Humans have killed off most of Earth's big mammals. In 200 years, cows could be biggest ones left.

Throughout history, humans and their ancestors have perfected the art of killing big animals. Now, a new study finds that as humans spread around the globe, extinction of large mammals soon followed.

Massive mammals such as wooly mammoths, elephant-sized ground sloths and various saber-toothed cats roamed the Earth between 2.6 million and 12,000 years ago. Now they — and most of the rest of the big ones — are extinct. 

"Species that went extinct tended to be two to three times bigger than mammals that survived, a trend that was evident globally," the study said. Thus, most mammals alive today are much smaller than the typical mammal was millennia ago.

This "downsizing" trend may continue as vulnerable and endangered animals go extinct, study authors said. In 200 years, the largest terrestrial mammal left may be the domestic cow. The average weight of mammals would also plummet to less than six pounds — roughly the size of a Yorkshire terrier. 

This would be down from an average of 37 pounds now and 216 pounds 125,000 years ago.

"Mammalian body size around the globe will revert to what the world looked like 40 million years ago," said study lead author Felisa Smith, a professor of biology at the University of New Mexico. 

The study was published Thursday in the peer-reviewed journal Science. 

The study also notes that size-selective extinction is a hallmark of human activities and not the norm in mammal evolution.

“It wasn’t until human impacts started becoming a factor that large body sizes made mammals more vulnerable to extinction,” said study co-author Kate Lyons of the University of Nebraska-Lincoln. The extinctions "occurred not very long after the birth of us as a species. It just seems to be something that we do."

One of the most surprising finds in the study was that the average body size of mammals on Africa was already 50% smaller than on other continents 125,000 years ago, Smith said. "We suspect this means that archaic humans and other hominins had already influenced mammal diversity and body size," she said.

Perhaps most striking is the reduction of mammals in the New World during the late Pleistocene, which coincided with humans' use of long-range weapons. 

“From a life-history standpoint, it makes some sense," Lyons said. "If you kill a rabbit, you’re going to feed your family for a night. If you can kill a large mammal, you’re going to feed your village.”