A speeding clock could solve Darwin’s mystery of gaps in animal fossil records

The oldest fossilised remains of complex animals appear suddenly in the fossil record, and as if from nowhere, in rocks that are 538 million years old.

The very oldest of these are simple fossilised marks (called Treptichnus) made by something worm-like with a head and a tail. A host of other animals appear rapidly, ancestors of the diverse animal groups we know today: ancient crab-like arthropods, shelled molluscs and the forebears of starfish and sea urchins.

The rapid arrival of animals so different to each other (and their absence in even slightly older rocks) was a headache for Charles Darwin because it seemed to go against his idea of gradual evolution – and it has confused scientists ever since. However, a recent paper may provide a solution.

In 1859, Darwin wrote in On the Origin of Species: “If my theory be true … during these vast … periods of time, the world swarmed with living creatures. To the question why we do not find records of these vast primordial periods, I can give no satisfactory answer.”

Today, scientists are in disagreement about when these ancient animals evolved. The problem stems from a late 20th-century invention called the molecular
clock.

As I explain in my book the Tree of Life, the molecular clock relies on the idea that changes to genes accumulate steadily, like the regular ticks of a grandfather clock. If this idea holds true then simply counting the number of genetic differences between any two animals will let us calculate how distantly related they are – how old their shared ancestor is.

For example, humans and chimpanzees separated 6 million years ago. Let’s say that one chimpanzee gene shows six genetic differences from its human counterpart. As long as the ticks of the molecular clock are regular, this would tell us that one genetic difference between two species corresponds to one million years.

The molecular clock should allow us to place evolutionary events in geological time right across the tree of life.

When zoologists first used molecular clocks in this way, they came to the
extraordinary conclusion that the ancestor of all complex animals lived as long as
1.2 billion years ago. Subsequent improvements now give much more sensible estimates for the age of the animal ancestor at around 570 million years old. But this is still roughly 30 million years older than the first fossils.

This 30-million-year-long gap is actually rather helpful to Darwin. It means that there was plenty of time for the ancestor of complex animals to evolve, unhurriedly splitting to make new species which natural selection could gradually transform into forms as distinct as fish, crabs, snails and starfish.

The problem is that this ancient date leaves us with the idea that a host of ancient animals must have swum, slithered and crawled through these ancient seas for 30 million years without leaving a single fossil. Researchers expect gaps in the fossil record but this one would be a whopper.

A popular explanation for the missing fossils is that, for 30 million years, complex animals were tiny and squishy and so hard to fossilise. And then, around 540 million years ago, so the theory goes, these tiny animals began to grow larger, perhaps due to increasing oxygen levels. It is this increase in size that some scientists have used to explain the sudden appearance of complex animals in the fossil record.

The new paper by palaeontologist Graham Budd and mathematician Richard Mann gives a different explanation for the chasm between the ancient ancestor predicted by the molecular clock, and the more sudden, later appearance of complex fossils. Budd and Mann suggest that the molecular clock may not tick quite as regularly as we thought.

The new idea is that the moment that any big group of organisms first appears, evolution speeds up.

To return to our example, for a period of a few million years our imaginary clock could have ticked not once per million years but twice. A faster ticking clock would make it appear as if more time was passing, like pressing fast forward on a video, and this would push the age of the animal ancestor further back into the past.

Faster changing genes would also allow the animals’ appearance to change more quickly. This solves Darwin’s dilemma as it would make it easier for the various branches of the animal tree to become different from each other. The first animal ancestor could quickly diversify into vertebrates, molluscs, arthropods and starfish.

The overall effect of the new idea is to bring the age of the ancestor of complex animals much more in line with the appearance in the fossil record of its immediate descendants.

While the speeding clock idea needs testing, it could explain other mismatches between molecular clocks and fossil record. Perhaps the first flowering plants really existed for tens of millions of years before finally leaving a fossil. And it could help settle scientific debates about whether early primates, carnivores and rodents really lived alongside the last dinosaurs.

For the origins of the animals at least, I feel sure that Darwin would approve.

This article features references to books that have been included for editorial reasons, and may contain links to bookshop.org. If you click on one of the links and go on to buy something from bookshop.org The Conversation UK may earn a commission.