There are almost no fossils left in the rocks of this age, so we have to guess what life forms existed. It is possible that sponges might have appeared around 700 mya.
Just as algae gained by living together in colonies, so protozoa began living together. The earliest many-celled animals might have been sponges, which are bottle-shaped animals which look like plants. A current of water is drawn in through small holes in their sides by the beating of tiny whip-like hairs (flagella). Food is filtered out of the water current, which leaves through a large hole at the top.
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