Around 13,000 years ago North America had a more diverse mammal community than modern-day Africa. There were multiple horse species, camels, llamas and a now-extinct animal called Glyptodon, which looked something like a Volkswagen bug–size armadillo. Smilodon, a saber-toothed cat around the size of today’s African lion, skulked across the grasslands in search of ground sloths and mammoths. Seven-foot-long giant otters chowed down on massive trees. And such massive creatures were not just found in North America. On every continent mammals on average were a lot larger in the late Pleistocene, the geologic epoch spanning from around 2.5 million until about 11,700 years ago.

Scientists have long debated what caused all these large-bodied critters to go extinct while many of their smaller counterparts survived. A team of researchers led by University of New Mexico biologist Felisa Smith analyzed evidence from millions of years’ worth of mammalian extinctions and found that on each continent large mammals started to die out around the same time humans first showed up. They announced their findings Thursday in Science.

If the extinction trend continues apace, modern elephants, rhinos, giraffes, hippos, bison, tigers and many more large mammals will soon disappear as well, as the primary threats from humans have expanded from overhunting, poaching or other types of killing to include indirect processes such as habitat loss and fragmentation. The largest terrestrial mammal 200 years from now could well be the domestic cow, Smith’s research suggests.

Some scientists lay the blame squarely on humanity’s shoulders, arguing overhunting doomed the planet's megafauna. After our hominid relative Homo erectus fanned out from Africa into Eurasia starting some two million years ago, Homo sapiens followed around 60,000 to 80,000 years ago and became widespread in Eurasia, joining our close cousins, the Neandertals and Denisovans. It is thought H. sapiens later reached Australia between 50,000 and 60,000 years ago and finally settled the Americas between 13,000 and 15,000 years ago. In the time line of mammalian extinctions, large animals started to disappear only after humans or their hominid cousins showed up. But could that be a coincidence? Others have argued the main culprit behind these die-offs was the changing climate.

In their new study Smith and her team compiled a database of all terrestrial mammals that lived from 65 million years ago until today. They divided that time line into one-million-year chunks, and analyzed extinction trends for each of them. “We found absolutely no effect of climate on mammalian extinction over 65 million years,” she says.

But starting around 125,000 years ago and continuing until today, large-bodied mammals have been more likely to go extinct than smaller ones, the researchers found. The average size of surviving mammals has decreased as a result. And those large-mammal extinctions are tightly coupled with the appearance of humans.

In North America the average mammal weighed around 98 kilograms before the ancestors of humans showed up. Today the average size is closer to eight kilograms. “We’ve lopped a couple orders of magnitude off the distribution of mammals’ [body sizes],” Smith says. For most of mammalian evolutionary history, an animal’s size was not predictive of its extinction risk. That link only appeared once hominids began to live alongside large mammals.

This finding does not mean climate-related changes could not have stressed some wildlife populations, enabling humans to more easily bring about their eventual downfall. Rather it suggests the greater likelihood of large-bodied mammals going extinct is tied to human activities. A suite of animals that evolved in Eurasia, Australia and the Americas without the risk of predation from tool-using, fire-making, group-living hominids were suddenly faced with a new threat. They simply could not adapt fast enough to survive the incursion of these omnivorous bipedal apes.

In addition, Smith’s analysis looked at the size distribution of African mammals prior to the hominid migration into Eurasia. She found African mammals were also smaller on average once hominids began appearing on the landscape there—and they evolved right alongside one another. “They have evidence that hominids in Africa had already been impacting the size distribution of mammals on that continent before Homo sapiens evolved,” says paleoecologist Emily Lindsey, assistant curator and excavation site director of the La Brea Tar Pits Museum in Los Angeles, who was not involved in the study. What that means, she says, is “these groups of hominid species were having impacts on a continental scale before the evolution of modern humans.” And it does not take all that many hominids to have such broad effects. Driving a large species to extinction does not mean killing every last one of its members. “You just have to kill slightly more than are being produced each year,” Lindsey says. If a population's reproduction rate cannot compensate for its losses each year, within a few hundred to a couple thousand years the species will simply die out.

Large-bodied mammals are especially vulnerable because they reproduce slowly. Mammoths and mastodons, for example, likely had a two-year gestation period, akin to modern elephants, and would have typically produced just one offspring at a time. It is therefore a lot easier to decimate a population of 100,000 mammoths than a population of 100,000 rabbits, which reproduce twice a year and birth by litter.

Massive animals have disproportionate impacts on their ecosystems. They disperse seeds, knock down trees and compact the soil with every step they take. The shape of the trails they carve into hillsides impacts water flow and erosion. Large animals also create living spaces for smaller critters. Elephant footprints, for example, produce critical habitats for pond-dwelling invertebrates.

Smith says the lesson to be learned from the new findings is that our hominid heritage prepared us to be extremely proficient killers. “What’s different now,” she says, “is that some of us are comfortable enough, have a high enough standard of living, that we can start thinking about our use of the Earth.” Rather than simply behaving as consumers, many of us are now in a position to become environmental stewards.