"What we're losing are our only known living companions in the entire universe," one study author said.
In what researchers call a "biological annihilation," human activities are driving entire groupings of vertebrate species to extinction at a rate 35 times what it would have been without human interference.
The study, published in the Proceedings of the National Academy of Sciences Monday, found that 73 genera—the next thickest branch from species on tree of life—had been lost since A.D. 1500. Without the mass exploitation of the natural world that took off around that date with European colonization, the number lost in the past 500 years would have been only two, and it would have taken 18,000 years to reach 73 extinctions.
"Such mutilation of the tree of life and the resulting loss of ecosystem services provided by biodiversity to humanity is a serious threat to the stability of civilization," study co-authors Paul Ehrlich of Stanford University and Gerardo Ceballos of the National Autonomous University of Mexico wrote in the abstract.
Or, as Ehrlich summarized it in all caps on social media, "New approach to extinction crisis, very bad news."
Previous attempts to grapple with the sixth mass extinction had focused on the number of species lost or at risk. But looking at genera can provide a clear view of the "magnitude and impact" of these losses, the study authors wrote.
Why? Because when one species dies, other species in the same genus can fill its niche in the ecosystem and preserve much of its genetic code, Ceballos toldStanford News. However, when a genus disappears, it leaves a larger gap in both the ecosystem and the genetic record—one that it can take evolution tens to millions of years to fill.
For example, when the passenger pigeon genus went extinct in 1914, the white-footed mouse lost its main food competitor. This combined with a decrease in large predators caused white-footed mouse populations to explode, which has been fatal for humans, because white-footed mice are the primary carriers of Lyme disease.
"We are alarmist because we are alarmed."
"By losing all these genera, we are losing the foundations of the planet to have life in general and human life in particular," Ceballos told The Guardian.
There's also an inherent sadness to the disappearance of so much unique life.
"What we're losing are our only known living companions in the entire universe," Ehrlich told Stanford News.
Ceballos and Eherlich expected genera extinction rates to be lower than species ones, but found in fact that they were about the same. The pair looked specifically at birds, mammals, reptiles, and amphibians. Beyond the 73 extinct genera, the planet also lost 10 families and two orders: the elephant bird and the New Zealand moa. Birds overall lost the most genera, with mammals coming in second.
What's more, if the climate emergency, the illegal wildlife trade, and habitat loss continue and all endangered genera go extinct by 2100, their extinction rate would jump to 354 times what it would have been without these human actions.
"People say that we are alarmist by saying that we expect a collapse," Ceballos told The Guardian. "We are alarmist because we are alarmed."
However, both authors emphasized that it was not too late to act.
"As dramatic as the results are, what is important to mention is that we still have time," Ceballos added. Though he noted that "the window of opportunity is closing rapidly."