Shifting Baselines

Who killed the megafauna is one of science's greatest debates. Starting roughly 50,000 years ago, where and when humans show up around the globe, large animals disappear. First in Australia, later in North and South America, and finally on islands in the Pacific and New Zealand. Whether the main driving factor was man or climate has been a long-standing debate ever since Paul Martin put forth the overkill hypothesis in the late 1950s. Actually, there are two debates. First, were the megafaunal extinctions caused by humans, climate, or some combination of factors? Second, if humans did play a role, did they cause the extinctions directly through hunting or indirectly via some habitat alteration or other change?

A new paper in the US Proceedings of the National Academy of Sciences contributes to answering this great mystery. Analyzing giant marsupial fossils in Tasmania, Australia, Chris Turney and colleagues manage to disentangle some of the complexity around Australia's megafaunal extinctions. On the Australian mainland, 90% of Australia's giant marsupials went extinct 46,000 years ago, soon after the human colonization of the continent. But on the nearby island of Tasmania, it was thought that the marsupial megafauna went extinct prior to human arrival - thus acquitting human hunting as the direct cause. WIth new data and analyses, Turney and colleagues show that some of the Tasmanian megafauna survived after the mainland extinctions and overlapped with humans on the island. The researchers also analyze climate and subsequent vegetational changes on the island over the past 100,000 years. While drastic changes occurred, no megafauna went extinct - strong evidence against the the cause of extinctions being climatic.

The seven bygone Tasmanian giants included 1,000 pound marsupial "rhinoceros" and "ground sloths", and a 300 pound antelope-like kangaroo that stood 6.5 feet high. That giant kangaroo survived in Tasmania through multiple changes in climate and vegetation over a period 84,000 years. And then disappeared a few millennia after humans arrived to the island. Turney and colleagues also did not find any evidence of changes in vegetation or fire after humans arrived - evidence against an indirect role in the extinctions, such as fire. This new research adds to the mounting evidence that humans likely played a direct role - via hunting - in the demise of the world's megafauna throughout the late Pleistocene.

Jared Diamond summed up the research eloquently in a commentary in Nature Magazine, "Humans who colonized Australia did not reach Tasmania until thousands of years later - granting the island's giant kangaroos a brief respite before joining their Australian brethren in oblivion."