methods that use genetic differences between living organisms to estimate when their ancestors diverged — require calibration against geological and fossil evidence. Change an unjustified age constraint, and the resulting estimates can change substantially. Using alternative constraints from older deposits, the researchers recovered a much earlier possible origin for animals. They have not discovered an 800-million-year-old animal, nor established a final date for the animal kingdom’s beginnings. They have shown why one argument for restricting those beginnings to a much later interval is unreliable.
For young-Earth creationism, neither chronology offers any comfort. The debate concerns events hundreds of millions of years ago, not whether animal history can be squeezed into the few thousand years allowed by a literal reading of Genesis. Uncertainty over precisely when animals originated does not make every proposed date equally credible, any more than uncertainty over a mountain’s exact height makes it plausibly a molehill.
There is also a lesson here for claims that an apparently abrupt appearance in the fossil record requires an abrupt act of creation. The first fossils we find need not mark the first organisms that existed. Understanding that distinction requires investigating the incompleteness of the record, testing assumptions and comparing independent evidence. That is what these researchers have done — and why a revision of evolutionary history is evidence of science at work.
Reading Evolution’s Clock^ How Genes Help Date the Past. A molecular clock, sometimes called a genetic clock, is a method for estimating when evolutionary lineages shared a common ancestor. It uses changes accumulated in DNA or protein sequences, together with independent dating evidence, to place branches of the evolutionary family tree on a timescale.The paper in Science Advances was supplemented by an Oxford University news report:
What makes the clock “tick”?
Mutations introduce new genetic variants. Many disappear, but some persist and eventually become established within a lineage. Over generations, descendants of a common ancestor accumulate different changes. Comparing corresponding DNA sequences — or the amino-acid sequences of their proteins — allows researchers to estimate how much evolutionary change separates them.
The relevant measure is usually the substitution rate: the rate at which sequence changes become established over evolutionary time. This is distinct from simply counting every new mutation arising in an individual.
Turning genetic differences into dates
Genetic differences alone cannot tell us how many years have passed. A given amount of change could reflect a relatively short interval with rapid evolution, or a longer interval with slower evolution. Researchers therefore need a rate estimate or dated reference points to calibrate the clock.
In a deliberately simplified example, suppose two descendant lineages have accumulated 0.02 substitutions per DNA position between them. If each lineage evolves at 0.001 substitutions per position per million years, their combined rate is 0.002. Their estimated divergence time would therefore be:
0.02 ÷ (2 × 0.001) = 10 million years.
The factor of two matters because changes accumulate along both branches after they separate. Real analyses use many sequences and statistical models, including corrections for repeated changes at the same DNA position, which can conceal earlier substitutions.
How fossils calibrate the clock
A securely dated and correctly identified fossil establishes that its lineage already existed by that time. It can therefore supply a minimum age for a relevant evolutionary divergence. A fossil belonging to one branch of a group implies that the split producing that branch happened before the organism lived.
A maximum age is harder to establish. The absence of a group from older rocks may mean that it had not yet evolved, but it may also mean that its members lived elsewhere, were not preserved, or have not been discovered. Maximum-age constraints based on absence consequently require careful justification.
This is the issue highlighted by the Oxford-led study. Mongolia’s Kheseen Biota lacks definite animal fossils despite dating from an interval when animals existed elsewhere. Its exceptional preservation therefore does not guarantee that animals would appear in the assemblage. That weakens the argument that a similar absence in China’s older Weng’an Biota provides a reliable maximum age for animal origins.
Does every lineage tick at the same rate?
No. Rates differ among genes and evolutionary lineages, and can change through time. Generation length, mutation processes and natural selection can all affect the relationship between sequence change and elapsed time.
A strict molecular clock assumes a constant rate across the branches being analysed. A relaxed molecular clock allows rates to vary. Modern dating analyses commonly use relaxed clocks and incorporate uncertainty in both rates and fossil calibrations, producing a range of plausible dates rather than an exact birthday.
What does an estimated “origin” actually mean?
