In the Padang Highlands of western Sumatra, a large island in Indonesia, there is a small cave called Lida Ajer that has long offered up clues about human history. Dutch paleoanthropologist Eugene Dubois first excavated the cave before 1890, and Lida Ajer has turned up plenty of preserved animal remains since, including teeth that were identified as human in 1948.
It’s only now that the cave has been carefully and thoroughly dated, providing a new line of evidence that our species was in the region more than 60,000 years ago. That’s 20,000 years older than the previous oldest skeletal evidence of humans in the area. But these new dates line up with existing genetic evidence, as well as with reconstructions of the climate and sea levels at the time.
In a paper published in Nature this week, Kira Westaway of Macquarie University in Australia and her colleagues report what they found when they revisited the discoveries of Lida Ajer. They re-examined the teeth, pointing to all the evidence that the teeth did indeed belong to anatomically modern humans rather than orangutans or other primates. And they carefully dated the cave site to establish how old the teeth were likely to be.
The maximum age of the layer in which the teeth were found was determined partly by dating a stalactite that was found in the layer itself. “Straw stalactites … detach and drop into the underlying sediments over relatively short timescales,” write the authors, “and therefore commonly represent ages close to the true time of sedimentary deposition.”
In this case, U-series dating—a technique based on uranium decay rates—placed the stalactite at around 84,000 years old, suggesting that this was the oldest that the human teeth could be. The margin of error was around one thousand years, so a conservative estimate would say 83,000 years. U-series dating of the stones overlying the crucial layer pointed to a minimum age of 71,000 years ago, with a margin of error of 7,000 years. This puts a conservative timeframe of the layer at between 64,000 and 83,000 years old.
Dating of quartz and feldspar grains provided estimates between approximately 62,000 and 85,000 years old, lining up neatly with the U-series dates. And dating of animal fossils found in the layer—including one found by Dubois himself—yielded dates between 60,000 and 86,000 years old. This ties in neatly with genetic evidence suggesting that humans were in the region around 75,000 years ago (and with new evidence of early human presence in Laos).
At that point, Sumatra would probably have been connected to surrounding land: reconstructions of the climate and sea levels at the time suggest that these regions would have been accessible to humans. What’s surprising, though, is that Lida Ajer would have been rainforest at the time.
Humans might live in rainforests now and have done for some time, but this is the first evidence that they had figured out how to survive in that environment that long ago. The coastline provided easier pickings for humans to live off, while rainforests have much more limited sustenance. Evidence of rainforest-dwelling humans more than 60,000 years ago points to the humans of that time already having developed a certain level of technological sophistication and flexibility, allowing them to exploit a variety of different environments.
Coherent evidence pointing in the same direction is always a good sign, but ongoing discoveries will help to flesh out this picture, including helping researchers work out the details of early human migration routes—and possibly turning up further evidence of humans exploiting a variety of habitats much earlier than previously thought.
More Info: arstechnica.com