When did the first complex animals, the meta-zoans, appear on Earth and what did they look like? How could complex, multicelled animals evolve from the undifferentiated single-celled organisms of most of the Pre-cambrian? Why did they take almost 4 billion years to appear? These questions have puzzled scientists, including Charles Darwin, for over two centuries. In the last few decades a range of multidisciplinary techniques, from molecular biology to X-ray tomography, has helped generate new testable hypotheses regarding the origins of our early ancestors. Apart from the fossil evidence of metazoan body and trace fossils, the investigation of minute fossil embryos, carefully calibrated molecular clocks and more recently biomark-ers have placed the investigation of Precam-brian life at the top of many scientific agendas.
The first metazoans: when and what?_
Life on our planet has been evolving for nearly 4 billion years. Molecular data suggest metazoans have probably been around for at least 600 myr (Fig. 10.1), during which time, according to some biologists, as many as 35 separate phyla have evolved. Five lines of evidence have figured prominently in the search for the earliest metazoans: body fossils, trace fossils, fossil embryos, the molecular clock and biomarkers.
Much controversy still surrounds the timing of their origin. Was there a long cryptic interval of metazoan evolution prior to the Edia-caran - a time when we do not find fossils preserved, either because the animals lacked preservable bodies, or they were small, or perhaps a combination of both? Or, as the recalibrated molecular clocks suggest, can animal origins be tracked back only to the Ediacaran, when there was also a sudden rise
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