The story of life on Earth bears witness to five mass extinctions, results of natural calamities. Biologists are now talking about a sixth wave of extinction, a result of human action.
Taxonomists have already described nearly two million species. In fact, their number varies, according to estimates, from 5 to 100 millions. 90 to 99% of the species that existed on the planet went extinct. The vast majority disappeared as part of a process of natural extinction of species, due to the limited duration of their biological existence. This duration varies from one million years in mammals to eleven million years in the case of certain marine invertebrates. Beyond this natural natural extinction, the fauna experienced five mass extinctions, during which 50 at 95% of the existing species disappeared within a historically limited time.
According to many experts, a sixth wave of extinction is underway, as a result of climate and environmental change and the disappearance of local biotopes. On the basis of the current average extinction rate of 40 species per day, in 16.000 years would disappear 96% of contemporary animal species, just as much as during the period of the disastrous extinction of the Permian. Scientists estimate that the life span of species among contemporary mammals and birds today is 100 1000 times lower than that of fossil forms: it would now be 10.000 years. And if the habitat continues to be destroyed at the same rate, the lifespan of these species would be only 200 to 400 years.