Six Mass Extinctions in Earth’s History. Are We on the Brink of a Seventh?
Scientists define a mass extinction as the extinction of about three-quarters of all species over a short geological period of time, which is less than 2.8 million years, according to The Conversation. Right now, humans are at the beginning of the last mass extinction, which is moving much faster than any other. Since 1970, vertebrate species populations have declined by an average of 68%, and more than 35,000 species are currently considered threatened with extinction, according to the International Union for Conservation of Nature (IUCN). During the 20th century alone, about 543 species of land vertebrates became extinct, according to a research paper in the journal PNAS.