At the end of the Cretaceous period 66 million years ago, mammals took over as the largest creatures on land. However, around 125,000 years ago during the late Pleistocene, these megafauna started disappearing, writes Ed Yong for The Atlantic. Today they are all gone, and according to a new study by Felisa Smith from the University of New Mexico, the blame lies with humans and our hominin relatives.
Smith and her colleagues examined how mammals have changed in size over time showing that when humans are around, the mammals that disappear are usually 100 to 1000 times bigger than those that survive. “Size-selective extinction is a hallmark of human activity,” Smith says. She believes that size-specific collapses started well before the ascent of Homo sapiens, probably dating back to the origins of Homo erectus, around 1.8 million years ago.
When hominins like Neanderthals, Denisovans, and modern humans spread through Europe and Asia, the average mass of mammals there halved, writes Yong. Later, when Homo sapiens entered Australia, the mammals there became ten times smaller on average. “When it got warmer or colder, it didn’t select for bigger or smaller mammals,” says Smith. “It’s only when humans got involved that being large enhanced your extinction risk.”
Modern humans have now become adept at killing medium-sized and smaller mammals. As Smith puts it, if all the species that are now threatened eventually go extinct, the largest mammal left on land will be the domestic cow.