91av

EARLY humans may have systematically massacred some of the world鈥檚 largest
wildlife.

Detective work by two groups of researchers has fingered humans as the chief
suspects in two of palaeohistory鈥檚 most intriguing murder mysteries鈥攖he
extinction of the giant animals and birds of North America and Australia more
than 10,000 years ago in the Pleistocene era.

A spectacular array of North American animals disappeared around 11,000 years
ago, including woolly mammoths, sabre-toothed cats and ground sloths. Tens of
thousands of years earlier, Australia experienced a similar extinction of
megafauna, including Genyornis, the heaviest bird known, and Thylacoleo
carnifex, a marsupial version of the lion. In each case, evidence suggests that
human colonisation pre-dated the die-offs.

One theory dubbed the 鈥渂litzkrieg hypothesis鈥� suggests that humans hunted
some species into oblivion. 鈥淭o some people that seemed intuitively wrong,鈥� says
John Alroy, an evolutionary biologist based at the University of
California, Santa Barbara. 鈥淗ow could a few humans arrive and kill millions of
补苍颈尘补濒蝉?鈥�

So Alroy decided to test the theory in a computer simulation where virtual
humans hunted dynamic populations of 41 North American species. To Alroy鈥檚
surprise, rather than it being hard to achieve some extinctions, it was hard to
avoid. With a wide variety of initial assumptions, the model predicted that even
fairly inept human hunters would devastate certain species. Often these were
larger animals whose reproduction couldn鈥檛 keep pace with the culling. The model
correctly forecast the fate of 32 out of 41 of the species.

In Australia, only a single extinction, Genyornis, had been
precisely dated to follow soon after humans colonised the land about 50,000
years ago. Dating the demise of other species has been difficult since
radiocarbon dating is unreliable for samples older than 40,000 years. So it
wasn鈥檛 clear if Homo sapiens were at the scene of the crime at the
right time to trigger most extinctions.

Now geochronologists Linda Ayliffe at the University of Utah in Salt Lake
City and Richard Roberts of the University of Melbourne in Victoria and their
colleagues have been able to accurately date the extinctions of 45 species from
28 sites. Rather than radiocarbon dating, they used two methods that rely on
crystal growth and exposure to sunlight to date the sediments buried with the
fossils, rather than the fossils themselves. Their work suggests that all these
species disappeared about 46,000 years ago.

鈥淲e find the extinctions happen across species and environments all at the
same time, shortly after humans arrive,鈥� says Ayliffe. The researchers say the
humans may have hunted the megafauna or drastically altered the ecosystem by
burning vegetation, for example.

鈥淓ach of these papers is a tour de force,鈥� says Ross MacPhee of the American
Museum of Natural History in New York City. But he believes neither places guilt
squarely on the shoulders of our species. MacPhee鈥檚 team thinks that emergent
diseases鈥攖he Ebola virus or HIV of their day鈥攃ould be to blame
(91av, 5 May, p 32).