Up in smoke

Did fires caused by the Aborigines drive creatures larger than humans to extinction?

 
Last Updated: Saturday 04 July 2015

-- the history of Australia is closely linked to the fires that have engulfed the continent for centuries. Captain James Cook who reached the Australian shores in 1770 described it as a "continent of smoke". Some describe it as a land shaped by smoke ( New Scientist , Vol 162, No 2184). It is believed fire played a crucial role in the lives of Aborigines who arrived in Australia some 60,000 years ago, leading to extinction of large animals that once inhabited the continent.

However, there has always been a disagreement among scientists who believe that climate change was the cause of the megafauna extinctions, and those who argue that humans were responsible. The root cause of the disagreement is the difficulty in dating the fossil remains and human artefacts . A recent study by Gifford Miller and his colleagues at the University of Colorado at Boulder in the us has given a new dimension to the debate.

For those like Miller, Aborigine actions led to the extinction of animals like Procoptodon, a kangaroo that was three metres tall, a wombat-like creature that was as big as hippo, Wanabe , a snake nearly a metre in diameter and seven metres long, and Genyornis , a flightless bird twice as heavy as the emu. It is believed that in the epoch that ended with the last ice age, Pleistocene, about 10,000 years ago, every creature larger than the human disappeared.There has been an ongoing debate among scientists on precisely when the megafauna disappeared and why? The study by Miller and his team is the first of its kind to date the remains of the megafauna.

They analysed hundreds of fossil eggshell of Genyornis newtoni from Australia's arid interior. Miller says he has found evidence in the chemistry of the ancient eggshell that the Aborigines burnt Genyornis and other megafauna into extinction. Due to Australia's severe climate, organic matter quickly leaches out of the bone. This makes it impossible to run radiocarbon or other dating procedure directly on most fossils. Instead, investigators date bits of organic sediment found with the bones, a strategy with many uncertainties, because anything from floods to the movement of termite colonies may move fossil remains closer to older or younger sediments.

But the case of eggshells is different, as they have a different mineral structure to bones and they retain traces of protein. Miller, by using the technique of amino acid racemisation ( aar ), which is more reliable over a longer period of time compared to radiocarbon dating, has been able to date the eggshell directly. When animals synthesis protein, their bodies use only the left-handed isomer of the amino acid isoleucine. After death, the amino acid molecules begin inexorably to revert to the right-handed form.

In aar, the ratio of left-handed to right-handed isomers is used to determine the age of a fossil. The aar measurements done by Miller on fossil eggshells from Lake Eyre, Lake Frome and the confluence of the Darling and Murray rivers found the big bird had disappeared from the Australian interior about 50,000 years ago. This was a time of mild climate, long before the peak of the last ice age brought the cold, arid conditions to Australia.

"It is one of the major breakthroughs we have had in Australian prehistory," says Tim Flannery, senior research scientist at the Australian Museum in Sydney. "It is unbelievable. He has 1,200 dates from three sites, the biggest data we have had." Eggshells not only reveal when birds lived and died, they can also indicate what they ate, because shrubs and other woody plants contain a lower ratio of the stable isotope carbon-13 than annual grasses. Miller's group compared the carbon isotope signatures of fossil emu and Genyornis eggshells.

The results indicate that the emu, which is still present had a more flexible diet than Genyornis. "There were plenty of grasses available prior to the extinction event because some of the emus ate nothing but grass. None of the Genyornis living at the same time ate that much grass,"says Miller.

He argues that when people arrived on a continent already rich in fire-adapted vegetation, they began to use fire for their own purposes -- to clear the land or to drive game towards places where they could be killed easily. Before people arrived, fires occurred in the interiors only during a few weeks each year, normally at the beginning of the monsoon season. In Miller's scenario, humans changed the timings and frequency of fires in a way that caused sweeping changes to Australia's vegetation. Perennial shrubs were replaced by annual grasses that can thrive frequent burning. Genyornis and many large animals had relied on those shrubs for their food and could not survive this shift, says Miller.

His theory is based on a complicated chain of evidence and inference. "There are basically two hypotheses," says Miller. "Either climate is the root cause of extinction or people are. In order to resolve the question you need three things. You need to know when the extinction occurred, when people first arrived and what was the climate at that time." Miller believes he has put the three pieces of the puzzle together. The evidence that people arrived in Australia 50,000 and 60,000 years ago fits in well with his finding that Genyornis died out 50,000 years ago. These findings come from two sites in Arnhemland, in the far north of the continent, where sediments surrounding the human artefacts have been dated to this time using a technique called optically stimulated luminescene or osl . This technique measures the release of electrons from the crystal structure of minerals. These extra electrons accumulate when artefacts are exposed to natural radiation while buried.
The other side There are some researchers that question the Arnhemland dates. "I am not convinced by the accuracy of luminescene dates, especially when taken on associated sediments," says Jim Mead, a geologist at the North Arizona University. These dates have not yet been confirmed using radiocarbon techniques and sceptics point out that no other archaeological find in Australia has been shown to be more than 40,000 years old. Another link in the chain is an intriguing sign that a sudden increase in burning may have coincided with the arrival of the first Australians and with ecological changes. A geological core from the Atherton Tablelands in northern Queensland shows a surge in charcoal deposits at the same time that a major change in the vegetation occurred.

Peter Kershaw of Monash University in Victoria, who analysed the core, used radiocarbon dating to place the increase in burning at 38,000 years ago. But the true date may be older if, as appears possible, the core samples are contaminated with younger carbons.

"By the time you get to 40,000 years before the present it becomes very difficult to say whether you have a 'real' date or a radio-carbon dead sample, with a bit of younger contamination," says Michael Bird of the Australian National University in Canberra. He adds "I think that all radiocarbon dates older than 35,000 years must be treated as suspect, and hence the increase in burning may be older, which might bring it into the line with the 50,000 to 60,000 arrival age. According to him, the increased burning may have nothing to do with people.

Moreover, fire is not the only way in which human activity could have caused by megafauna's demise. Flannery for once is convinced that people hunted the megafauna to extinction, and that it was this that led to the fires. Most of the lost animals were browsers that fed on shrubs and other vegetation. He believes that dry, dead plants accumulated after the extinction of the megafauna, and that with this increase in natural fuel, fires became hotter and more dangerous. This could have led the Aborigines to set frequent, less intense blazes as forms of self-defence.

Kershaw and Bird are now testing new core samples from the Atherton site, using improved techniques which can ensure the removal of contamination and produce reliable dates going back earlier than 40,000 years. They are also in the process of re-dating Australia's oldest archaeological sites, hoping to resolve the controversy over the time of human arrival. "Miller's scenario seems plausible but it requires people to have actually arrived before the extinction occurred, and I think this is still an open question," says Bird.

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