When the first modern humans ventured beyond Africa during the late Pleistocene, roughly 120,000 years ago, they stepped into a world filled with giants: the six-ton giant ground sloth in South America, the two- to three-ton wooly rhino in Europe and northern Asia, the 350- to 620-pound sabertooth cat in North America, and the six-ton wooly mammoth in Eurasia and North America. It’s hard to imagine a world filled with animals that large. The giants of the Pleistocene quickly vanished, and the animals that survived were generally two or three times smaller than those that went extinct. A new study indicates that the late Pleistocene decrease in mammal size coincided with the geographical spread of humans around the world—and the authors say that’s not just happenstance.

Human involvement in the disappearance of the Pleistocene megafauna is still the subject of intense debate, but this is hardly the first time we’ve been implicated. To provide a different perspective on these extinctions, a team of biologists led by Felisa Smith of the University of New Mexico, Albuquerque, decided to look for changes in the pattern of extinctions since the beginning of the Cenozoic period 65 million years ago—the end of the dinosaurs and the beginning of the rise of mammals. Species go extinct all the time at a steady background rate of about one to five species per year. If that rate or the kinds of animals dying off changed after humans started colonizing the world beyond Africa, that could imply we had something to do with it.

The biologists examined two large datasets. One listed the global distribution and body size of animal species in the late Pleistocene and Holocene, starting 125,000 years ago. The other listed similar information for species spanning the whole Cenozoic. Starting at around 125,000 years ago, the datasets traced a decrease in both the mean and the maximum body size of mammals on every continent, coinciding with the spread of humans into each region. Wherever humans went, mammals got smaller, and big ones tended to die off.

Spreading humans, diminishing megafauna

In Africa at the dawn of human dispersal, mammals were already smaller, on average, than those living elsewhere in the world, with a mean body mass about half that of Eurasian mammals. Smith and her colleagues say that was probably the legacy of thousands of years of interactions between hominins and other African mammals, which drove larger species extinct and exerted pressure that reduced the size of the surviving species. Not only modern humans, but also our ancestors, had a major impact on mammal populations.

“This finding suggests that the homogenization of natural ecosystems was a consequence of hominin behavior in general and not specific to H. Sapiens,” Smith and her colleagues wrote.

But as humans spread into Eurasia around 120,000 years ago, mean mammal size there decreased by half. And it dropped by an order of magnitude shortly after humans arrived in Australia between 40,000 and 60,000 years ago. But nothing changed in the Americas until the terminal Pleistocene, between 10,000 and 20,000 years ago—and humans didn’t get there until sometime around 15,000 years ago.

Once the first people reached the Americas, their activities seem to have taken a steeper toll on the local megafauna than on other continents. About 11.5 percent of the mammal species in North America at the time, and 9.7 percent of those in South America, didn’t survive long after the arrival of humans, and it was disproportionately the large species that were weeded out. The mean body mass of North American mammals dropped by a factor of ten, from 216 pounds to 16.8 pounds, during the Terminal Pleistocene.

That’s probably because by the time humans arrived in North America, they had developed much more efficient long-range weapons for taking down prey.

Circumstantial evidence

Human hunters have usually tended to seek out the largest prey—which means large mammal species get hunted to extinction, and smaller species find their larger members removed from the gene pool. This pattern stands in stark contrast to the rest of the Cenozoic—and, in fact, it contrasts with all five mass extinction events recorded in the fossil record since 540 million years ago, including the one that killed the dinosaurs. Before the late Pleistocene, a species’ size didn’t seem to give it better or worse odds of extinction.

But starting 125,000 years ago, larger mammal species started dying off at a disproportionately high rate. Smith and her colleagues say there’s no other trend like it in the fossil record.

The researchers ruled out climate change as a potential cause. They compared changes in body mass distribution throughout the Cenozoic and found that although climate shifts did trigger extinction events, neither small nor large mammals were more vulnerable to dying out. That leaves humans as the most likely culprit, the biologists claim.

Mammals keep getting smaller

Mammals have kept getting smaller because we’re still at risk of losing larger species and selecting for smaller individuals in others. Around 10,000 years ago, smaller-bodied mammals started to find themselves in the crosshairs, mostly thanks to urbanization and the booming populations of invasive predators like domestic cats. But it’s still the largest mammals that are most at risk, primarily because they most often find themselves in conflict with humans and their livestock, the dominant players in land ecosystems today.

Today, the mean body mass of North American mammals is 16.8 pounds, about half the size of a coyote. The largest mammal in North America today is the 2,000-pound wood bison. But if all currently threatened species went extinct in the next 200 years, the average North American mammal would weigh about 10.8 pounds, roughly as much as a fox or an armadillo. That would be the same on other continents, according to the Smith and her colleagues, and the largest land animal left would be the domestic cow, weighing in at roughly two tons.

“The last time the body size distribution of terrestrial vertebrates was similarly disrupted was about 66 million years ago, during the end-Cretaceous mass extinction,” they concluded.

Animals at the small end of the spectrum, especially rodents, would be the winners in that scenario, and that’s going to have implications for ecosystems worldwide. “As mammals play a critical role in shaping ecosystems, the downsizing trend will have a cascading impact on other organisms,” Smith and her colleagues wrote.