Skeleton Found In Submerged Mexican Cave Sheds New Light On Earliest People In America

Skeleton Found In Submerged Mexican Cave Sheds New Light On Earliest People In America
A cave diver holding a human skull in Chan Hol Cave. (Image: Eugenio Acevez)

An extraordinary 9,900-year-old skeleton found in the submerged caves of Tulum is both enhancing and complicating our understanding of the first humans to settle in the Americas.

Mexico’s Yucatán Peninsula is typically associated with the Maya civilisation, but emerging archaeological evidence suggests this region was settled thousands of years earlier by some of the first people to set foot in the Americas.

New research published today in PLOS One describes an important new discovery that’s adding to our understanding of this period in human history: a 9,900-year-old skeleton found in the submerged Chan Hol cave near the Tulum archaeological site in Mexico’s Quintana Roo state.

The skeleton, which belonged to a woman who died in her 30s, has some unique characteristics that suggest the region was inhabited by at least two different groups of early Mesoamerican settlers, who made the area heir home roughly 8,000 years before the Maya first appeared on the scene.

A striking feature of this part of the Yucatán Peninsula is the large complex of submerged caves and sinkholes. Thousands of years ago, these caves and sinkholes served as shelters, and only later did they become inundated. In recent years, archaeologists have dared to dive to the bottom of these dark pools, an effort for which they’ve been suitably rewarded. To date, archaeologists have discovered 10 human skeletons in these underwater caves, including the new one, designated Chan Hol 3.

The Chan Hol 3 skull as it was found at the bottom of the submerged cave. The red arrow indicates a broken stalagmite nearby. (Image: W. Stinnesbeck et al., 2020/PLOS One.)

The story these fossils are telling is nothing short of extraordinary. Back in 2014, Tulum divers found the skeletal remains of a young girl in a cave called Hoyo Negro, which is Spanish for “black hole.” Using carbon dating, scientists dated these remains to 10,976 years ago. During the 2000s, archaeologists working in Naharon cave, also near Tulum, found a skeleton that was dated to 11,570 years ago.

These are some of the oldest human fossils to be found anywhere in the Americas—but there’s a major problem, and it has to do with the dating method used. Bones that have been submerged in water for a long time are stripped of their organic tissue, namely collagen. That makes carbon dating a precarious proposition at best.

Scientists analysing the skeleton at Liverpool John Moores University in the UK. (Image: Jerónimo Avilés Olguín)

Wolfgang Stinnesbeck, the first author of the new study and an archaeologist from Heidelberg University in Germany, used a different approach to date the Chan Hol 3 skeleton, which is 30 per cent complete.

“We used an indirect dating technique from physics,” Stinnesbeck told Gizmodo. “This method is based on the radioactive decay of uranium and its conversion into thorium. We dated the uranium-thorium isotopes of a lime crust that had grown on the finger bones when Chan Hol cave was still dry.”

The solid calcite crust that formed on the finger bones was caused by water dripping from the cave ceiling, according to the paper. Study co-author Norbert Franck and his team from the Institute of Environmental Physics at Heidelberg University performed the dating, coming up with a minimum age for Chan Hol 3 at 9,900 years old—with “minimum” being the key word. Clearly, the body had already become “skeletonized,” in the words of Stinnesbeck, before the crusts could appear, so the fossil is likely much older.

Similar encrustations appeared on Chan Hol 2, a skeleton previously found in the same cave. Stinnesbeck’s team used the same uranium-thorium technique to date this fossil back in 2015, coming up with a minimum age of 11,300 years but a likely age of 13,000 years, given the amount of crust seen on the skeleton. The Chan Hol 2 individual is thus one of the oldest skeletons to have ever been found in the Americas.

Other archaeological evidence from Chan Hol cave has produced similar timeframes. In 2018, the same team dated bits of charcoal from ancient fire pits, resulting in a date range between roughly 9,100 and 7,900 years ago.

“These charcoal concentrations are interpreted by us as ancient illumination sites,” Stinnesbeck told Gizmodo. “They provide strong evidence that the Chan Hol cave was dry and accessible and that humans used the cave for at least 1,200 years during the early and middle Holocene, before access was successively interrupted by global sea level rise and flooding of the cave system.”

Analysis of the Chan Hol 3 skeleton points to a woman who was around 30 years old when she died. A comparative analysis involving over 400 ancient skulls found across the Americas, including Tulum, revealed a “mesocephalic” skull pattern indicative of a round head. This stands in contrast to skulls found elsewhere, including those belonging to Paleoamericans from Central Mexico and North America, which feature “dolicocephalic” skull patterns indicative of long and narrow skulls. The Chan Hol 3 individual also suffered from tooth decay, likely caused by a sugar-rich diet. Dolicocephalic individuals don’t tend to have cavities and instead feature badly worn teeth, which is caused by chewing on tough foods.

Together, this evidence points to the presence of at least two physically distinct human groups who lived at roughly the same time in the Mexican region as the Pleistocene gave way to the Holocene. This presents one of two possibilities: At least two different groups arrived to North America from Eurasia, or the two groups are offshoots of a single group that arrived to the Americas but subsequently diverged and developed distinctive physical characteristics over time.

“In either case, the early settlement history of the Americas appears to be more complicated and may date back thousands of years earlier than commonly believed,” said Stinnesbeck, who pointed to his own work and a recent study co-authored by Ohio State University scientist Mark Hubbe as evidence. “In the absence of DNA data, nevertheless, we cannot say where these people originally came from and how they came to the Americas,” he said.

Indeed, the scenarios presented in the paper don’t preclude the possibility that other groups spilled into North America from Eurasia around the same time but weren’t connected to the two groups described in the new study. And the DNA evidence that does exist—scant as it is—points to a complicated story of ancestral splits, multiple migrations, and the reunification of diverged groups.

But the analysis of Chan Hol 3 doesn’t end there. This ancient woman endured skull trauma and disease.

Signs of trauma on the skull. (Image: W. Stinnesbeck et al., 2020/PLOS One.)

Evidence of at least three serious injuries were found on her skull. The woman appears to have been struck by a hard object, or multiple objects, which broke the bones in her head. It’s not clear if the woman died from these injuries, but no signs of healing were found on the skull. It’s “likely” that these wounds resulted in her death, but there’s “no positive evidence” to support this scenario, cautioned Stinnesbeck.

Chan Hol 3 also appears to have contracted a bacterial disease, as evidenced by dents and crater-like deformations on her skull. Specifically, she may have been infected with Treponema peritonitis, which can lead to osteitis (inflammation of bone) or severe periostitis (inflammation of connective tissue that surrounds bone). The researchers ruled out the possibility that these skull deformations were caused by erosion.

“They are thus of anthropological importance, in particular when it comes to the possibility that Treponema may be involved—a group of bacteria which causes syphilis,” said Stinnesbeck, who made it clear that “we did NOT present evidence for this disease [syphilis]” in the new paper.

The submerged caves at Tulum are steadily showing their immense value as archaeological sites. These chambers undoubtedly have many fascinating stories still to tell—we just have to dive right in.