by Mary Caperton Morton Tuesday, June 16, 2015
If not for the megafaunal extinctions that wiped out many large animals at the end of the Pleistocene, the world might be a very different place today — with humans coexisting alongside still-living saber-toothed tigers, woolly mammoths and 3-meter-tall birds. The agents of these mass extinctions have been debated for decades: Were shifting climates, or our spear-wielding ancestors mainly responsible? A new study of the receding shorelines of Australia’s largest lake has found that a substantial drying of the environment, more so than human pressure, is mostly to blame for the loss of megafauna down under.
Because the Pleistocene extinctions coincided with the window in time when humans were spreading into North and South America and across Australia, some researchers have accused our ancestors of massacring, or at least out-competing, large land animals to extinction wherever we went. In North America and Eurasia, lake sediment and pollen evidence suggests that ongoing climate change — namely, widespread drought — may have also played a role in the extinctions, but in Australia the evidence for a dramatic climate shift during the Late Pleistocene has been scant, says Tim Cohen, a paleoclimatologist at the University of Wollongong in Australia and lead author of the new study in Geology.
Cohen and his colleagues focused on 9,300-square-kilometer Lake Eyre, Australia’s largest lake during the rare occasions when it is filled by floodwaters. The lake drains approximately 1 million square kilometers, although arid conditions in modern times mean the lake is usually a dry shadow of its former self. By excavating and dating paleoshorelines and ancient river deposits in the Lake Eyre basin, the team found that the once expansive lake entered a catastrophic drying phase about 48,000 years ago, around the same time that the giant bird Genyornis newtoni and other Australian megafauna went extinct.
Because of the vast size of its source area, climate records from Lake Eyre don’t represent just “a local climatic blip,” Cohen says. The team’s results “tell us that in the time period between 50,000 and 45,000 years ago, this lake system experienced a massive change in hydrology,” he says. It went from a lake with perennial rivers flowing into it to one with source rivers “that run rarely, like we see today.”
Previous studies have also identified this drying trend in Australia using data collected from smaller lakes, but this is one of the first to use the expansive Lake Eyre to confirm the hypothesis, says Brett Murphy, a paleoecologist at the University of Melbourne in Australia who was not involved in the new study. And it appears to tip the balance of responsibility for the disappearance of Australian megafauna toward natural climate variations.
Previously, the extinction of G. newtoni had been attributed to overhunting and burning of the landscape by humans, who are thought to have arrived in Australia about 60,000 years ago, Murphy says. “This study really helps put to rest the idea that humans arrived in Australia, started burning the landscape … and that’s what drove megafauna to extinction,” he says.
But, Murphy cautions against letting humans off the hook altogether. The fact that the continent was drying out “is not direct evidence that climate change was the sole cause of the megafaunal extinctions,” he says. “This is just a snapshot of a few thousand years, whereas these species had been around for hundreds of thousands of years or longer. Whether there were similar dry periods they survived before this one, we don’t know.” If Genyornis newtoni had survived previous major drought cycles, its earlier resilience could lend support to the idea that additional stressors — such as humans — must have contributed to their extinction, he says.
The next step will be to study Lake Eyre and other lake basins over longer time periods, “to see whether this dry period was unprecedented or whether this was part of a long-term wet-and-dry cycle,” Murphy says.
Cohen says he would also like to see other teams apply his group’s work to address questions related to human migration across Australia. “After people arrived on this continent … what conditions were conducive to people migrating across such a vast territory? Understanding what the climate was like during this time period is important for many fields of study.”
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