According to the natural background rate, just nine vertebrate species should have gone extinct since 1900, the researchers found. But, using the conservative, modern rate, 468 more vertebrates have gone extinct during that period, including 69 mammal species, 80 bird species, 24 reptile species, 146 amphibian species and 158 fish species, they said.

Each of these lost species played a role in its ecosystem, whether it was at the top or bottom of the food chain.

“Every time we lose a species, we’re eroding the possibilities of Earth to provide us with environmental services,” Ceballos told Live Science.

Researchers typically label an event a mass extinction when more than 5 percent of Earth’s species goes extinct in a short period of time, geologically speaking. Based on the fossil record, researchers know about five mass extinctions, the last of which happened 65 million years ago, when an asteroid wiped out the nonavian dinosaurs. [Wipe Out: History’s Most Mysterious Extinctions]

“[The study] shows without any significant doubt that we are now entering the sixth great mass extinction event,” study researcher Paul Ehrlich, a professor of population studies in biology at Stanford University, said in a statement.

Here’s More Proof Earth Is in Its 6th Mass Extinction.