There has been a long and heated debate in Australia about the timing and cause of the extinction of the Australian megafauna, large animals which includes a variety of large marsupials and the giant flightless bird, and which happened ca. 50,000 to 45,000 years ago.
The debate has concerned whether early humans are the direct cause (through hunting) or were indirectly responsible (by setting fires causing burning of the landscape and changing vegetation). Or was there a shift of climate that led to the disappearance of the large animals, or a combination of any of these factors?
Different environment without big animals
Researchers from NIOZ and their Australian colleagues, reconstructed past climate conditions as well as vegetation for South East Australia, home for a large part of the Australian fauna. They used specific molecules present in sediments from the coast of South East Australia which were deposited there by the large Murray-Darling rivers. Using this, it was possible to determine the relative importance of the so-called C3 vegetation (shrubs, trees) and C4 vegetation (mainly grasses).
This revealed an extensive period (68,000-31,000 years) of generally high C4 plant abundance that was punctuated by an abrupt increase in C3 vegetation at around 43,000 years. This sharp increase in C3 vegetation lasted for approximately 5,000 years and directly followed the period in which the large animals in Australia became extinct.
More shrubs and trees
The cause for the vegetation change was not climate change, as no evidence was found for any abrupt, strong changes in temperature or precipitation. Instead, the extinction of the megafauna allowed the expansion of shrubs and trees, which were normally consumed by these large animals.
The researchers also found specific molecules which indicated that large parts of the vegetation were burned at this time. These fires were likely caused by humans and helped by the fact that the trees is more prone to fire than grass.