9,900-year-old human skeleton found in a Mexican underwater cenote cave illustrates the complexity of the first settlers in the Americas.

New research published yesterday in the journal PLOS One details the discovery of a 9,900-year-old human skeleton, Chan Hol 3, found submerged in the Chan Hol cave, near the Tulum archaeological site in Quintana Roo state, on Mexico’s Yucatán Peninsula.

The skeleton belonged to a woman who had died in her 30s and she is referred to in the paper as one of the “first people to set foot in the Americas,” and her remains prove this region was inhabited by at least two different groups of early Mesoamerican settlers at least 8,000 years before the Maya culture first emerged.

Fingerprints of Ancient Violence

The Yucatán Peninsula is maze of submerged caves and sinkholes (cenotes), which before filling with water served America’s first settlers as shelters and archaeologists have now discovered 10 human skeletons in these underwater caves, including the newly discovered Chan Hol 3. According to a 2014 Gizmodo article, Tulum divers discovered the skeletal remains of a young girl in a cave called Hoyo Negro (black hole) dating to 10,976 years ago. Additionally during the 2000s, archaeologists working in Naharon cave, near Tulum, found another skeleton that radiocarbon dated to 11,570 years ago.

This new study not only successfully dates the ancient woman’s skeleton, but it shows she had suffered a bacterial disease, which had caused pitting and deformations on her skull. Furthermore, she had sustained three serious head injuries inflicted with a hard object, or multiple objects, that shattered the bones in her skull.

Searching For Thor

The woman’s skeleton is among the oldest human fossils to be found anywhere in the Americas, but the scientists identified a “major problem” with the usual radiocarbon dating method. Dr Wolfgang Stinnesbeck, the first author of the new study and an archaeologist from Heidelberg University in Germany , said in the paper that bones that have been submerged in water for thousands of years lose much of their “collagen”, which is the most abundant protein in the human body that holds the body together, and without collagen, accurate carbon dating is virtually impossible.

The Chan Hol 3 skeleton is “30 percent complete” and Dr Stinnesbeck told Gizmodo that his team had used an “indirect dating technique from physics” based on the radioactive decay of uranium and its conversion into thorium, which is a naturally-occurring radioactive metal discovered in 1828 by the Swedish chemist Jons Jakob Berzelius , who named it after Thor, the Norse god of thunder.

The uranium-thorium isotope samples were taken from a solid calcite (lime) crust that had formed on the skeleton’s finger bones, having dripped from the cave ceiling at a time the Chan Hol cave was still void of water.

In 2018 the same team of scientists gathered charcoal samples from ancient fire pits dating to about 9,100 and 7,900 years ago. This provided evidence that the Chan Hol cave was free of water and that humans used the cave for living in for at least 1,200 years during the early and middle Holocene, before a rise in global sea levels, which eventually flooded of the cave system. The study’s co-author, Norbert Franck, and his team of researchers from the Institute of Environmental Physics at Heidelberg University dated Chan Hol 3 to being a “minimum of at least 9,900 years old” and according to Dr Stinnesbeck, because the body had already become skeletonized, before the crusts formed, the fossil is likely “much older.”

Analyzing Skull Patterns

A comparative analysis of over 400 ancient skulls found across the Americas indicates what the scientists call a “mesocephalic”, or round headed, skull pattern which is different to skulls of Paleoamericans from Central Mexico and North America, which are longer and narrower skulls (“dolicocephalic” skull patterns).

In conclusion, this observation tells scientists “at least two physically distinct human groups” lived at roughly the same time in the Mexican region as the Pleistocene gave way to the Holocene. But it is unclear whether two different groups arrived in North America from Eurasia at the same time, or the two groups emerged from a single group and developed distinctive physical characteristics over time.

A recent study , co-authored by Ohio State University scientist Mark Hubbe, said that in the absence of DNA data, nevertheless, we cannot say where these people originally came from and how they came to the Americas. But the little DNA evidence they gathered suggests a complex series of “ancestral splits, multiple migrations, and the reunification of diverged groups.”

With modern technology, submerged cave systems like those in Tulum are beginning to share their archaeological secrets. Even without, DNA scientists can study proteins, and it is these little building blocks of life that are divulging their fossilized truths regarding the first people to populate the Americas.