The discovery of animal embryos in cell-by-cell detail, together with the perfectly preserved tissues of advanced seaweeds, in 570-m.y.-old rocks in China, will change our understanding of the truly ancient past. These remarkable, exquisite fossils are reported in the 5 February 1998 Nature by Shuhai Xiao and Andrew Knoll of Harvard University, and Yun Zhang of Beijing University.

There is an apparent 'explosion' in evolution in the early Cambrian, when, in an an interval of about 10 m.y., the fossil records of almost all groups of animals seem to start, with a frank abruptness. Before the 'Cambrian Explosion' there is almost nothing. Just before the Cambrian, around 550 Ma, was a flowering of strange creatures of no readily discernible affinity to animals or plants that have lived since. Before that, faint trails and obscure traces hint at animal life.

Many feel that the Cambrian Explosion is illusory, and that creatures diversified much earlier. These early creatures may not have fossilized readily, perhaps because they were very small; perhaps because conditions did not suit the evolution of hard, fossilizable shells and armour; perhaps because fossils of any kind in rocks that ancient stand little chance of surviving to the present day in any recognizable state.

Without fossils, researchers have resorted to molecules. By comparing sequences of genes and proteins of modern creatures, Gregory Wray of SUNY Stony Brook and colleagues suggested in Science in 1996 that the major diversification of animal life (in particular, life any more complex than a jellyfish or a flatworm) took place over 1000 m.y. ago. This astonishingly ancient divergence time has since been challenged. Work by Francisco José Ayala of Pennsylvania State University and colleagues, published in January in PNAS, sets the date at around 670 Ma: much more recent, but still 130 m.y. before the start of the Cambrian.

All the while, palaeontologists have been able to do nothing but sit on their hands and wonder. The latest fossils will change all that. They all come from the 570-m.y.-old Doushantuo Formation of southern China, and owe their preservation to the special properties of calcium phosphate, which can preserve fossils in extremely fine detail.

The animal embryos will attract most attention. They seem to have come straight out of the pictures in a biology textbook, preserved as single fertilized cells, as two-, four- eight-cell and later stages, all preserved uncrushed and in microscopic detail.

What would these embryos have become, had they grown up? That is harder to say, and any answer will contain a measure of speculation. But details of the way the cells are arranged resemble similar arrangements in the cells of relatively complex invertebrates such as crustaceans. This suggests that relatively complex animals existed at 570 Ma, which in turn implies that the divergences between the lineages of the major animal groups must have happened much, much earlier.

These results confirm the molecular data, inasmuch as the divergences of the major animal lineages happened well before the Cambrian Explosion. But they do more than just confirm, they add flesh to the theory, providing solid evidence to support the claims made by molecular biologists. Greater and more wonderful discoveries will surely follow.

© Macmillan Magazines Ltd 1998 - NATURE NEWS SERVICE

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