How did the ice ages play a role?
Some of the biggest finds have come from an ancient seabed in China, called the Doushantuo Formation, where unusual conditions preserved some extraordinary fossils. Layers between 550 and 580 million years old, during the last part of the Ediacaran, contain tiny spheres consisting of anything from one to dozens of different cells - just like the early embryos of animals. Some have suggested they are the remains of giant bacteria, but a series of studies over the past decade have left little doubt that they really are animal embryos.
In 2007, for instance, Leiming Yin of the Nanjing Institute of Geology and Paleontology in China reported finding embryos encased inside hard, spiky shells - unlike anything produced by bacteria. What is more, shells that are identical apart from the lack of preserved embryos on the inside can be found in rocks as old as 632 million years - the dawn of the Ediacaran - suggesting that the animal embryos themselves go back this far.
More pieces of the puzzle.
"This glaciation reset the chemistry of the oceans," he says. Ice caps covering the continents halted delivery of sulphur to the oceans and cut off production of hydrogen sulphide. "You have changes in ocean chemistry like an increased availability of molybdenum and zinc," says Ariel Anbar, a biogeochemist at Arizona State University in Tempe, "all of which play into making the world more hospitable for eukaryotes and ultimately, metazoans."
Sponges or something like them would have been the first animals on the scene. They lack a nervous system and have no need for circulatory systems. Animals like jellyfish might also have evolved early.
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