During the Ediacaran period, complex life had begun to take form for the first time on Earth. Tiny bacteria had evolved into the more complex and specialized Eukaryotes, some of which grouped together to increase their chances of finding food and avoiding becoming food. Most of these odd creatures did not leave a record because they had no skeletons; they were soft and tended to rot when they died rather than fossilize. Only in peculiar circumstances could fossils form, such as a creature lying on soft mud which suddenly hardened and left an imprint. These few fossils tell us of seas full of strange and alien creatures who resembled modern worms, sponges, and jellies. However, these creatures were dependent upon oxygen, as are we. The oxygen levels began to fall and world-wide extinctions occurred 542 million years ago. Over 50% of all species died. The huge numbers of dead creatures decomposed and make up some of today’s fossil fuels. The exact cause of the lowering oxygen levels is unknown, however, this mass extinction made room for the Cambrian explosion, a sudden diversifying of complex creatures beyond mere worms.
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