A human skeleton — probably one of the Americas' oldest — was stolen from the Chan Hol Cave in Mexico soon after it was discovered in 2012.  Credit: Nick Poole/Liquid Jungle

A human skeleton that was stolen from an underwater cave in Mexico in 2012 may be one of the oldest ever found in the Americas. Scientists have now put the age of the skeleton at more than 13,000 years old after analysing a shard of hip bone — left behind by the thieves because it was embedded in a stalagmite.

Cave divers discovered the remains in February 2012 in a submerged cave called Chan Hol near Tulúm on Mexico's Yucatán peninsula, and posted photos of a nearly complete skull and other whole bones to social media. The posts caught the attention of archaeologists Arturo González González at the Desert Museum in Saltillo, Mexico, and Jerónimo Avilés Olguín at the Institute of American Prehistory in Cancún.

By the time researchers visited the cave in late March, the remains were gone — except for about 150 bone fragments and a pelvic bone that had been subsumed by a stalagmite growing up from the cave floor. On the basis of these bones, the researchers think that the skeleton belonged to a young man who died when sea levels were much lower and the cave was above ground.

Dating techniques

To determine the age of human remains, researchers often measure levels of a radioactive isotope of carbon in collagen protein within bones. But in this case, most of the collagen had been leached out by water while the bones were submerged, making this method unreliable, says Wolfgang Stinnesbeck, a palaeontologist and geoscientist at the University of Heidelberg, Germany, who led the efforts to date the remains.

Instead, Stinnesbeck’s team collected a fleck of the pelvis bone and surrounding stalagmite, which contains a mineral called calcite. The team then dated the rock using the relative levels of uranium and thorium isotopes in the calcite. The deeper into the stalagmite the researchers sampled, the older the dates turned out to be; stone just 2 centimetres from the bone was 11,300 years old. Calcite closer to the bone gave conflicting results, Stinnesbeck says.

The team determined that the skeleton was older than 13,000 years by analysing the rate at which calcite had formed around the bone, and by matching the shifts in stalagmite isotope levels to those in other caves. The findings were published on 30 August in PLoS ONE1.

A diver collects a portion of a cave stalagmite found in cave that contains ancient human bones. Credit: Eugenio Acevez Nunez

Alistair Pike, an archaeological scientist at the University of Southampton, UK, notes that the stalagmite set over the bone during a time of profound climate change, which could have altered the stalagmite's rate of growth. He says he is therefore more comfortable considering the bones to be a minimum of 11,300 years old — still “very significant”, he notes.

Ancient company

Few other human remains from the Americas are older than 13,000 years. The skeleton of a teenage girl recovered from a different Yucatán cave was carbon-dated to more than 12,000 years old, and a skeleton found in another submerged cave near Tulúm was deemed to be around 13,500 years old, also using radiocarbon dating.

“They’ve done a really nice job determining the age of this thing,” says David Meltzer, an archaeologist at Southern Methodist University in Dallas, Texas. There is convincing archaeological proof that humans colonized the Americas before 14,000 years ago, but very old remains are precious. “These sites are rare as hen’s teeth,” Meltzer says.

Apart from the Yucatán finds, the next-oldest skeleton from the Americas is that of a 12,600-year-old boy found in Montana, whose sequenced genome places him on a lineage leading to present-day Native American groups. Researchers have sequenced only a few other human skeletons from the Americas that are older than 10,000 years, hindering efforts to unravel the region's ancient population history.

A stalagmite had grown around a shard of ancient pelvic bone (bottom portion). Credit: Wolfgang Stinnesbeck

Getting DNA from what remains of the Chan Hol skeleton will be hard. A sample sent to one of the world’s leading ancient-DNA labs, the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, did not contain enough DNA, Stinnesbeck says. He hopes to find DNA in the few teeth not taken by the thieves.

The theft still boggles Stinnesbeck, whose team is continuing to study the cave and its remains. The researchers recently reported the discovery of fossils in the cave that are of a new species of peccary2 — a hoofed mammal related to pigs — as well as evidence that the cave's human inhabitants made fires.

“What would you want with a skeleton? Would you take it home?” Stinnesbeck asks. “If they had known it was very old, maybe just to have a souvenir, to have something special.”

“We went to the police and they did some inquiries,” he adds. “They never came up with anything substantial.”