Diprotodon, human, Pleistocene & modern wombat skeletons

Imagine a bird three times the size of an ostrich, or a burrowing animal as big as an elephant. How about a kangaroo three metres tall? Such creatures were all Australian megafauna, Alive during the Pleistocene.

Fifteen million years ago, 55 species of megafauna were widespread in Australia, the largest of which was the marsupial diprotodon, weighing around 2700 kilograms (5952 lb).Giant snakes, crocodiles, and birds were also common. Wombats and kangaroos reached more than 200 kg (440 lb), and even koalas weighed 16 kg (35 lb). Then, rather suddenly, around 46 thousand years ago (46 kyr), all these animals became extinct. Some scientists claim this was due to environmental pressures, like climate change or fire; others favour predation.

At the end of the Pleistocene, humans reached Australia via Indonesia, and, according to the archaeological record, by 45 kyr their settlement was widespread. One hundred and sixty archaeological sites in Australia and New Guinea have been much surveyed. There is some disagreement about the dates of these sites; meantime, a forceful movement aims to push human settlement back before 45 kyr.

Dating the rare bones of megafauna was highly controversial until 20 years ago, when a technique called optically stimulated luminescence (OSL) was developed. With OSL, the age of minerals up to 200 kyr can be established with +/ - 10% accuracy.

The largest OSL dating of megafauna was carried out in 2001 by Roberts, who put the extinction date for megafauna at around 46 kyr, very early on in the time of human habitation.

Megafaunal bones are rare enough, but, at archaeological sites with human habitation, they are extremely rare with fewer than 10% of the 160 sites containing them. Bones that show cutting, burning, or deliberate breaking by humans are virtually non-existent, and thus far, not one megafaunal skeleton shows conclusively an animal was killed by humans. There are no ‘kill sites' either whereas in New Zealand, where the giant moa bird became extinct in the 18^th century due to hunting, there are sites with hundreds of slaughtered creatures. As a result, many scientists still believe that humans were not responsible for megafaunal extinction – especially as the weapons of Australian Aborigines at 45 kyr were only wooden clubs and spears.

There is, perhaps, a cultural record of megafauna in Aboriginal myths. The Adnyamathanha people of South Australia tell of the Yamuti, something like a diprotodon. An ancient rock painting in Arnhem Land shows an extinct giant echidna. But this record is small and open to interpretation.

If the Aborigines were not technologically advanced enough to kill them, what else might have destroyed megafauna? One theory has been climate change - perhaps there was a relatively hot, dry period between 60-40 kyr. Research suggests otherwise. Indeed, at 40 kyr, the climate was moderate and Lake Eyre, in central Australia, grew. If there was desertification, scientists would expect megafauna to have moved towards the coast, looking for food and water. But instead, the fossil record details an equal distribution of the dead inland and on the coast.

In addition, changes in specific vegetation occurred after the extinction of the megafauna. Trees that relied on large animals to eat their fruit and disperse their seed covered far smaller areas of Australia post 40 kyr. These plants were not threatened by climate change; rather, they died off because their megafaunal partners had already gone.

Typically, climate change affects almost all species in an area. Yet, around 46 kyr, only the megafauna died. Previously, there had been many species of kangaroo, some as heavy as 200 kg (440 lb), but, after, the heaviest weighed only 32 kg (70 lb). This phenomenon is known as dwarfing, and it occurred with many animals in the Pleistocene.

Dwarfing has been studied extensively. In 2001, Law published research related to fish farming. Despite excellent food and no predators, farmed fish become smaller as generations continue. This adaptation may be a response to their being commercially useless at a smaller size, meaning they hope to survive harvest.

Of the dwarf marsupials, the most notable development over the giants was their longer reproductive lives, which produced more young. They were better runners as well, or, those that were slow-moving retreated to the mountainous forest, beyond the reach of humans.

If climate change isn't a credible factor in extinction, what about fire? Fire is caused naturally by lightning strikes as well as by humans with torches. Surprisingly, the charcoal record for many thousands of years does not show a marked increase in fire after human habitation of Australia - there is only a slow increase over time. Besides, it could be argued that forest fires aid megafauna since grass, their favoured food, invariably replaces burnt vegetation.

Johnson, an archaeologist, has proposed that the Aborigines could have wiped out all 55 mega- faunal species in just a few thousand years. He believes that the 45 kyr human settlement date will be pushed back to make this extinction fit, and he also maintains that 700 years are enough to make one species extinct without large-scale hunting or sophisticated weapons. Johnson used computer modelling on a population of only 1000 animals to demonstrate this. If just 30 animals are killed a year, then the species becomes extinct after 520-700 years. Human populations in Australia were small at 45 kyr - only 150 people occupied the same 500 square kilometres as 1000 animals. However, at a rate of killing just two animals a year by each group of ten people, extinction is highly likely.

A recent study on the albatross has shown the bird has almost disappeared due to females occasionally being hooked on fishing lines. A large number of animals do not need to be killed to effect extinction especially if an animal breeds late and infrequently like the albatross and like megafauna.

With Johnson’s model, it is easy to see that the archaeological record need not be filled with tonnes of bones. Megafaunal skeletons are not visible because hunting them was a minor activity, or because they are yet to be found.

The mystery of the rapid extinction of Australian megafauna may be over. These animals probably became extinct because they were large, slow, easy victims whose birth rates never exceeded their death rates. Their disappearance is consistent with predation rather than environmental change. Although hard evidence of hunting is lacking, it remains the simplest explanation.