Some 40,000 to 50,000 years ago, a band of intrepid Southeast Asians became the first humans to reach Australia, and without a single glance at a GPS unit. Now new research suggests that perhaps 3000 people—many more than previously thought—made that foray into the unknown to become the continent's founding population and the ancestors of Australia's aboriginal people.
The study is among the first to specify just how many adventurers weathered the trip to become the original Aussies, although researchers are divided on whether the new numbers are accurate. The study has also stirred debate on exactly when and why the aboriginal population ballooned to hundreds of thousands of individuals.
"People remained at low levels, we believe, for 40,000 years and suddenly, for no apparent reason, we see … their numbers start to build," says study author and archaeologist Alan Williams, a graduate student at the Australian National University in Canberra. "We need to ask what changed."
To determine the head count in Australia's founding population, Williams compiled the most complete estimate to date of the continent's prehistoric population. He turned to a database of 5000 cooking pits, human burials, shell heaps, and charcoal deposits. All were from Australian archaeological sites, and all had been assigned a date using radiocarbon dating methods. As the human population grows, the number of archaeological sites and artifacts available for radiocarbon dating grows, too. That makes radiocarbon dates a yardstick of population, or so think Williams and some other researchers who employ this technique.
Relying on the radiocarbon-date database, Williams worked out the rates at which the population changed over time. Then he back-calculated from the aboriginal population at the time of the first European settlement in 1788. He found that for the aboriginal population to reach the estimated 770,000 to 1.2 million at the time of settlement (it's roughly 460,000 today), the founding population that arrived in Australia roughly 45,000 years ago must have been between 1000 and 3000 people. Other recent estimates—based on genetic diversity in aboriginal Australians—range from "small" to "hundreds" to "more than 1000."
Williams says his larger numbers may speak to the mindset of those Pleistocene-era pioneers. "It's not just a family that got stuck on a raft and washed away," he says. "It's people with the intention to move, to explore."
As to motivation, Australia's first colonizers may have left home because of competition for resources, says archaeologist James O'Connell of the University of Utah in Salt Lake City, who has written extensively about Australian prehistory. Later waves of immigration to Southeast Asia explain why current populations there don't much resemble aboriginal Australians, he says.
Williams also found that Australia's population stayed below 20,000 for many millennia. But his analysis, online today in Proceedings of the Royal Society B, shows that population numbers began to grow roughly 12,000 years ago, some 7000 years earlier than previously proposed, and peaked at approximately 1.2 million people 500 years ago. Those who put the population spike at 5000 years ago say the growth may have been triggered by the arrival of the dingo, whose semidomestication could've led to major cultural changes, or by the adoption of new tools. Williams says that one possible driver for a growth spurt 12,000 years ago is climate change. The global climate was warming at that time, which should've encouraged plant growth, a benefit to hunter-gatherers such as the aboriginal Australians.
An early population spike "makes much better sense to me" because of the favorable climate changes, O'Connell says. He also thinks a founding population in the thousands is reasonable and could force scholars to rethink their ideas about early seafaring. "It means purposeful marine voyages going back 50,000 years, and for a lot of archaeologists, that's a surprise."
A large colonizing force also "implies the presence of watercraft that are bigger and more seaworthy than you'd expect from the Australian historical record," O'Connell says. Most Australian watercraft at the time of European contact were relatively small, fragile rafts made of reeds or branches, he says, whereas the trip to Australia would've required a proper boat, perhaps with an outrigger.
Other scientists, while praising some aspects of the study, raise questions about its methodology and some of its conclusions.
The number of radiocarbon dates might fluctuate because of factors other than population change, says Simon Holdaway of the University of Auckland in New Zealand, who has studied the radiocarbon method. For example, archaeological sites are more readily preserved in some landscapes than in others, which could lead to a high number of radiocarbon dates at a site with good preservation and low numbers of dates at a site with poor preservation.
Also, the study relies on a low number of data points from the early years of human occupation, says Barry Brook of the University of Adelaide in Australia, who has found evidence for a late population rise. The sparse record means that the paper's models can't distinguish between a founding population of 5000 that grew little and a founding population of 100 that quickly multiplied to 5000, he says. "For the most recent times, (the model) is reasonably realistic," he says. "For a long time ago, the model is pretty uncertain." He agrees with Williams that there was a population explosion in the more recent past, but he thinks the continent was settled by at most a few hundred people.
Williams doesn't consider his paper the last word on the subject. "There are still a lot more questions than answers," he says.