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Animals may have evolved hundreds of millions of years before their oldest recognized fossils. The clearest fossils discussed by researchers are about 574 million years old, yet a new calibration strategy places animal origins somewhere between 850 and 715 million years ago. The estimate is plausible, not settled: it depends on how scientists interpret rocks that could have preserved animals but contain none.
The fossil record’s apparent beginning is not necessarily the beginning of animals
Darwin identified a long-standing problem: complex animal fossils appear relatively abruptly in rocks more than 500 million years old. That pattern can mean animals evolved rapidly, but it can also mean an earlier history existed in bodies or environments that rarely fossilized.
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What the oldest recognized fossils show
The oldest animal fossils discussed by Ross Anderson of the University of Oxford and Orin Lole Durbin of Virginia Tech are about 574 million years old. They come from rocks in the United Kingdom and Canada and preserve organisms that are visibly complex even though they lacked hard skeletons. Their appearance marks the oldest widely recognized physical evidence in this discussion—not necessarily the first animals.
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Why molecular clocks produce very different dates
Molecular clocks use DNA differences among living organisms to estimate when their lineages diverged. They need calibration points, usually fossils assigned to particular branches of the animal family tree. Change the fossils used as anchors, and the estimated mutation rate changes too.
For animal origins, different calibration choices have produced estimates separated by hundreds of millions of years. A clock anchored only to the oldest visible fossils tends to place origins closer to the fossil record. A clock that allows for a long, hidden pre-fossil history can produce substantially older dates. DNA can therefore test whether an early origin is biologically plausible, but it cannot independently reveal a precise date without assumptions about the calibration fossils.
Why an empty deposit is useful—but not decisive
A rock formation with no animal fossils supplies negative evidence only under strict conditions. It must be old enough, exposed well enough, and formed in an environment capable of preserving the kinds of animals scientists are seeking. Even then, the result applies to that deposit, not automatically to the entire planet.
“An empty fossil deposit like the one in Mongolia can tell us that animals are absent from that deposit, but not necessarily that animals were absent from the entire planet,” Anderson and Durbin wrote.
Mongolia: a local absence
Mongolian rocks about 550 million years old formed under conditions that could promote fossilization, yet they contain no animal fossils. That absence can help constrain molecular-clock models, but it cannot prove animals had not evolved elsewhere. Animals could have lived in another region, occupied a different environment, or been destroyed before burial.
China: an existing calibration with a narrower view
Chinese rocks about 590 million years old have also been used in some calibrations because they lack animal fossils. Their value depends on how confidently scientists can establish that the rocks offered a realistic opportunity for animal remains to be preserved and found. Treating a single region as a global cutoff risks making the inferred origin too young.
| Evidence | Approximate age | What it can establish |
|---|---|---|
| Recognized animal fossils in the United Kingdom and Canada | 574 million years | Direct evidence that complex animals existed by this time |
| Fossil-free Mongolian rocks with fossilisation-promoting conditions | 550 million years | Animals were not found or preserved in that deposit |
| Fossil-free Chinese rocks used in some calibrations | 590 million years | A regional absence that can inform, but not by itself settle, a global origin date |
The proposed solution: replicate the absence across the planet
Anderson and Durbin propose comparing several deposits of similar age from different parts of the world. Each site should have conditions favorable to fossil preservation and enough exposure and study to make a missing fossil meaningful. A repeated absence across independent regions is stronger evidence than an empty site considered alone.
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- Identify suitable deposits. Geologists select rocks whose age is securely constrained and whose chemistry, burial history and sedimentary setting could preserve animal remains.
- Check the fossil record carefully. Researchers distinguish a genuinely well-sampled deposit from one that has simply received little fieldwork or has poor exposure.
- Compare sites globally. Deposits on different continents reduce the chance that a local environment or regional extinction explains every absence.
- Use the combined interval as a molecular-clock calibration. Multiple fossil-free, preservation-capable deposits can place an older boundary on when animals likely existed, while known fossils provide a younger boundary.
The authors identify an interval from about 900 to 720 million years ago in which enough suitable fossil-free deposits might be assembled for this approach. Using the older calibration, they estimate that animals originated between about 850 and 715 million years ago.
What the 850–715 million-year estimate means
The interval is a model-based origin window, not a newly discovered animal fossil. It says that, given the selected deposits and their preservation potential, an origin during this period fits the combined evidence better than an origin only shortly before the 574-million-year-old fossils.
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The estimate also carries a substantial qualification. The authors put about 70% confidence in the evidence-of-absence approach, leaving a 30% chance that animals had already evolved but were not recorded in the deposits used for calibration. That figure describes the reliability of the method, not a 70% probability that one particular origin date is correct.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How Snowball Earth fits into the story
The proposed origin window overlaps the broad span of “Snowball Earth” episodes, when geological evidence indicates that ice may have covered much or nearly all of the planet. If animals did arise between 850 and 715 million years ago, their earliest evolution could have occurred during, between or shortly after these severe glaciations.
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What would make the timeline more secure?
Future work must improve both sides of the argument: find older animal fossils where preservation allows, and test more fossil-free deposits for whether they truly offered a fair chance of preserving animals. Better radiometric dating, broader geological mapping and molecular-clock models that make calibration assumptions explicit can narrow the range.
The central lesson is methodological. The sudden appearance of fossils at 574 million years ago may be a visibility threshold rather than an evolutionary starting line. As Anderson and Durbin put it, “The timing of animal origins remains uncertain.” Comparing many well-preserved, globally distributed rocks is intended to reduce that uncertainty; only then might scientists have more confidence in the result.
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