Walking on eggshells

Many of Australia's big animals died out when the first human settlers turned up, writes Sanjida O'Connell

Guardian

Thursday May 10, 2001

Thousands of years ago, Australia was a fearsome place to live. The marsupial lion, the largest meat-eating marsupial, had slicing cheek teeth and a huge thumb claw for disemboweling its prey, all the better for hunting diprotodon.

At three metres long, this peaceful vegetarian was twice the lion's size and lived like a peaceful hippo near river banks. If the lion didn't get you, the quinkana might, a seven-metre long crocodile that lived on land and sprinted after prey on its long legs. Today's red kangaroo would be dwarfed by the kangaroo of the Pleistocene. Weighing in at 200 kg, it had grappling hooks instead of paws and hoofed feet. But at some point in Australia's history, 85% of this so-called megafauna became extinct. One man believes he knows exactly when Australia's big animals died out and why; he has found out by looking at eggshells.

For more than a century, the timing of this extinction has been controversial, mainly because radiocarbon dating from bones is not accurate enough for "early" dates. However, eggshells from flightless birds, preserved in the hollow scoops the birds made in lieu of nests, are surprisingly common. Professor Gifford Miller, from the University of Colorado, Boulder, dated eggs from two birds, emus and those of the two-metre tall thunder bird, Genyornis newtoni, which is believed to have gone the way of the marsupial lion at about the same time.

Miller's team used radiocarbon dating as well as amino acid racemisation, a technique that involves studying changes in amino acids over time and which can provide an accurate time clock. Professor Miller and his colleagues found eggs from both birds in the same places, suggesting that the two species co-existed and nested close to one another. That is until 50,000 years ago, give or take five thousand years.

At this point there is an abrupt lack of genyornis egg shells but the emu egg shells remain. Because Professor Miller found emu egg shells dating to the present day, he thinks the results are not due to a sudden paucity of egg shells - rather, at that time genyornis became extinct.

It may have been due to a change in climate. The most complete record of moisture change comes from the Willandra lakes in south-western New South Wales, which were dry for a long period until about 60,000 years ago, when the lakes filled and were surrounded by lush vegetation. This idyll continued, (though the land started drying out about 40,000 years ago) until 20,000 years ago when a serious drought set in. Lake Mungo parched and has remained dry ever since.

Throughout this time emus managed to live by the lake, but genyornis eggs were not present when the lake dried up. Professor Miller says: "It seems that genyornis was able to survive through the range of natural environmental changes caused by the Pleistocene climate oscillations. At the time of genyornis extinction, modest aridity to slightly wetter-than-present conditions prevailed. Consequently climate change as an explanation for genyornis is unlikely."

Miller goes on to postulate that since genyornis bones are found with other megafaunal bones, such as the giant kangaroo and marsupial lion, it is likely to have died out at the same time as the rest of Australia's megafauna.

The other reason why these animals are extinct could be due to human intervention. Humans are known to have had a severe impact, not just on flora and fauna in general, but on flightless birds in particular. The dodo is a classic example; in New Zealand the eradication of the moa has been well documented and was due to people hunting the birds and starting fires.

Could humans have killed off the megafauna? Again, the answer is controversial and depends on the date modern humans first arrived in Australia. Professor Miller believes the most accurate dates are those taken from quartz grains enclosing human artefacts and this puts their arrival at exactly the same time as the demise of genyornis.

Did early peoples hunt the birds to extinction exactly as they were later to do to the moa?

There is only evidence for a genyornis kill by humans in one site, and direct evidence that humans killed other megafauna is equally rare. This is not to say that humans did not hunt these creatures, but Professor Miller believes the greatest impact humans may have had was by altering the landscape. By reexamining the egg shells, Professor Miller was able to reconstruct what these birds might have eaten. Living birds, such as quail and emu, incorporate their food into their eggshells. Professor Miller was able to work out the ratio of carbon from food compared to other egg-building materials.

Depending on what type of food they ate, and therefore what kind of carbon the birds were ingesting, this ratio altered and changed the weight of the eggs. Using this contemporary data, Professor Miller was able to record the types of carbon in the fossil eggs, and estimate the levels of carbon that would have come from food. He concluded that emus were able to eat a wider variety of foods than genyornis.

There is another clue in the skull of genyornis: it looks as if they used their beaks in a shearing motion, a character istic of birds that browse. Professor Miller concluded that genyornis was eating shrubs and was highly dependent on these plants. He believes that human-started fires ravaged a landscape unaccustomed to burning, which resulted in a dramatic decrease in trees and shrubs. This put an enormous stress on fauna already surviving in impoverished soil. He says: "This stress, possibly coupled with modest drying that occurred simultaneously, and perhaps some direct human predation, led to megafauna extinction." Most of the 22 out of 38 species of megafauna that died out were browsers, and the predators, with too few animals to prey upon, were also unable to survive.