The earliest known complex animal communities existed around 570 million years ago, the Neoproterozoic era. Paleontologists used to believe that these early animals were relatively simple creatures but they are finding more and more evidence that points to a greater complexity of ecosystems and biological systems in early animals than previously believed. As more research is done, new evidence may extend the known history of animals on earth.
570 million years (or 635 million; see link) is the time for the oldest shelly (hard-bodied) and/or relatively large complex animals. Some researchers think the earliest animals – probably microscopic – began about 1.2 billion years ago (see the book Rare Earth, by Ward & Brownlee, 2004, p. 104.)
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