Life is a struggle for survival. Animals live under constant stress to obtain enough food to eat by being as well adapted to their environments as they can. Animals who are poorly adapted will, in times of hardship, starve, fail to reproduce, and eventually die out completely. Throughout Earth’s history, life has constantly been taking new forms which are immediately tested for survival. When the climate or environment changes drastically, many animals who are poorly adapted for their new situation become extinct. Mass extinctions are when a substantial proportion of Earth’s life has vanished completely, leaving no further fossils or descendants. These events have been occurring since nearly the first appearance of life itself. All the animals alive today are merely the descendants of creatures who have been lucky enough to have met the adaption requirements each time their world changed. Here we look at ten of the biggest extinction events in Earth’s history.
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.