Of all the species that have lived on the Earth since life first appeared here three billion years ago, only about one in a thousand are still living today. All the others have became extinct usually within ten million years after they first appeared. This in turn has contributed greatly to the current level of biodiversity on the planet. .
Extinction is the loss of populations of interbreeding organisms. It can be caused by habitat destruction, predation and the inability to adapt to changing environments. It can also be caused by changes in the climate and environment or by the evolution of new, better-adapted types of organisms that have displaced earlier forms. Commercial exploitation, damage caused by non-native plants and animals introduced in to an area and pollution also contribute to the extinction of species According to many biologists the human population is the primary reason for the increase in the rate of extinction. Since 1600 about 1100 species have became extinct. The rapid disappearance of species was ranked as one of the planet's gravest environmental worries, surpassing pollution, global warming and the thinning of the ozone layer, according to the survey of 400 scientists commissioned by New York's American Museum of Natural History. .
Since the 1600s, worldwide commercial exploitation of animals for food and other products has caused many species to become extinct or endangered. The whaling industry, in which whales are slaughtered for oil and meat, has led many whale species to the brink of extinction. The African black rhinoceros, killed for its horn, which is prized as a medicine and aphrodisiac, is also critically endangered. The great auk and the passenger pigeon became extinct in the 19th and early 20th centuries because of over hunting, and the Carolina parakeet also died as a species because of the combination of over hunting and habitat destruction.
