“They have evidence that hominids in Africa had already been impacting the size distribution of mammals on that continent before Homo sapiens evolved,” says paleoecologist Emily Lindsey, assistant curator and excavation site director of the La Brea Tar Pits Museum in Los Angeles, who was not involved in the study. What that means, she says, is “these groups of hominid species were having impacts on a continental scale before the evolution of modern humans.”
And it does not take all that many hominids to have such broad effects. Driving a large species to extinction does not mean killing every last one of its members. “You just have to kill slightly more than are being produced each year,” Lindsey says. If a population’s reproduction rate cannot compensate for its losses each year, within a few hundred to a couple thousand years the species will simply die out.
Around 13,000 years ago North America, continues Jason G. Goldman in Scientific American, had a more diverse mammal community than modern-day Africa. There were multiple horse species, camels, llamas and a now-extinct animal called Glyptodon, which looked something like a Volkswagen bug–size armadillo. Smilodon, a saber-toothed cat around the size of today’s African lion, skulked across the grasslands in search of ground sloths and mammoths. Seven-foot-long giant otters chowed down on massive trees. And such massive creatures were not just found in North America. On every continent mammals on average were a lot larger in the late Pleistocene, the geologic epoch spanning from around 2.5 million until about 11,700 years ago.
Scientists have long debated what caused all these large-bodied critters to go extinct while many of their smaller counterparts survived. A team of researchers led by University of New Mexico biologist Felisa Smith analyzed evidence from millions of years’ worth of mammalian extinctions and found that on each continent large mammals started to die out around the same time humans first showed up. They announced their findings in Science.
If the extinction trend continues apace, modern elephants, rhinos, giraffes, hippos, bison, tigers and many more large mammals will soon disappear as well, as the primary threats from humans have expanded from overhunting, poaching or other types of killing to include indirect processes such as habitat loss and fragmentation. The largest terrestrial mammal 200 years from now could well be the domestic cow, Smith’s research suggests.
Image credit: World Wildlife Fund
Most Popular Space & Science Headlines