But by the time hominins left Africa, the average mammals there were about 50 percent smaller than the average ones in either Eurasia or the Americas. By around 15,000 years ago, the average mass of North America’s mammals had fallen from 216 pounds to just 17. “People make this assumption that large animals are more at risk,” says Smith. “But large animals also have larger geographic ranges, which buffers them against extinction. “When it got warmer or colder, it didn’t select for bigger or smaller mammals,” says Smith.