Earth has supported life for 3.5 billion years, but its hospitality is hardly consistent. Natural disasters have triggered at least five mass extinctions in the past 500 million years, each of which wiped out between 50 and 90 percent of all species on the planet. The most recent occurred about 65 million years ago, when an asteroid ended the reign of dinosaurs and opened new doors for mammals.

Now it's happening again. A 2015 study reported the long-suspected sixth mass extinction of Earth's wildlife is "already under way." A new study published in the journal Science also suggests this sixth mass extinction is killing off large ocean dwellers (like sharks, whales, giant clams, sea turtles and tuna) in disproportionately greater numbers than smaller animals. That's a reversal from past extinctions, when there was a slight connection between smaller size and going extinct.

And while previous extinctions were often linked to asteroids or volcanoes, this one is an inside job. It's caused mainly by one species — a mammal, ironically. The current crisis is the handiwork of humans, and we have a "unique propensity to cull the largest members of a population," the authors of the 2016 study write.

Many scientists have been warning us for years, citing a pace of extinctions far beyond the historical "background" rate. Yet critics have argued that's based on inadequate data, preserving doubt about the scope of modern wildlife declines. To see if such doubt is justified, the 2015 study compared a conservatively low estimate of current extinctions with an estimated background rate twice as high as those used in previous studies. Despite the extra caution, it still found species are disappearing up to 114 times more quickly than they normally do in between mass extinctions.

Here are six important things to know about life in the sixth mass extinction:

1. This isn't normal.

Once abundant in the U.S., passenger pigeons were driven extinct by a surge of hunting and habitat loss. Once abundant in eastern North America, passenger pigeons were driven extinct by a surge of hunting and habitat loss. (Photo: Seabamirum/Flickr)

Extinction is a natural part of evolution, having already claimed an estimated 99 percent of all species in Earth's history. But things can get ugly when too many species die out too quickly, creating a domino effect capable of bringing down ecosystems. In the new study, researchers used a background rate of two mammal extinctions per 10,000 species per 100 years (2 E/MSY), which is double the background rate used in many previous studies. When they compared that to a conservative estimate of modern-day extinctions, they found no way to avoid calling this a mass extinction.

"Even under our assumptions, which would tend to minimize evidence of an incipient mass extinction, the average rate of vertebrate species loss over the last century is up to 114 times higher than the background rate," the study's authors write. "Under the 2 E/MSY background rate, the number of species that have gone extinct in the last century would have taken, depending on the vertebrate taxon, between 800 and 10,000 years to disappear. These estimates reveal an exceptionally rapid loss of biodiversity over the last few centuries, indicating that a sixth mass extinction is already under way."

2. Space is at a premium.

Aerial view of deforestation Deforestation is one of the primary threats for endangered species. (Photo: Fedorov Oleksiy/Shutterstock)

The No. 1 cause of modern wildlife declines is habitat loss and fragmentation, representing the primary threat for 85 percent of all species on the IUCN Red List. That includes deforestation for farming, logging and settlement, but also the less obvious threat of fragmentation by roads and other infrastructure.

And even where habitats aren't being razed or divided, they're increasingly altered by other human activities. Invasive species now threaten a variety of native plants and animals around the world, either by killing them directly or by outcompeting them for food and nest sites. Pollution is pervasive in many places, from chemicals like mercury that accumulate in fish to the plastic debris that slowly kills sea turtles, sea birds and cetaceans. Entire ecosystems are now migrating due to climate change, leaving behind less mobile or adaptable species. And in some parts of the world, poachers are obliterating rare species to meet demand for wildlife parts like rhino horn and elephant ivory.

3. Vertebrates are vanishing.

Lemur tree frog The lemur tree frog is critically endangered and listed on the IUCN Red List. (Photo: G.J. Verspui/Shutterstock)

The number of vertebrate species that have definitely gone extinct since 1500 is at least 338, according to the new study. (That doesn't include the less stringent categories of "extinct in the wild" (EW) and "possibly extinct" (PE), which push the total up to 617.) More than half of those extinctions have occurred since 1900 — 198 in the "extinct" (EX) category, plus another 279 in EW and PE.

Even under the most conservative estimates, the extinction rates for mammals, birds, amphibians and fish have all been at least 20 times their expected rates since 1900, the researchers note (the rate for reptiles ranges from 8 to 24 times above expected). Earth's entire vertebrate population has reportedly fallen 52 percent in the last 45 years alone, and the threat of extinction still looms for many — including an estimated 41 percent of all amphibian species and 26 percent of mammals.

"There are examples of species all over the world that are essentially the walking dead," Ehrlich says.

4. It's probably still worse than we think.

Insecticides can weaken native pollinators like bees, raising concerns about food supplies. Insecticides can weaken native pollinators like bees, raising concerns about food supplies. (Photo: Bjorn Watland/Flickr)

The new study is intentionally conservative, so the actual rate of extinctions is almost certainly more extreme than it suggests. "We emphasize that our calculations very likely underestimate the severity of the extinction crisis," the researchers write, "because our aim was to place a realistic lower bound on humanity's impact on biodiversity."

The study also focuses on vertebrates, which are typically easier to count than smaller or subtler wildlife like mollusks, insects and plants. As another recent study pointed out, this leaves much of the crisis unexamined. "Mammals and birds provide the most robust data, because the status of almost all has been assessed," write the authors of that study, published June 8 in the Proceedings of the National Academy of Sciences. "Invertebrates constitute over 99% of species diversity, but the status of only a tiny fraction has been assessed, thereby dramatically underestimating overall levels of extinction."

By incorporating data on terrestrial invertebrates, they add, "this study estimates that we may already have lost 7% of [contemporary] species on Earth and that the biodiversity crisis is real."

5. No species is safe.

Fishing boats About 1 billion people rely on fish as their main source of animal protein, according to the World Health Organization. (Photo: Albert Pego/Shutterstock)

Humans are hardly an endangered species, with a global population of about 7.2 billion and growing. But fortunes can change quickly, as we've demonstrated in recent decades with lots of other wildlife. And despite our best efforts to buffer ourselves against the whims of nature, civilization remains reliant on healthy ecosystems for food, water and other resources. Adjusting to mass extinctions would be a challenge under any circumstances, but it's especially daunting in the context of climate change.

"If it is allowed to continue, life would take many millions of years to recover, and our species itself would likely disappear early on," says Gerardo Ceballos of the Universidad Autónoma de México, lead author of the new study. "We are sawing off the limb that we are sitting on," Ehrlich adds.

6. Unlike an asteroid, we can be reasoned with.

An artist's rendering of the asteroid widely credited with wiping out the dinosaurs. An artist's rendering of the asteroid widely credited with wiping out the dinosaurs. (Photo: NASA)

Previous mass extinctions may have been inevitable, but it's not too late to stop this one. While the authors of the new study acknowledge the difficulty of curbing lucrative destruction like deforestation, not to mention climate change, they note it is still possible. It's even gaining momentum, thanks to growing public awareness as well as high-profile attention from governments, corporations and even the pope.

"Avoiding a true sixth mass extinction will require rapid, greatly intensified efforts to conserve already threatened species," the study's authors write, "and to alleviate pressures on their populations — notably habitat loss, over-exploitation for economic gain and climate change."

That won't be easy, but at least it's more of a chance than the dinosaurs got.

Editor's note: This story has been updated since it was originally published in June 2015.

Russell McLendon ( @russmclendon ) writes about humans and other wildlife.