I’M OK WITH THIS: Mammals are smaller than they used to be, and it’s our fault. “Human dispersal coincided with a reduction in the average body size of mammals.”

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.

Making your competition (and potential predators) smaller is a good survival strategy, even if it’s inadvertent.