– Thirteen thousand years ago Southern California was crawling with enormous mammals — all of which are extinct today.

There were mammoths three times bigger than modern-day elephants, giant ground sloths up to 20 feet in length, and strange, armadillo-like beasts known as glyptodons that were roughly the size of a VW bus.

And don’t forget the llamas, camels, dire wolves, cave lions and saber-toothed cats that all called the area home as well.

Today, the largest local land mammal is the bighorn sheep, which weighs about 300 pounds.

And a similar trend can be found on all the continents of the planet.

Over the past 100,000 years, the mean body mass of mammals in Eurasia dropped by 50 percent and by an order of magnitude in Australia. More recently, there was a tenfold drop in the average size of mammals in the Americas.

So, what led to this dramatic shift in mammal size worldwide?

According to a recent study published in Science, the answer is us.

“When we look at the fossil record, what we find is that every time hominids get to a new continent there is an extinction event, and that extinction is always large-bodied animals,” said Felisa Smith, a paleoecologist at the University of New Mexico, who led the work.

Her research also revealed that if this pattern continues, in a mere 200 years the largest land mammals left on Earth will be the size of a domestic cow.

“And it shouldn’t escape your notice that we take care of cows,” she added. “If they survive, it’s because we want them here.”

Scientists have long known that the big land mammals were the first to disappear in extinction events that occurred in the past 125,000 years, but there was disagreement about why that might be the case.

Some argued that the biggest animals may have been more susceptible to changes in climate or the environment. Others thought the increasingly skilled hunting prowess of Homo sapiens was the culprit.

What Smith and her colleagues found is that it wasn’t just our own species that was responsible for these global changes in animal size; instead, it was the rise and dispersal of hominids in general.

“We are not the only species of homos that ever hunted,” she said. “Homo erectus, Homo heidelbergensis, Neanderthals and Denisovans all used tools and hunted, as far as we know.”

Smith found that the extinction events were swifter and more dramatic as time went on. The extinction event was slow and long in Eurasia and much speedier and deadlier in the Americas. This suggests that as humans developed more advanced weaponry, they were more effective at eradicating large animals quickly, she said.

However, Smith cautions that we shouldn’t take these findings to mean that climate change won’t influence future extinctions of big mammals.

“In the past, large mammals were able to adapt to climate change by moving to different regions,” she said. “But we have urbanized most of the land, so they can’t move to cope with the changes.”