Bear-size beavers, mammoths, horses, camels and saber-toothed cats used to roam North America, but by 11,000 years ago most such large mammals had died off. To this day, experts debate what caused this late Pleistocene extinction: climate change, overhunting by humans, disease—or something else? Eric Scott, curator of paleontology at the San Bernardino County Museum in Redlands, Calif., suggests it was something else: namely, the immigration of bison from Eurasia.

Armed with data from his own ongoing excavations as well as from those dating back as far as the 1800s, Scott says that bison appeared in North America as early as 220,000 years ago and spread across the continent throughout the remainder of the Pleistocene, a time when climate change had made food and water scarce. He first formally suggested the idea last spring  in the journal Quaternary International, speculating that bison may have won enough battles for food and water during that time to share the blame with climate change as the major cause of the large mammal extinctions.

Scott’s initial “aha!” moment came while excavating near the town of Murrieta, Calif., in the early 1990s. Years before, digging nearby in strata 760,000 years to 2.5 million years old, he had found no evidence of bison, only horses, and wondered: “What did horses think when bison showed up and ate their food?” So when his team excavated in late Pleistocene strata at Diamond Valley Lake near Murrieta, just miles away from where he had found no bison, and turned up fossils of bison and other mammals, he thought he might have an answer: “This brought home to me the idea that as bison immigrated into areas and their numbers grew, their effect on other large mammal populations might have reached tipping points.” Scott is now collecting data from other parts of the U.S. to make sure the pattern he has observed in the Southwest holds up elsewhere.

Scott speculates that bison would have had multiple advantages over other large herbivores. For example, their multiple stomachs probably allowed them to extract maximum nutrition from their food. And they need not have won every battle they engaged in. Instead they might have, for example, malnourished nursing mothers just enough to cause population collapse. With no large herbivores to eat, dire wolves, American lions and other carnivores would have starved as well.