The Earth is undergoing a mass extinction, which is caused by human activity that destroys habitats and kills off species.

In a new study, Danish researchers concluded that so many mammal species will go extinct in the next 50 years that the planet's evolutionary diversity won't recover for at least 3 million years.

The scientists said their findings could be used to help determine which endangered species are the biggest priorities for conservation efforts.

Humans will cause so many mammal species to go extinct in the next 50 years that the planet's evolutionary diversity won't recover for 3 to 5 million years, a team of researchers has found.

The Earth may be entering its sixth mass extinction: an era in which the planet's environments change so much that most animal and plant species die out. The International Union for the Conservation of Nature predicts that 99.9% of critically endangered species and 67% 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.

Evolution can't keep up

Evolution is the planet's defense mechanism against the loss of biodiversity. As habitats and climates change, species that can't survive die, and new species slowly emerge. But it takes a long time for new species to fill the gaps — and that process is far slower than the rate at which humans are causing mammals to go extinct.

For their calculations, the Aarhus University researchers used a database containing existing mammal species and mammals that already went extinct as humans spread across the planet. They combined that data with information about extinctions expected to come in the next 50 years, and used advanced simulations of evolution to predict how long recovery would take.

Their estimates are based on an optimistic assumption that people will eventually stop ruining habitats and causing species to die out, and the extinction rate will go back down. But even in that best-case scenario, the timeline depends on how quickly mammals start recovering. If the extinction rate doesn't start falling for another 20-100 years, more species will likely disappear, causing greater diversity loss, the study said.

Litopterns, like this one discovered by Charles Darwin, were a strange-looking group of prehistoric South American mammals that were not closely related to any species alive today. When they went extinct at the end of the Ice Age, the mammal Tree of Life lost one of its deepest branches. Robert Bruce Horsfall via Wikimedia Commons

The researchers noted that in their model, certain species were given more importance than others. Matt Davis, a paleontologist at Aarhus University who led the study, cited the shrew as an example. There are hundreds of species of shrew, so if one or two go extinct, that would not kill off all shrews on Earth.

But there were only four species of sabre-toothed tigers on the planet. So when they all went extinct, many years of evolutionary history disappeared with them.

"Large mammals, or megafauna, such as giant sloths and sabre-toothed tigers, which became extinct about 10,000 years ago, were highly evolutionarily distinct," Davis said in a press release. "Since they had few close relatives, their extinctions meant that entire branches of Earth's evolutionary tree were chopped off."

The research could help conservationists

Today, other large animals like the black rhino are facing extinction. Asian elephants' chance of making it to the 22nd century is less than 33%, the study found. These elephants are one of only two remaining species from a group of mammals that once included mastodons and mammoths.

"We now live in a world that is becoming increasingly impoverished of large wild mammalian species," Jens-Christian Svenning, an Aarhus University professor who researches megafauna, said in the press release. "The few remaining giants, such as rhinos and elephants, are in danger of being wiped out very rapidly."

He noted that the planet no longer boasts giant beavers, giant deer, or giant armadillos.

Though the researchers' findings are dire, the scientists said their work could be used to figure out which endangered species are evolutionarily unique, which might help conservationists decide where to focus their efforts to prevent the most devastating extinctions.