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Dating the Origin of Animals

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Science  20 Dec 1996:
Vol. 274, Issue 5295, pp. 1993-1997
DOI: 10.1126/science.274.5295.1993f

The Research Article by G. A. Wray et al. (25 Oct., p. 568) suggesting that the diversification of major animal phyla occurred between 1 and 1.2 billion years ago, finds indirect support from Proterozoic fossil stromatolites. These structures underwent a dramatic decline in diversity over a long period that may be explained if the view of this early origin of animals is accepted.

Earlier published diversity curves of stromatolite abundance (1) indicated that peak abundance was reached about 1.5 billion years ago, persisting at this level until about 700 million years ago. Then followed an apparent rapid late Precambrian decline, attributed to increased animal diversity. Such data provided support for those who argued for animal origins very late in the Precambrian.

More recently published Proterozoic stromatolite abundance data (2) show, however, a substantially different pattern, with a decline in diversity initiated much earlier and persisting for over 1 billion years. These show peak diversity 1 to 1.3 billion years ago. It then declined to about 75% of this level between 1 billion and 700 million years ago, dropping to less than 20% at the beginning of the Cambrian.

If, as seems likely, the decline in stromatolite diversity was a result of the increase in animal diversity during the late Proterozoic, then the time of onset of this decline supports the suggestion, based on molecular sequence divergence times, of animal origins at least 1 billion years ago.

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