Australian Deserts: Australia
T he hard, sprawling deserts of Australia and the remarkable people who have lived in them for 50,000 years offer fascinating clues to human evolution, climate shifts, and the intricate connections between living things and the ecosystems they inhabit (an example is shown in the color insert on page C-8). Although Australia boasts tropical forests along great sections of coast, much of the interior is divided into three great deserts, the 317,800-square-mile (823,000 sq km) Great Sandy, the 164,000-square-mile (424,400 sq km) Great Victoria, and the 56,000-square-mile (145,000 sq km) Simpson.
These mostly flat, sandy, scrubby deserts occupy most of the interior of this island continent and are sustained by the same atmospheric circulation patterns that created the deserts of the Northern Hemisphere like the Sahara. However, the vagaries of the southern oceans and the shifting monsoons make conditions in Australia less harsh and the temperatures less extreme than in the great waste of the Sahara. Moreover, Australia's long isolation from any other landmass has ensured that its plants, animals, and human cultures have evolved along distinctive and unexpected paths, creating a biological cast of characters found nowhere else.
The isolated island-continent of Australia has long provided a bizarre proving ground for theories of evolution and ecology. Some 200 million years ago, all the present-day continents were gathered into a single landmass called Pangaea. Convection currents deep within the molten Earth caused this continent to split apart and set the continents adrift, embedded on crustal plates. As the dinosaurs peaked in the Jurassic period, Australia was an island drifting out to sea, cut off from most of the plants and animals on the other continents. As a result, Australia developed a unique set of creatures. In particular, Australia remained the land of the marsupials, mammal-like creatures including kangaroos, wombats, bushbabies, and others whose young are born as tiny, squirmy, utterly helpless near-embryos who mature while attached to a mother's nipple tucked safely inside a protective pouch. Although such marsupials once lived all over the world, outside of Australia they generally could not compete with placental mammals, which kept their young inside the womb until they are much more developed. Only a few oddball marsupials like the opossum survived outside of Australia.
Human beings are relative newcomers to this complex web of living things. Several varieties of relatively big-brain, upright-walking, big-bodied primates with opposable thumbs spread throughout the world in the roughly 5 to 8 million years after their line diverged from their nearest primate relatives, like the chimpanzees and gorillas. Once, paleontologists thought these scattered hominids were the ancestors of modern humans. But more recent analysis of the DNA shared by all living human beings suggests that modern Homo sapiens spread out of southern Africa between 70,000 and 180,000 years ago. Perhaps as these tool-using early humans spread they drove into extinction other upright-walking primates competing for the same food resources. In any case, the first humans walked out of Africa, perhaps following the great game herds, and spread across Asia and then across ice age land bridges into North and South America.
Some 50,000 years ago, these early humans somehow crossed the open ocean to reach the shores of Australia. No one knows how they managed such a feat, but they must have fashioned rafts or dugout canoes and made their way from island to island until they reached this strange land. Of course, like most aspects of human evolution, the details of that migration and the impact the newly arrived hunters and gatherers had on the remarkable, isolated creatures of Australia remain deeply controversial. But timing, as they say, is everything.
For instance, fossil experts quickly concluded that the arrival of the first human beings in Australia coincided with the disappearance of most of the continent's strange megafauna. Roughly 85 percent of the Australian animals weighing more than 100 (45.4 kg) pounds disappeared at this time, including 19 marsupials weighing over 220 pounds (99.8 kg). That included bizarre animals like a wombat the size of a hippopotamus, a 25-foot-long (7.62 m) snake, a 25-foot-long lizard, and a horned tortoise the size of a car.
Scientists have debated for years whether the newly arrived human beings hunted down the giant marsupials, whose great size was no protection from these top-of-the-food-chain predators. On the other hand, perhaps human beings introduced other changes into the environment that doomed the larger animals, like the early human practice of setting fire to grasslands. Or perhaps humans just happened to arrive at the same time that overall climate changes doomed many of the existing species. Scientists have tried to solve the riddle to better understand how human beings both cause the changes that produce deserts and then survive the harsh demands of the changed conditions. But answering the many questions about this mass extinction first required a better grasp of the timing of both the arrival of the first human beings and the crash of the big marsupials.
At one time, fossil experts argued that the first humans reached Australia about 62,000 years ago. That early date was based on a complex method for dating the fossilized remains of Mungo Man and Mungo Lady, the fragmentary bones of two people who died on the shores of long-vanished Lake Mungo. At that time, an ice age gripped the planet, creating a great chain of freshwater lakes in areas long since turned to desert (which can be seen in the color insert on page C-8). The original 62,000-year-old date for those fossils stirred a hornet's buzz of debate. It placed the first humans squarely on the scene of the extinction of the continent's large marsupials. It also created great difficulties for the DNA experts who argued that modern human beings didn't start their spread out of Africa until about 80,000 years ago. It did not seem possible for humans to have so quickly spread all the way from Africa into Asia and hop islands across the Indian Ocean to Australia.
