DNA leaves little doubt: All non-Africans alive today descend from a single wave of migration out of Africa, perhaps sometime between 50,000 and 60,000 years ago. But over the years, scientists have found a handful of modern human sites that suggest our species may have wandered from its birthplace even earlier. A study published today in Nature Communications provides some of the strongest evidence yet of such an early dispersal, thousands of kilometers from Africa. Inside a cave in Laos, scientists have dated a pair of modern human bones to between 68,000 and 86,000 years ago. Although these precocious migrants likely didn’t contribute much genetically to modern populations, they blazed a trail into Southeast Asia followed by later generations.
“This [finding] makes the dispersal of modern humans more interesting,” says Russell Ciochon, a biological anthropologist at the University of Iowa who wasn’t involved with the work. “It also begs the question of why the later migration was demonstratively more successful.”
Earlier hints of very ancient human presence in Southeast Asia and Australia include stone tools and charcoal from a shallow cave in northern Australia called Madjedbebe, dated to about 65,000 years ago by optically stimulated luminescence (OSL). That method estimates how long it’s been since bits of sediment were last exposed to light and gives a reliable date only if the sediment around an artifact is undisturbed. Some scientists argue that burrowing termites may have shuffled Madjedbebe’s ground, and question its date. In 2017, Sumatra’s Lida Ajer Cave yielded teeth identified as Homo sapiens and dated to between 70,000 and 46,000 years ago. Scientists have also identified stone tools in central India as humanmade and dated them to about 74,000 years ago. But in each of these cases, other researchers have questioned the evidence.
The newly dated bones come from a massive, sloping cave named Tam Pà Ling, high in the verdant mountains of northern Laos. It’s one of many such grottos in the region, says one of the study’s lead authors, Laura Shackelford, an anthropologist at the University of Illinois Urbana-Champaign. “You cannot trip without falling into a cave or sinkhole, sometimes literally.”
Scientists, including archaeologists from the Laotian Ministry of Information, Culture and Tourism, began excavating Tam Pà Ling in 2009. The cave wasn’t anybody’s permanent home. Every year, seasonal floods wash sediment and sometimes bones from the surface into its depths, building up a layered record. There, researchers in 2010 found most of a H. sapiens skull and jawbone, which they dated by OSL to about 46,000 years old. Then in 2017, in an even deeper layer, they discovered a forehead or frontal bone and a piece of shin bone, both identified from their characteristic bone shapes as belonging to our species. These bones were found in the same layer as animal teeth, probably from goats or sheep.
Shackelford and colleagues, including geochronologist Kira Westaway, dated the sediment around the forehead and shin by OSL. They dated the animal teeth with a pair of techniques known as uranium-series dating and uranium-series/electron spin resonance dating. Both measure how much radioactive uranium, naturally present in soil, has migrated into a tooth’s enamel over time. By comparing the ratio of uranium isotopes in a sample, which decay at different rates, researchers can estimate how long a specimen has been buried.
Based on these techniques, the researchers concluded the human frontal bone and tibia were buried in Tam Pà Ling sometime between 86,000 and 68,000 years ago. Because genetic evidence suggests that all living non-Africans left that continent later, the bones must represent a population that ventured out from Africa in an earlier wave and didn’t leave their genetic mark in living people.
“People might say, ‘Oh, yes, but they were unsuccessful … [and didn’t leave a legacy in] our DNA,’” Westaway says. “But it doesn’t discount the fact that they were there. It’s still an incredible achievement” to voyage halfway around the world and carve out life in a new environment.
No tools have been found at the site, Shackelford adds, possibly because people here crafted implements with readily available bamboo, which decays over time, rather than stone.
Maxime Aubert, a geochemist and archaeologist at Griffith University, says the paper’s dating methods are “state of the art” and reinforce the previous claims from Sumatra. This summer, he and colleagues are excavating caves in Borneo and Sulawesi that may add to the evidence of humans in the region around this time.
The paper convincingly “demonstrates that there was an earlier, unsuccessful migration of modern humans into Asia that left no descendants,” Ciochon says. The combination of dating methods that all converge on a similar age range inspires confidence, he adds.
Shackelford says her team isn’t done exploring the Laotian mountains. “You go 5 feet and there’s another site,” she says. “Every one of those we need to explore.”