Published online 4 February 1999 | Nature | doi:10.1038/news990204-4

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Pushing back the origins of animals

If a recent analysis of animal evolution is correct, then the famed “Cambrian explosion” in the evolution of multicellular animals was not so much a Big Bang as simply the end of a long, slow, crawl.

In the most recent issue of The Proceedings of the Royal Society of London, Daniel Wang and colleagues from Pennsylvania State University in the USA have looked at the molecular evidence for the timing of animal evolution. By looking at the molecular clocks represented by differences in the DNA sequences of some 50 genes in present-day animals, they have pushed the time of origin of the major animal groups (phyla) hundreds of millions of years further back into the Precambrian.

The apparent Cambrian explosion is one of the most intriguing events in evolutionary history. The Cambrian period began 543 million years ago: about 530 million years ago, most of the main groups of present-day animals appear for the first time in the fossil record. Animals as diverse as worms, arthropods, and our own chordate ancestors all appear within a few tens of millions of years. With no apparent fossil forebears in the Precambrian, a vast amount of evolution and diversification of animal body plans seems to have been packed into a period of around 50 million years.

Such a rapid and dramatic spurt of evolution is not easy to explain. Nothing like it has been seen since, and evolutionary biologists have often wondered whether there might be some alternative explanation. Is the fossil record in the Cambrian just a record of the appearance of animals with hard skeletal parts that fossilized well? Is it just a multitude of tips of a flotilla of hidden icebergs of evolution, stretching back hundreds of millions of years?

Wang and colleagues’ analysis supports the second contention. Instead of requiring arthropods (insects and crustaceans) and chordates (vertebrates and allies) to have evolved from a common ancestor in a mere few tens of millions of years, they estimate that these two lines diverged as far back as 993 million years ago. This gives these two very different groups of animals an extra 400 million or so years of evolution to play with, which is in many ways a much more likely scenario.

Throwing their net wider, they place the fundamental time of divergence between animals, plants and fungi at around 1500 million years ago. Nematode worms come along at around 1200 million years ago, on their estimate, again much earlier than the fossil record would suggest. The most primitive animals, the sponges and coelenterates, would therefore have evolved somewhere between 1500 and 1200 million years ago.

But although molecular clocks work fine in theory, no-one knows just how reliable they are in reality. The principle is simple. Given the same gene in two different animals, an insect and a mammal for instance, one can count the differences in the nucleotide sequences to estimate how long ago the ancestors of those two animals diverged. But previous estimates of animal origin times using these sorts of methods have yielded some wildly different timescales.

Fully aware of the problems, Wang and colleagues used a large number of genes in their analysis, far more than in previous analyses, and chose the genes carefully to avoid those that might not count time accurately. They calibrated their molecular clocks against well-established and uncontroversial timelines in animal evolution, such as the divergence of the reptilian ancestors of birds and mammals about 310 million years ago. And their results are indeed in rough agreement with at least some of the previous studies, which also place the time of animal origins far back in the Precambrian.

So the evidence is slowly mounting that animals were around a long time before the Cambrian. And this of course raises an almost equally difficult question - where were they? Why is there no fossil record of these arthropod and chordate ancestors in the supposed 400 million years of their early evolution? Were they all soft-bodied animals that left no traces? Or, as Wang and colleagues suggest, might they simply have been very small? In which case, their remains will only be found in extremely fine-grained rocks, of which there are some examples of suitable age. The search is now on.