The International Union for the Conservation of Nature predicts that 99.9 percent of critically endangered species and 67 percent of endangered species will be lost within the next 100 years.
The five other times a mass extinction has occurred over the past 450 million years, natural disasters were to blame. But now, human activity is killing mammal species.
In a study published Monday in the journal PNAS, scientists from Aarhus University in Denmark calculated how fast extinctions are happening, and how long it would take for evolution to bring Earth back to the level of biodiversity it currently has.
The scientists concluded that in a best-case scenario, nature will need 3-5 million years to get back to the level of biodiversity we have on Earth today. Returning to the state Earth’s animal kingdom was in before modern humans evolved would take 5-7 million years.