A molecular clock usually dates a branching point: the common ancestor from which sampled lineages descended. When researchers estimate the origin of a crown group, they mean the most recent common ancestor of all its living members and the beginning of their subsequent diversification.
That need not date the first appearance of every characteristic associated with the group. Some features may have evolved earlier along its ancestral stem lineage. Nor does an estimated ancestor necessarily correspond to a particular fossil species.
In the animal-origins study, alternative calibrations support an origin around 700–800 million years ago. These are model-based estimates, not dates obtained from newly discovered animal fossils of that age. Their sensitivity to calibration is part of the result: better genetic data cannot, by themselves, eliminate uncertainty about the geological timescale.
Brief glossary
- Mutation
- A change in genetic material that introduces a new variant.
- Substitution
- A sequence change that becomes established in an evolutionary lineage.
- Divergence
- The separation of ancestral lineages whose descendants subsequently evolve along different branches.
- Phylogeny
- An evolutionary tree representing inferred relationships among organisms or genes.
- Calibration
- Dating information used to translate genetic change into elapsed time.
- Crown group
- The most recent common ancestor of a group’s living members, together with all its descendants, living and extinct.
- Stem group
- Extinct relatives closer to a particular crown group than to its nearest living relatives, but outside that crown group.
- Credible interval
- A range assigned a specified probability in a Bayesian analysis, conditional on the data, model and prior assumptions.
New study suggests animals may have evolved vastly earlier than fossil evidence suggests
Animals may have evolved up to 200 million years earlier than their first appearance in the fossil record, according to a new study led by the University of Oxford. The findings, published in Science Advances, challenge key assumptions scientists have used to date the origins of animal life.
The findings could transform our understanding of one of evolution's greatest mysteries: why animals seem to appear relatively suddenly in the fossil record just before the Cambrian Period (539 to 487 million years ago), despite other evidence suggesting that their evolutionary history began much earlier. At the heart of the study is a problem with the way scientists have recently used the fossil record to set a maximum age for the origin of animals. A roughly 590-million-year-old fossil deposit known as the Weng'an Biota in rocks of Ediacaran age in China preserves microscopic organisms in extraordinary detail, none of which are definitive animals. Scientists have therefore inferred that the first animals must have originated after Weng'an, assuming that this exceptional fossil deposit would have preserved them if they existed at this time.
How new fossils from Mongolia challenge assumptions
The research team, which included members from the University of California Berkeley, ETH Zürich, and Yale University, carried out a comprehensive survey of exceptionally well-preserved microfossils from the Kheseen Biota in Mongolia. The Kheseen Biota is more than 40 million years younger than Weng’an, even though they share some species. By this time animal fossils are known from sites elsewhere around the world, including Namibia and South China.
The researchers studied over 140 samples, including some from previously undocumented localities, and examined them using scanning electron microscopy. This revealed exquisite new microfossil species including acritarchs (tiny spherical organisms with spines and branching projections) as well as embryo-like fossils containing internal cells. Yet, despite this remarkable level of preservation, none can be confidently identified as an animal.
Animals must have lived in different environments from those that preserved the fossils in the Kheseen Biota, or the chemical conditions there failed to preserve animal remains.
Asterocapsoides wenganensis, Kheseen Formation, latest Ediacaran/early Cambrian. Scale bar: 0.1 mm.Credit: Derek Briggs.The Kheseen Biota breaks the argument that the exceptional microfossils of Weng'an mean we would have seen animal fossils in the assemblage had they existed at the time. The Kheseen microfossils are just as well-preserved, yet animals continue to be absent – despite the fact we know at that point they existed.
Associate Professor Ross Anderson, senior author.
Museum of Natural History
Oxford University
Oxford, UK.
A hidden history of animal life
Given that the Kheseen Biota was preserved when animals were present but they are nonetheless not preserved as fossils, the researchers argue that Weng’an does not automatically qualify as evidence that no animals existed at that time and only appeared later than 590 million years ago.
Instead, they looked much further back, to several deposits dating from roughly 850 to 730 million years ago. These included the Svanbergfjellet Formation (Norway), Bitter Springs Group (Australia) and Chuar Group (Arizona, USA) - all fossil-rich deposits with the potential to preserve animals, even though they have eluded discovery. These deposits have previously been used as maximum dates for the origin of animals.
