Any animals existing before this period must have obtained oxygen for their tissues by diffusion, which is a slow process. Those animals would have had no heart—at least, no pump—nor would they have possessed a circulatory system. They would have been tiny, gossamer-like creatures, so it’s little wonder they left no trace in the fossil record. But then, for some reason that’s not entirely clear, the atmospheric oxygen level rose yet again in the Cambrian period. Several key evolutionary developments took place—gills, hearts, haemoglobin in blood—allowing marine animals to make much more
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