When Dutch archaeologist D. A. Hooijer first saw a pair of weathered teeth recovered from a remote cave on the Indonesian island of Sumatra, he noted that they were about the right size and shape to belong to modern humans. But in 1948, he couldn’t be sure of their identity or their age. Now, harnessing cutting-edge science, a group of researchers has confirmed what Hooijer had suspected: Modern humans lived in Southeast Asia as far back as 73,000 years ago—about 20,000 years earlier than previously thought. The earlier timeline helps fill in the blanks on the migration routes of our early ancestors and bolsters an emerging theory that humans may have dwelled in rainforests much sooner than researchers had assumed.
Previous studies suggested that after evolving in Africa, modern humans eventually made their way to Southeast Asia, but researchers have argued whether they arrived about 50,000 years ago or earlier. Recent studies put modern humans in Australia by about 65,000 years ago, but there has been little direct evidence of an early presence in Southeast Asia.
To unravel the mystery, researchers led by geochronologist Kira Westaway of Macquarie University in Sydney, Australia, decided in 2008 to give the Sumatran teeth another look. She and her team used new techniques, including micro–computed tomography scanning to precisely measure the thickness of the enamel, and luminescence dating to determine when minerals in the rock surrounding the teeth were last exposed to sunlight. They found thick enamel, confirming that the teeth are from modern humans, and pegged the date to between 63,000 and 73,000 years ago, they report today in Nature.
The findings offer new hints about how our early ancestors spread across the world, paleoanthropologist Russell Ciochon of the University of Iowa in Iowa City wrote in an email. “[The teeth] show greater affinity to east Asian humans than later southeast Asian specimens, which may give us some clues about the early dispersal routes of modern humans.” he wrote. The researchers, he says, “have definitively and superbly demonstrated the presence of modern humans in Southeast Asia 20,000 years earlier than previous estimates.”
Though just a pair of teeth may seem like insubstantial evidence, the new analysis is convincing, says paleontologist Patrick Roberts of the Max Planck Institute for the Science of Human History in Jena, Germany. “If they show they are clearly human, which I think they do, then it is enough to document humans in this part of the world.”
And the earlier timestamp also means early modern humans may have overlapped with the hobbit (Homo floresiensis), a tiny early human species that lived more than 60,000 years ago on another Indonesian island called Flores. Yet it’s unlikely the two crossed paths, Westaway says, because strong currents would have made travel to Flores difficult.
The new findings also suggest that these early colonizers may have been the first to live in a rainforest setting. That’s significant, because researchers have long thought that early humans would have found rainforests unappealing: Why hunt clever monkeys in the treetops when easy-to-catch shellfish and other resources await on the coast?
But Roberts of isn’t fully convinced that these early human colonists did live in rainforests. Fossils from rainforest animals were found at the site but don’t bear the marks of a human kill and may not have coexisted with modern humans.
Still, the study shows that “tropical terrestrial habitats were crucial resources for humans expanding beyond Africa, and our species was flexible enough to adapt to them,” Roberts wrote in an email. “Perhaps it is this environmental plasticity that characterizes our species and has left it the last remaining hominin in the world.”