Barnosky and his colleagues found sparse evidence outside Australia that humans were the sole cause of extinction. Data are sketchy for the Australian continent, Barnosky cautioned, but little climate change was going on at the time of extinction between 40,000 and 50,000 years ago. However, humans were certainly on the scene, and some scientists think that fires set by humans had as much to do with extinction as direct hunting. Over a few thousand years, Barnosky said, an extended sitzkrieg may have led to the extinction of large mammals such as kangaroos, wombats, the marsupial lion (Thylacoleo carnifex) and the largest ever marsupial, the 2 1/2-ton Diprotodon.
Elsewhere, human activities combined to a greater or lesser degree with climate change to lead to extinctions. In Europe and parts of Asia, mammals such as the giant Irish deer or Irish elk died out broadly toward the end of the late Pliestocene, in some areas before humans were present. Earlier, though, warm-adapted megafauna such as straight-tusked elephants (Palaeoloxodon) and hippos, which were abundant during preceding interglacials, disappeared with the cooling of the last ice age, starting around 45,000 years ago and persisting up to the height of the glacial period 20,000 years ago.
"This is a very clear case of climate-caused extinction without the significant input of humans," Barnosky said.
Similarly, in Alaska and the Yukon, the disappearances of short-faced bears, such as the grizzly-like Arctodus simus, the largest land carnivore ever to inhabit North America; mammoths; and two horse species occurred before apparent signs of human contact.
A second pulse of climate-caused extinctions began in Europe and Asia about 12,000 years ago as cold-adapted animals
Contact: Robert Sanders
University of California - Berkeley