When humans arrived on New Zealand some 700 years ago, they discovered a plethora of unusual birds that could be found nowhere else in the world, having evolved in isolation on the island. There was the giant moa, which could reach a height of seven feet, the Haast’s eagle, an enormous predator that hunted the moa, the laughing owl, known for its piercing cries. All of these birds, along with half of New Zealand’s avian taxa, have since gone extinct. And according to a new study in the journal Current Biology, it would take 50 million years to recover the bird biodiversity that has been lost.

The impact of humans on New Zealand’s avian species is relatively well understood. Because many of the nation’s birds were large and flightless, they were particularly susceptible to habitat alteration, introduced predators and hunting—first by the Polynesian Maori, then by European settlers. Today, some of the nation’s most iconic species remain at risk. The kākāpō, a chunky, flightless parrot, is considered critically endangered, pushed to the brink of extinction due to predation by cats and rats. The kiwi, New Zealand’s unofficial national emblem, is similarly threatened by dogs and ferrets, and is listed as vulnerable by the IUCN.

But according to the authors of the new study, “little is known about the long-term macroevolutionary impact of anthropogenic extinction. In other words, how far have humans perturbed this unique and isolated biological assembly from its natural state?”

To find out, the research team looked at previously collected archaeological and paleontological data, which pinpoint the times that many of New Zealand’s birds went extinct, according to Roni Dengler of Discover. The researchers also collected DNA sequences for extinct New Zealand birds, published in earlier studies. They then used computers to simulate a variety of human-induced extinction scenarios, estimating natural rates of extinction, speciation (the formation of new species) and colonization (the spread of a species into a habitat or ecological niche).

The team’s simulations showed that it will take an astonishing 50 million years to recover the number of bird species lost since humans arrived on New Zealand, which, as the study authors point out, “far exceed[s] the amount of time that humans have existed.” Additionally, the researchers found that if all of the nation’s threatened avian species were to go extinct, it would take around 10 million years for New Zealand’s birds to bounce back to today’s numbers.

“The fact that it is such a huge amount of evolutionary time lost really puts into perspective the impact that humans have already had on natural isolated systems,” Luis Valente, evolutionary biologist and lead author of the new study, tells Dengler.

Highlighting the scope of the loss is a 2017 study, also led by Valente, that showed it would take eight million years to recover the biodiversity of Caribbean bats—a relatively small amount of time compared to the situation in New Zealand. In fact, the study authors say their findings contradict the notion that species diversity would quickly recuperate if humans would simply leave nature well enough alone.

"[T]he reality is that, at least in New Zealand, nature would need several million years to recover from human actions—and perhaps will never really recover,” Valente says.

Moving forward, the study authors plan to estimate the evolutionary return times for other island nations, with the ultimate goal of helping conservationists prioritize areas where unique evolutionary histories have faced significant pressures. The good news is that New Zealand has in fact been working hard to save its threatened birds—and these efforts have been successful. Kākāpōs, for example, have been moved to predator-free islands off New Zealand’s coast, reported the Washington Post's Marylou Tousignant in March. Scientists keep a close eye on the birds through nest cameras, microchips and transmitters, and eggs are frequently removed to be raised in the safety of incubators. (The moms get a 3D-printed replacement egg to prepare them for hatch time.) In April, the country’s Department of Conservation announced that kākāpōs had their best breeding season on record.

“Regardless of the [conservation] path we choose, our results caution that the policy decisions we make today will have implications far into the future,” the authors of the new study conclude. “Luckily, New Zealand’s pioneering bird conservation efforts may yet prevent millions of years of evolutionary history from further being lost.”