Extinction is an inevitable consequence of evolution. Environments change, new species arrive and crowd out the old, any number of factors make a formerly successful species unsuccessful. No less an authority than Charles Darwin wrote in The Origin of Species, “Natural Selection almost inevitably causes much Extinction of the less improved forms of life and induces what I have called Divergence of Character.”
Even without the 19th-century capitalization for emphasis, extinction is a big deal. Earth has experienced at least five major mass extinction events, of which the end of the dinosaurs wasn’t even the largest. (The end of the dinos that hadn’t evolved into birds, of course.) Now the planet is experiencing the sixth mass extinction, and growing evidence points to the culprit. It’s not asteroids or volcanoes or methane this time.
To be fair, scientists have suspected humans are the reason for the Sixth Extinction for some time. It’s even the subject of several books. However, it’s difficult to assign numbers and rates of extinction over human history: It’s easiest to see extinctions long after they happened, rather than in process. The key is quantifying how many extinctions have happened on our watch versus the normal rate of species death.
After all, species go extinct without our help, much less without catastrophes like the asteroid at the end of the Cretaceous Period. An analogy is with climate change: Even though climate changes happened before we came along, the current rapid change is due to human activity. Similarly, extinctions happened before we arrived on the scene, but with the accelerated loss of species, we have to consider the possibility that we’re responsible for that one too.
To try to settle the issue, Gerardo Ceballos and colleagues looked specifically at mammals, which have better historical records (from roughly 1500 through today) than plants, fish, reptiles, amphibians, and so forth. Previous studies estimate that for every 10,000 mammal species, two go extinct in a century in the absence of mass extinction events.
In other words, a rough average of 0.1 percent of mammal species die out in a 500-year period, based on the fossil record. Assuming that rate is reliable for other organisms, we have a baseline against which we can measure actual species death. Since the year 1500, 77 known mammal species have vanished completely—none surviving in the wild or in captivity—with 35 of those disappearing since 1900.
These estimates are conservative, because scientists are cautious about declaring a species extinct. (There are even a few times when everyone thought a species was gone forever, only later to find a group holding on somewhere.) Even with that cautious assessment, mammal extinction rates are 14 times higher than expected. Since 1900, the rate is 28 times higher than the normal extinction average. The rate of bird extinctions is roughly the same.
For amphibians and fish, we only have reasonable data for the 20th century, but the number of species that have vanished is still very high. Even if we pretend the 66 known fish species that died out in the 20th century are all that went extinct since 1500, the rate is still five times over the expectations.
Matters get worse when species that are probably extinct or surviving only in captivity are included. Since 1500, 617 known vertebrate species have died out; 477 of those vanished since 1900. That rate is 53 times the normal extinction expectation. Taking all this information together, it means Earth is definitely experiencing the sixth mass extinction right now. (The previous five are: the Ordovician-Silurian event about 450 million years ago; the Devonian event 375 million years ago; the Permian extinction 251 million years ago, which was the most devastating in all history; the Triassic extinction 200 million years ago; and the Cretaceous extinction 65 million years ago.)
But are humans responsible? Some cases are easier to pin on humanity than others: We know human-related activity killed off the dodo, the Stellar’s sea cow, and other species. We can measure humanity’s impact on threatened species like tigers, rhinos, lemurs, and so forth. Studies have shown how activities like deforestation and wide-scale fishing have impacted life. Climate change is another factor in extinctions, of course, but it’s not the only one, and possibly not even the most significant one.
Thanks to all that research and the conservative estimates on extinction, it really does look like we’re the only cause big enough to wipe out so many species. Ceballos and his coauthors point out that the situation is likely worse, since they deliberately chose lowball numbers on extinctions. Conservation efforts are essential to stave off the worst, but there’s little reason to hope we can turn things around completely, given how many species are already gone.
The Police song “Walking in Your Footsteps” warned of humans extincting ourselves through nuclear annihilation, comparing us to the dinosaurs. Today it seems more that we’re not the dinosaurs: We’re the asteroid.