The two studies support the 'overkill' or 'blitzkrieg' theory of extinction, developed in the 1970s by Paul Martin at the University of Arizona. Martin blamed the extinctions on the arrival of humans, and likened their passage down the continent to a "bloody wave," resembling the passage of German tanks through Europe during World War Two. At the time, other scientists objected to Martin's thesis, sugesting that climate change or a sudden virus was a more likely culprit in the extinction of the giant animals.
It's those darn spear carriers
In the first of the two new studies, John Alroy of the University of California, Santa Barbara, used a computer simulation to determine whether overkilling by human was a conceivable cause of the wave of extinction, which occurred about 1200 years after humans are believed to have arrived in North America 13,400 years ago. The simulation took into account factors ranging from population density to hunting ability, and was able to correctly predict the extinction of 32 out of the 41 species studied.
The second study, which examined the disappearance of large animals in Australia, sought to specify the time frame of the extinction rather than the cause. According to the study, every Australian land mammal, reptile and bird heavier than 100 kg and most of those weighing 45 to 100 kg died out 46,400 years ago. The study, which used optical dating to pinpoint the age of fossils from sites across Australia, narrowed the previously fuzzy timeframe of the extinction. But the new dates makes human influence a more likely factor because scientists believe humans arrived on the continent between 52 and 60 thousand years ago.
"It is clear that the downward spiral of these animals was after the arrival of humans," said Linda Ayliffe of the University of Utah in Salt Lake City, one of the researchers on the project.
The studies have their detractors. Some scientists cite the lack of archeological evidence as one of many problems with Alroy's study, and Ayliffe says it is unlikely that hunting was the sole cause of the Australian extinction. She suggests that the human use of fire, combined with hunting, may have been too much for the species to bear.