Ask when animals first appeared and you get two answers. The rocks point to about 574 million years ago, the age of the oldest known animal body fossils. Genes point much further back.

A new study in Science Advances, led by Orin Lole Durbin of Virginia Tech and Ross Anderson of the University of Oxford, widens that gap. By re-examining how molecular clocks are calibrated, it places the origin of animals at roughly 800 to 700 million years ago. That is between about 125 and 225 million years before the first fossils.

Molecular clocks turn genetic differences into time

After two lineages split, their DNA slowly collects different changes. Count the differences, estimate how fast they build up, and you can work out roughly when the split happened.

Think of two copies of a manuscript copied by hand in separate monasteries. The more the copies differ, the longer ago they parted, as long as you know how many errors copyists make per century.

The rate of change isn’t fixed, so clocks need anchors from the rock record:

  • A minimum age: a group is at least as old as its oldest fossil.
  • A maximum age: a group is assumed to be no older than some well-preserved rock layer where it should show up but doesn’t.

The usual upper limit rested on a missing fossil

Researchers often use the Weng’an Biota in southern China, about 590 million years old, as a maximum age for animals. It preserves microscopic life in remarkable detail, yet contains no definite animals. The logic: if animals had existed then, they would be there.

The team tested that logic. They examined more than 140 samples from the Kheseen Biota in Mongolia, which shares several species with Weng’an and preserves them in a similar way, but is tens of millions of years younger.

It contained no confirmed animals either. Yet by then animals certainly existed, because their fossils are known from rocks of that age elsewhere.

If a deposit can miss animals when they definitely existed, it can’t prove they were absent earlier.

With older limits, animals moved back by about 200 million years

The authors argue that the most reliable maximum ages come from older, well-preserved deposits from before the Ediacaran period, such as rock formations in Norway, Australia and Arizona dated to roughly 850 to 730 million years ago.

With those anchors, their analyses put the last common ancestor of all living animals at roughly 800 to 700 million years ago, depending on the calibration. Chemical fossils point the same way: fat molecules linked to sponges suggest animals were around before 635 million years ago.

To make the timescale concrete (our calculation): squeeze Earth’s 4.6 billion years into one calendar year, so each day covers about 12.6 million years.

  • The oldest animal fossils appear around 15 November.
  • The new estimate moves the first animals to between about 28 October and 5 November.
  • Our own species arrives in the last half hour of 31 December.

If the dates are right, early animals lived through Snowball Earth

From about 720 to 635 million years ago, during the Cryogenian period, Earth went through extreme glaciations, with ice that may have reached the tropics. An origin 800 to 700 million years ago means early animals would have faced them. Whether they survived, and how, is unresolved.

Why no fossils? Early animals may have been tiny, soft-bodied or rare. They may have lived in environments that rarely preserve anything, or researchers may simply not have searched the right rocks yet.

The bigger lesson is about evidence. A fossil shows that something existed by a certain time. A missing fossil shows much less. The two dating methods disagree partly because they answer different questions: when animals became common and preservable, and when the lineage began.

What the study doesn’t show

  • It doesn’t prove animals existed 800 million years ago. No animal body fossil from before the Ediacaran has been found, as the authors themselves stress.
  • The result depends on which calibrations you trust, and other researchers may defend the younger maximum ages.
  • Molecular clocks rely on assumptions about how fast DNA changes, and rates vary between lineages and over time.
  • The range of about 100 million years reflects real uncertainty, not a precise date.
  • The study is peer reviewed.

The fossil record is a lower bound, not a birth certificate

The oldest fossil of a group tells us it existed by then, not when it began. Genes and rocks are both telling the truth about different things.

Read every “first appearance” in the fossil record as “no later than”.