Vanishing life

Extinction is the disappearance of a species; that is, an entire species of animals or plants has died and can never return. When the environment changes, species must adapt to the new environment to survive. Species that adapt survive, others become extinct. Extinction is not an unusual thing; species disappear continually, and new species appear. In the past, most extinctions were caused by changes in climate or physical surroundings, but one of the main causes of extinctions today is human activity such as the destruction of forests. Half the world's species could disappear within a few decades if we do not change our ways.

When the number of extinctions is very large compared to the number that normally occurs, it is called a mass extinction. In mass extinctions there are few survivors and many victims. Mass extinctions have four important features: (1) many types of species go extinct; (2) large numbers of species go extinct; (3) extinction happens throughout the world - on land and sea; and (4) extinction occurs in a geologically short period.

Throughout the history of Earth there have been five big mass extinctions, one each in the Ordovician, Devonian, Permian, Triassic and Cretaceous periods: in the late Ordovician (438 million years ago); in the late Devonian (380 million years ago); at the end of the

Figure 10: Mass extinctions through time. (Graph based on the work of David Raup and John Sepkoski.)

Permian (245 million years ago); in the late Triassic (208 million years ago); and at the end of the Cretaceous (65 million years ago).

How do we know there were five big mass extinctions? One way of finding out about mass extinctions is to draw a graph of the rate of extinction over time. The peaks in the graph show mass extinctions. In the 1980s, after studying fossils of thousands of species of invertebrates, two American scientists, David Raup and John Sepkoski, drew similar graphs. Their study showed fifteen mass extinctions, of which five clearly towered above the others. Their study also showed a curious pattern: the fifteen mass extinctions seem to be spaced about 26 million years apart. Their conclusion that the mass extinctions have a 26-million-year cycle leads to the question: what causes the cycle? Raup and Sepkoski declared that they favoured 'extraterrestrial causes'. It did not take astronomers long to come up with fanciful ideas, some of which we will soon investigate.

The worst destruction of life in Earth's history took place at the end of the Permian 245 million years ago. Palaeontologists call this extinction the Great Dying because it nearly wiped out most of life on Earth. The death toll included 95 per cent of species in the oceans, 70 per cent of reptiles and amphibians, and 30 per cent of species of insects on land. So many trees and other forms of vegetation disappeared that for a brief period most of the land was covered with fungi. The wiping out of the ruling vertebrates opened the doors for the arrival of the dinosaurs in the Triassic that followed.

What caused this spectacular extinction? The long line-up of suspects includes changes in global climate, sudden drop in sea levels, toxic concentrations of carbon dioxide in the oceans, reduced oxygen and increased carbon dioxide in the atmosphere, decreasing supplies of nutrients in the oceans, and huge volcanic eruptions; but the prime suspect is a massive extra-terrestrial object the size of Mount Everest that slammed into Earth.

The most famous of the Big Five is the one when the dinosaurs died at the end of the Cretaceous. The end of the Cretaceous is often called the Cretaceous-Tertiary, or K-T, boundary (Cretaceous is shortened to 'K' to avoid confusion with the Carboniferous and Cambrian).

Dinosaurs were not the only species to die in the K-T

extinction; possibly 75 per cent of the species living at the time disappeared. Virtually all land and sea animal and plant groups lost species. The main survivors were some land plants, crocodiles, alligators, frogs, salamanders, turtles, birds and mammals. Most of the surviving animals were much smaller than the dinosaurs; they crawled into burrows or hid in water to escape the catastrophe. There was another reason: the surviving animals' place in the food chain. Most of the land animals that died lived in food chains that relied directly on plants. These plants were the first to die during the catastrophe. The surviving animals were in a different food chain. These animals ate insect larvae, worms and other small animals which, in turn, fed on dead and decaying plants.

Fossil records do not tell us whether the K-T extinction was sudden, with everything over in a few minutes, or whether it lasted several million years. The fairest answer is that scientists do not know.

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