Recently discovered prehistoric embryos are throwing previous theories into a tizzy
evolutionary biology has long rested in the belief that the major animal forms of today -- known as phyla -- date from an 'explosive' event or series of events that occurred about 550 million years ago at the start of what is known as the Cambrian period. The belief is based on fossil data: animal fossils tended to be sparse in deposits formed prior to the Cambrian period, and abundant thereafter. The Cambrian explosion, if true, is also extremely puzzling, because conventional thinking does not fit in well with the possibility of radical evolutionary developments taking place within relatively brief intervals of time (here 'brief' is to be understood in a geological time-frame: 10 million years would certainly qualify).
A spectacular finding, made recently by Shuhai Xiao and colleagues at the Harvard and Beijing Universities, undermines the sanctity of the Cambrian explosion theory. The researchers have reported the existence of superbly preserved animal embryos in the Doushantuo phosphate deposits in southern China. The stratum is 570 million years old approximately, and the complexity of the embryos suggests that their evolutionary ancestry must be far older than would have been guessed hitherto. This may also mark the beginning of a rapprochement between palaeontologists, whose best estimates of the time of divergence of the major animal phyla was 565 million years ago, and molecular biologists, who had inferred (on the basis of dna sequence analysis) that the time of divergence had to be at least 1,000 million years ago.
The fossils identified by Xiao and colleagues are spherical and about half-a-millimetre in diameter. They are broken into 2, 4 or 8 sub-units that look exactly like dividing animal embryos. The overall size of the fossils is the same irrespective of how many sub-units they contain, reinforcing the inference that they were indeed embryos -- initial embryonic development is not accompanied by an increase in size -- and not crystalline aggregations of inert matter, for example.
These fossilised embryos confirm in their appearance to an old theory due to Haeckel, that in the beginning multi-cellular animals must have been microscopic creatures that resembled the embryos of the animals of today. Multi-cellular algal fossils have been dated to about 1,000 million years ago, and the present work indicates a similar age for animal fossils too. As the authors point out, the Doushantuo embryos constitute the first geological evidence in support of the hypothesis that the chief lineages of multi-cellular life diversified much before large animals came on the scene. If verified by future studies, implication will be that evolutionary pressure for an increase in size must have come only long after an extensive exploration of shapes and forms had taken place.
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