The Great Extinctions
Biologists divide living organisms into a hierarchy of classifications. The broadest classification is the kingdom, such as animals, plants, fungi and bacteria. Next comes the phylum. Each phylum corresponds to a distinct, basic body plan. Phyla include the chordates (which includes vertebrates, like us), mollusks, roundworms, seed plants and so on. Through all of the history of the Earth there have been almost 100 phyla. About 30 of these still exist today.
Within phyla, organisms are divided into class, order, family, genus and, finally, species. The total number of species that have ever existed is about 30 billion. There are about 30 million species today. That is, 99.9% of all species are extinct. Nevertheless, there are more species today than at any other time in the Earth's history.
It used to be thought that phyla and species came and went in a more-or-less uniform pattern. But this is far from the truth as told by the fossil record.
Up to the early Cambrian Period, about 530 million years, life on Earth existed in a limited variety of forms. There were bacteria, archea and some soft-bodied marine creatures. Then, in a period of possibly as little as 5 million years, between about 530 and 525 million years, all of the phyla that have ever existed suddenly appear in the fossil record.
Over the following 525 million years, many of these phyla disappeared, but the number of species within the remaining phyla increased enormously.
This did not happen in a steady pattern but in a number of "extinction events" of which six are known as the "great extinctions". Each of these wiped out at least 15% of all species in existence at the time. (There have, of course, also been many lesser and localized extinctions).
1. The End of the Ordovician (440 million years ago)
Late in the Ordovician Period, the super-continent of Gondwana drifted over the north pole. This caused enormous glaciation and, consequently, lowering of sea levels.
During this period, life existed only in the seas. The lowering of the sea levels drastically reduced the area of shallow seas on the continental shelf which devastated the marine life which was established there. More than 60% of marine invertebrates genera disappeared, including numerous groups of brachiopods, trilobites and reef-building fauna.
2. The End of the Devonian (365 million years ago)
Another period of glaciation and falling sea levels occurred about 365 million years ago, late in the Devonian period. It is possible that a large meteorite may have struck the Earth at about the same time.
70% of marine species were wiped out. The impact on reef-building organisms was so severe that reef building was uncommon until the evolution of modern corals in the Mesozoic Period over 100 million later. Trilobites, brachiopods and jawless fish were also severely effected.
3. The End of the Permian (225 million years ago)
Huge volcanic eruptions (a million times more powerful than Mount Saint Helens) about 225 million years ago produced worldwide clouds of ash and greenhouse gasses. The result was the greatest of all the mass extinction.
Between 90 and 95% of all marine species were wiped out. Trilobites, blastoids and some early forms of fish were wiped out. Crinoids, ammonoids, brachiopods (like clams) and bryzoans were severely affected.
4. The End of the Triassic (210 million years ago)
The cause of the extinction at the end of the Triassic Period is not not clear but it may have the result of a comet shower striking the Earth.
It resulted in the extinction of all marine reptiles except icthyosaurs and some types of amphibians. Mammal-like reptiles, mollusks, brachiopods (like clams) and gastropods (like snails) were severely affected.
5. The End of the Cretaceous (65 million years ago)
About 65 million years ago, a meteorite struck the Earth in the area of the Yucatan Peninsula of Mexico.
Among the species wiped out were the dinosaurs, marine reptiles, blemnoids, ammonoids and many species of plants. Brachiopods, mollusks and many species of fish were severely affected. On the other hand, mammals, birds, turtles, crocodiles and amphibians were not much affected.
6. The Impact of Man (current)
In the past 10,000 years, the human population of the earth has soared from about 6 million to 6 billion. This has had a severe impact on other species. It is estimated that as many as 30,000 species are becoming extinct every years.
In terms of the fossil record, the impact of man can be clearly seen in Australia. When man first arrived in Australia about 60,000 years ago, there were about 50 species of large animals; only four (all kangaroos) survive. Those which have become extinct include a marsupial lion, a giant carnivorous lizard, a rhino-like marsupial, a tapir-like marsupial and ground sloths.
A similar pattern has occurred in every other continent except Africa. In Africa, man evolved along with the large animals but on every other continent, the arrival of man has meant the extinction of most of the large animals.