However, after years of debate, scientists now mostly agree that Mungo Man lived only about 40,000 years ago, which is more consistent with other studies that date the oldest stone tools found so far in Australia back to about 50,000 years, according to a study published in the journal Nature. The firm dating of Mungo Man in the midst of the last ice age now makes it possible to study the way in which early human beings responded to dramatic and relentless changes forced by a shift in climate, which converted much of Australia from a wet, lake-speckled mosaic of grasslands and woodlands into an unrelenting desert.
Much of the evidence suggests that human beings sometimes adapted to those changes and sometimes forced changes of their own. One fascinating attempt to measure the impact of human beings depended on a careful analysis of the fossil eggshells of several giant, ostrichlike birds, the emu and the extinct genyornis, according to an article published in the journal Science.
The scientists used certain elements in eggshell fragments dating back some 65,000 years to figure out what these two giant, flightless birds were eating. For instance, plants that depend on summer rains use a slightly different method of converting the Sun's rays into energy, which affects the type of carbon molecules in the plants' tissues. By contrast, plants that live in desert areas that do not get summer rains will produce a different ratio of carbon. The emus and genyornis eat grass and shrubs, which means that distinctive ratio of different isotopes of carbon are reflected in the composition of their eggshells. Therefore, by looking at the types of carbon stashed in the fossilized eggshells, scientists can tell whether those long dead emus and genyornis were eating grass and shrubs growing in a wet grassland or a dry desert.
The study yielded a fascinating glimpse of the shift from the wet ice age climate to the arid deserts of today. The scientists concluded that summer storms brought plenty of rain deep into the interior of Australia between 65,000 and 45,000 years ago, coinciding with the ice age arrival of the first human beings. But sometime between 30,000 and 15,000 years ago, everything changed. The summer rains failed, and the forests and grasslands changed into desert. That probably reflected climatic shifts that moved the summer monsoon storms away from Australia to now drench India. As a result, even though many areas of Australia's interior get 10 inches of rain a year, which makes them almost lush by desert standards, the long, dry, rainless summer creates a harsh environment for both plants and animals.
The sad fate of genyornis suggests that human beings may have dramatically increased the impact of the underlying shifts in climate. This speedy, 200-pound (91 kg) flightless bird lived in the vast interior of the continent until about 50,000 years ago, just when human beings arrived. The analysis of its eggshells shows that before human beings arrived, genyornis mostly ate bushes and shrubs. By contrast, the more flexible emu also ate grasses. But the eggshells show that 50,000 years ago, the grasslands expanded and the areas with shrubs dwindled sharply, leading to the extinction of the grass-shunning genyornis. So the expansion of the grassland and the extinction of many of the megafauna actually came a little ahead of the climate change.
Some experts say that means human beings must have contributed to the extinction of the genyornis and many other species. Gifford Miller, a geochronologist at the University of Colorado at Boulder, suspects that the newly arrived human beings started deliberately setting fire to the grasslands, which was a common practice among Native Americans as well. These hunters and gatherers observed that fires usually stimulate the spread of grasslands, which yield many foods and support key species they liked to hunt. Miller's analysis of the eggshells and climates shifts suggest that the effect of the newly arrived human hunters was heightened by their tendency to set fires to expand grasslands, which then became deserts as the climate shifted.
However, other scientists continue to debate the impact of humans. One study published recently in the Proceedings of the National Academy of Sciences concluded that many large creatures like a 6,000-pound (2,722 kg) wombat, a fierce marsupial “lion,” and the biggest lizard in history did not die out until 30,000 years ago, which means they probably coexisted with human beings for at least 15,000 years. That would mean the shift in the summer monsoon patterns did more to cause their extinction than the arrival of human beings. These findings were based on a new method of measuring rare earth elements absorbed by fossil bones from the soil in which they are embedded, which supported a much younger age for certain key fossils than earlier methods.
Of course, more recent, drastic ecological changes are theoretically easier to document than ancient shifts. For instance, a study of 300-yearold emu eggs documented another shift from grasslands toward desert that took place when the first Europeans brought sheep and cattle to the continent. Once again, the scientists used radio carbon dating to figure out how old the eggshells were, then analyzed the carbon in the amino acids in the shells to figure out what the birds were eating. The study demonstrated that the introduction of domestic livestock resulted in a sharp drop in the total biomass of plants. Essentially, emus had to give up eating grass entirely, since the cows and sheep gobbled the surviving grasses down to the nub.
Those studies document the dramatic transformation of a grassland into a harsh desert, shoved along first by the introduction of fire by the first humans and then by livestock brought in by the second wave of immigrants.