Molecular-clock studies give different dates for the origin of animals because DNA does not contain a timestamp. Researchers infer when lineages split by combining genetic differences with assumptions about evolutionary rates, the animal family tree, and fossil ages used to calibrate the clock. Change those inputs—or ask about a different meaning of “origin”—and the estimate can shift. The dates are model-dependent estimates, not precise birth certificates for the animal kingdom.
What does “the origin of animals” mean?
There is more than one event a study or headline might call the origin of animals. A molecular-clock estimate usually targets a branching point in an evolutionary tree. A fossil date records when an organism lived, while the rise of familiar, diverse animal forms is a further biological and ecological event. These milestones need not coincide.
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- Crown Metazoa: the last common ancestor of living animals and its descendants. Estimates for this node are often described informally as dates for the origin of animals.
- A subgroup split: the divergence of a major branch within animals, such as Bilateria or Deuterostomia. It is later than, or equal to, the origin of the crown group being discussed.
- First known fossil occurrence: the age of the oldest fossil currently recognized as belonging to a group. It shows the group existed by that time, not necessarily when it first evolved.
- Animal diversification: the expansion and ecological spread of animal groups, which is not identical to the first lineage split.
Before comparing two dates, check which of these events each study actually estimates. A date for a deep lineage divergence and a date for recognizable animal fossils are answers to different questions.
How a molecular clock estimates a divergence
DNA sequences accumulate changes over generations. If researchers estimate how many changes separate lineages and model how quickly those changes accumulated, they can infer how much time may have passed since the lineages shared an ancestor. The rate is not known simply by reading the sequence: it must be estimated, and fossils or other evidence help anchor the timescale.
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- Build a dataset and evolutionary tree. Researchers select species and genetic sequences, estimate their relationships, and measure sequence differences along branches.
- Choose a rate model. A strict clock assumes a common rate across branches. A relaxed clock permits rates to differ, but must estimate those differences from limited evidence.
- Calibrate the timescale. Fossils associated with particular branches provide age constraints. The analysis combines these constraints with the sequence data and model to estimate divergence times, with uncertainty.
The result depends on the interaction of the sequence data, tree, rate model, and calibrations. Time and rate can be difficult to separate: a branch with many genetic changes might reflect more elapsed time, a faster rate, or both.
Why estimates differ
Fossils constrain dates; they rarely reveal the first split
A fossil generally establishes that a lineage existed by the fossil’s age, making that age a minimum constraint on the lineage’s history. It does not usually show that the lineage began exactly then. Which fossil is assigned to which branch, how its age is represented, and whether an additional maximum-age bound is justified can all affect the result.
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A 2005 methodological critique argued that some unusually young estimates arose from treating fossil calibrations as maximum limits without sufficient justification, as well as from problems with particular rate models. Its argument was not that Bayesian analysis itself is invalid.
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Evolutionary rates vary
A strict clock can be a poor fit if different lineages evolve at different rates. Relaxed-clock methods address that possibility, but their estimates still depend on how rate variation is modelled and on the finite sequence data available. Different models can therefore translate similar genetic differences into different amounts of elapsed time.
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Sequences and partitions change the signal
Studies may use different genes, species, and ways of dividing genetic data into partitions with separate evolutionary parameters. In a 2015 sensitivity analysis, changing the partitioning strategy significantly affected some deep estimates. Nodes near the root of the tree, or without a direct calibration, were especially variable.
Tree topology and branch lengths matter
The assumed branching order determines which lineages are treated as close relatives; estimated branch lengths describe their genetic separation. Both feed into divergence dates. In the 2015 analysis, competing phylogenetic hypotheses produced very different times, showing why a date cannot be interpreted independently of the tree used to calculate it.
The fossil record can start after the lineage did
Early animals may have been small, soft-bodied, rare, or otherwise unlikely to be preserved. Fossils can also be difficult to identify, and incomplete sampling may leave older evidence undiscovered. As a result, the first known fossil appearance can postdate the lineage’s origin without either record being wrong.
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| Source and estimate | What the figure describes | How to interpret it |
|---|---|---|
| Earlier molecular-clock studies summarized by dos Reis and colleagues in 2015: crown Metazoa estimates ranged from 1,298 to 615 million years ago (Ma). | A spread across historical studies of the same broad animal group. | This is not a confidence interval from one unified analysis; it shows how much estimates differed across studies. |
| dos Reis and colleagues, 2015: crown Metazoa, 833–650 Ma; crown Eumetazoa, 746–626 Ma; crown Bilateria, 688–596 Ma; crown Deuterostomia, 662–587 Ma; crown Protostomia, 653–578 Ma. | Several animal-group divergence estimates from one sensitivity analysis integrating uncertainties it tested. | These are ranges from that analysis, not settled dates. Its authors concluded that the resulting timescale was not precise enough to distinguish among causal explanations for animal diversification. |
| Cunningham and colleagues, 2017: modern clock analyses placed animal origins somewhere between about 850 and 650 Ma. | A review’s synthesis of molecular-clock estimates. | The review also discussed biomarker evidence interpreted as possible animal presence by about 635 Ma and reasonably convincing fossil evidence from about 565 Ma onward. The biomarker interpretation is debated, and neither figure is an exact origin date. |
| Live Science report, October 2, 2026: a newer estimate shifted to roughly 800–700 Ma under older geological constraints. | A reported model-based proposal for animal origins. | The report quotes first author Orin Lole Durbin cautioning that the analysis “does not prove that animals existed 800 million years ago.” The primary article’s detailed methods and uncertainty bounds are not established here, so this figure should not be treated as a confirmed fossil age or a consensus date. |
The 2015 sensitivity analysis is particularly instructive because it tested how results changed with calibration strategy, relaxed-clock model, data partitioning, and phylogenetic hypothesis. Its key lesson is not that one number must be chosen from the ranges above, but that deep dates can move materially when reasonable analytical choices change.
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Are molecular clocks more reliable than fossils?
They answer different parts of the timeline. Fossils provide direct evidence of organisms at particular times, but the oldest known specimen may be younger than the lineage itself. Molecular clocks can estimate an earlier divergence that left no recognized fossil, but that estimate depends on models and calibrations rather than a direct observation of the organism.
A 2017 review argued that the gap between the fossil and molecular records is smaller than it is often portrayed: both lines of evidence support a pre-Cambrian animal history, while leaving its timing imprecise. Neither evidence type should be treated as a substitute for the other.
How to evaluate a new animal-origin date
When you encounter a claim that animals evolved at a particular time, use these checks before comparing it with another study:
- Identify the event: Is the estimate for crown Metazoa, a subgroup split, a fossil occurrence, or a diversification event?
- Inspect the calibrations: Which fossils were assigned to which branches, and are their ages used as minimum constraints, maximum constraints, or both?
- Check the clock model: Does it assume one rate or allow rates to vary among branches?
- Look at data and tree choices: Which sequences and taxa were sampled, how were sequences partitioned, and what phylogenetic relationships were assumed?
- Read the uncertainty and sensitivity tests: A point estimate without its uncertainty or evidence about how alternative assumptions affect it can give a misleading impression of precision.
- Separate inference from observation: A clock estimate for a divergence is not proof that a fossil organism lived at that exact date.
The evidence supports a pre-Cambrian history for animals, but it does not establish a single precise date for their origin. The 2026 estimate is a proposal under particular geological constraints, not proof of an 800-million-year-old animal fossil record.
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