The history of the first animal life remains somewhat confused. Ediacaran fossils are clearly multicellular, but lack many of the features shared by all modern animals. In the ensuing period, the Cambrian, all of these organisms are gone, and most of the groups we’re familiar with—along with a few unfamiliar ones—are present. The transition between the two is murky.
Spectacular fossils from Doushantuo in China appeared to resolve this issue. The tiny remains date from the Ediacaran, but appeared to share features with animal (more properly, metazoan) embryos, suggesting that metazoans were around for many millions of years, even though we’ve been unable to identify any fossils of their adult forms. Since their initial announcement, however, this interpretation has been challenged, with some even suggesting that the fossils were little more than clusters of bacteria.
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