Cave excavation pushes back the clock on early human migration to Laos

"When we first started excavating the cave, we never expected to find humans in that region," said University of Illinois Urbana-Champaign anthropology professor Laura Shackelford, who led the research with Fabrice Demeter, a professor of anthropology at the University of Copenhagen. "But beginning that first season when we started work there, we found our first modern humans. At the time, that made them the only early modern human fossils in the region."

While the remains of modern Homo sapiens dating back roughly 197,000 years have been recovered in Israel, genetic studies suggest the main phase of early human migration out of Africa and into Asia occurred much later -- around 50,000 years ago, Shackelford said. Her team's earliest excavations in Tam Pa Ling found bone fragments from modern human remains dating to about 40,000 years ago. But as the excavations dug deeper, the age of sediments and animal remains found alongside human bones dated back much earlier.

In 2019, the team had excavated as far as they could in the cave, reaching bedrock about 23 feet (7 meters) below the surface. The excavations yielded dozens of animal bones and many fragments of human skeletal remains. The deepest human bone recovered -- a partial tibia -- was resting on bedrock near the bottom of the trench. Analyses of sediments taken not far above this bone indicate the soil was deposited there between 67,000 and 90,000 years ago.

"The entire section of the trench goes from about 30,000 years ago to 80,000 to 100,000 years ago," Shackelford said. "Flood season after flood season, the sediments and bones washed into the cave and were deposited. They've been sitting there ever since."

The team used a variety of techniques to date the sediments and nonhuman mammal bones found at different depths in the cave. For the soil samples, the researchers relied primarily on a technique called optically stimulated luminescence, which reveals how much time has elapsed since sediments were last exposed to light. Numerous soil samples were collected from the cave in total darkness and taken back to the laboratory for analysis.

The researchers used several other techniques to date the soils, human and animal bones and teeth found at different depths in the trench.

The human remains were fragmentary, but the site yielded two pieces of skull -- both from the frontal region of the head -- one of which dated to about 35,000 years ago and the other much older, dating to roughly 67,000 years ago, Shackelford said.

At first, a comparison of the physical characteristics of the fossils baffled the researchers, Shackelford said.

"What we saw was that the youngest fossils, the first ones we found, looked old," she said. "And the oldest fossils that we found at the very bottom looked younger. It was the opposite of what we would expect to find because as you get deeper in time, you think of things becoming more archaic, more like ancestral populations."

There are two potential explanations for the differences between the older and younger fossils, Shackelford said. One, the older fossil may be from an ancestral population that had more modern characteristics than their descendants, who interbred with people with more archaic features. The other possibility is that the two skulls represent two distinct migrations into the region.

"We don't know how they're related, but we have two very different looking groups of humans," Shackelford said. "The older fossil was most likely part of an early migration -- a failed migration because it did not give rise to people who are still alive today."

The team was unable to extract viable DNA from the human remains, a common problem in studies of ancient remains from tropical sites, Shackelford said.

"I think the only way to settle our questions is with genetic data," she said. The team is preserving the human fossils in a freezer in the hopes that more refined DNA extraction and analysis techniques will one day yield more answers.

The National Geographic Society supported this research.