In just 50 years, we will witness dozens of mammals going extinct unless we make major global changes, says a new study published Tuesday. And it will take up to 5 million years before evolution recovers a similar level of diversity on Earth, according to the study in Proceedings of the National Academy of Sciences.

It’s all thanks to humans, says the study: Our human-driven climate change, habitat destruction, and poaching has led to the sixth mass extinction event, a period where creatures start to disappear at a rate much higher than normal.

The natural rate of extinction, called background extinction, is a part of evolution and is typically pretty low. New species emerge, other species die out; it's always happened, and it will continue to happen as long as there's life on Earth.

But studies estimate the current rate of extinction is 22 times higher than it should be. This is causing the number of species to plummet, and biologists estimate 50 percent of species will disappear by the end of the century if the current trend continues. The problem isn’t just that the total number of species is dropping, but also that we’re losing unique evolutionary history, called phylogenetic diversity, as species die off.

Mammals are particularly susceptible to this kind of diversity loss, according to the study, because many large mammals have few close relatives. That means when one of these species (like the elephant, which only has two remaining species) dies off, “that entire branches of Earth's evolutionary tree [is] chopped off," paleontologist Matt Davis from Aarhus University, who led the study, said in a press release.

There is a glimmer of hope in the study, which is that nature can (and will) recover, though it will take millions of year. Even better, if we prioritize phylogenetic diversity when working on conservation efforts, we can preserve more of the evolutionary diversity that’s been gained, and shorten the amount of time needed for nature to evolve and be as diverse as it once was.

Unfortunately, that would require humanity doing something to curb climate change, slow the mass extinction, and protect endangered species—an outcome even the study authors were not banking on.

“There is little reason to expect that humans will be able to bring extinction rates down

to background levels within the next century with a rising human population and increasing anthropogenic climate change,” the study reads.