PASSAGE 3: The Lost Animals of Australia

The history of Australia's animals over the past 50,000 years has been largely one of extinction. The time has been too short for new species of large animals to evolve and, over this period, nearly one-third of Australia's mammal species have become extinct, along with nearly all the large reptiles and many of the flightless birds. The cause of these extinctions is still hotly debated. One school of thought suggests that humans caused the extinctions, and another that they were due to changes in climate. The greatest problem in evaluating these theories is that we do not know exactly when these animals became extinct. Fossils from a number of well-dated cave and lakeside sites suggest that all the now-extinct species had already vanished 35,000 years ago, but a few other sites suggest that giant marsupials might have survived until 25,000 years ago, or even as late as 6,000 years ago.

One certainty is that Australia's climate has changed greatly over the past 40,000 years. Between 25,000 and 15,000 years ago, the sea level dropped by more than 100 metres and the continent experienced an extremely arid phase. Clearly, the timing of the extinctions is of critical importance. If they can be shown to coincide with the arrival of humans, about 50,000 years ago, then circumstantial evidence would point to humans as the cause. If, however, the giant marsupials survived until 25,000 years ago, the increased aridity would seem to be the more likely cause.

Before these extinctions, Australia was a very different place. Naturally occurring fires were probably less frequent than they are now, because large marsupial herbivores (of which there were about forty species) reduced the standing crop of vegetation. Rainforest plants were widespread in the parts of Australia which are now very dry. The largest marsupial species was the diprotodon, which, like other Australian marsupial giants, was about one-third the weight of its ecological equivalent elsewhere. It probably weighed between 1,000 and 2,000 kilograms, while the elephant of Afro-Asia weighed about 5,000 kilograms. The relatively small size of the Australian marsupial giants has been attributed to Australia's extraordinarily poor soils and erratic climate, both factors which typically limit plant growth.