Using these older geological constraints, the researchers used a technique called molecular clock analysis. This method compares genetic differences between living species and, using dates from the fossil record as reference points combined with rates of evolution, works backwards to estimate when their ancestors lived. Compared with analyses constrained by the younger Ediacaran Weng’an deposit, this approach shifted the estimated origin of animals backwards by around 200 million years, to between 800 and 700 million years ago.
There are already clues that such a hidden history exists. Chemical fossils, or biomarkers, preserved in ancient rocks provide evidence consistent with sponges living at least 650 million years ago, tens of millions of years before the oldest definitive macroscopic animal fossils.
These early animals would have been small and soft-bodied, without the shells, bones or other hard structures that make later animals easier to preserve. Whether such creatures were fossilised depended on their environment and on an unusual combination of chemical conditions after death.
Estimated timeline for the origin of animals based on molecular clock analyses. The grey bar on the right of the image is the origin of animals based on an Ediacaran upper calibration. The grey bar to the left is the origin based on older deposits in the Tonian period, as suggested by the new study.
Credit: Orin Lole Durbin.
Could animals pre-date Snowball Earth?
An origin between 800 and 700 million years ago raises striking possibilities: animals may already have evolved before Earth entered some of the most extreme ice ages in its history or even appeared during that inhospitable interval.
During the Cryogenian Period, which began around 720 million years ago, enormous glaciers spread across the planet in episodes commonly known as Snowball Earth. The study reopens questions about whether the conditions associated with Snowball Earth played a role in the origin and early evolution of animal life.
Pre-Ediacaran animal body fossils still elude us, and this analysis does not prove that animals existed 800 million years ago. However, our new fossil evidence from Mongolia undermines one of the main arguments for restricting animal origins to the Ediacaran interval. Meanwhile, our molecular-clock analyses show how much further back their evolutionary history could extend.
Orin M. Lole Durbin, first author
Department of Earth Sciences University of Oxford
Oxford, UK.
The researchers say that future work should explore fossil deposits from different parts of the world, representing different environments and modes of fossilisation, in the search for animals. This should also consider all available evidence for animals, including body fossils, traces of their activity and chemical biomarkers.
Until that evidence becomes available, the precise birth date of the animal kingdom remains uncertain.
Associate Professor Ross Anderson, senior author.
Museum of Natural History
Oxford University, Oxford, UK.
Publication:
The significance of this study goes beyond the possibility that animals originated substantially earlier than some recent estimates suggested. It demonstrates how science examines its own assumptions. Exceptionally preserved fossils had been used to argue that animals should have been recorded if they existed. Evidence from another exceptionally preserved assemblage shows why that inference is unsafe. The appropriate response is to reconsider the constraint and investigate how changing it affects the results. That is how scientific understanding improves.
The researchers have not established an exact birthday for the animal kingdom, and their estimates of an origin around 700–800 million years ago remain dependent on the evidence and models used. Acknowledging those limitations is part of the strength of the work. Scientific confidence comes from making assumptions explicit, testing their consequences and allowing others to challenge them. Declaring an answer beyond revision would contribute nothing to resolving the uncertainty.
For young-Earth creationists, however, the uncertainty offers no escape. Even the younger chronology places animal origins hundreds of millions of years before the supposed creation week. Moving the estimated origin further into the past compounds an already overwhelming chronological problem. A debate over which ancient timescale best fits the evidence cannot reasonably be presented as support for a timescale that fits neither the fossils nor the genetic data.
The study also reinforces the danger of treating gaps in the fossil record as evidence of supernatural intervention. An organism’s first preserved appearance need not mark its evolutionary beginning, and exceptional preservation does not necessarily produce a complete inventory of life. The productive questions concern ancestral relationships, environments, preservation and rates of genetic change. By pursuing those questions, science continues to uncover a history that was neither planned in advance nor obliged to leave us a complete record. Its willingness to revise that history is precisely what makes its explanations increasingly reliable.
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