A prehistoric human skeleton found on the Yucatán Peninsula in southern Mexico is at least 10,000 years old and most likely dates from the end of the most recent ice age, the late Pleistocene. An international research team led by geoscientists from Heidelberg University studied the remains of the approximately 30-year-old woman. The uranium-thorium dating technique was used to determine the age of the fossil record, which provides important clues on the early settlement history of the American continent.
The skeleton was discovered near the city of Tulúm in the Chan Hol cave, which is now water-filled as the result of global warming and sea-level rise approximately 8,000 years ago. Nine other prehistoric skeletons had already been discovered in this intricate submerged cave system near the coast in the eastern part of the peninsula. According to Prof. Dr Wolfgang Stinnesbeck, the leader of the research team, not all of the ten skeletons were complete, but they were well preserved. They offer valuable archaeological, palaeontological and climatic information about the American continent and its first inhabitants, the Paleoindians. The Tulúm skeletons exhibit round-headed - mesocephalic - cranial characteristics different to the long-headed - dolicocephalic - morphology of Paleoindians from Central Mexico and North America, explains Prof. Stinnesbeck, who teaches and conducts research at the Institute of Earth Sciences of Heidelberg University.
To the researchers, the head shape is an indication that two morphologically different groups of Paleoindians must have lived in America at the same time. They may have reached the American continent from different geographical points of origin. Or a small group of early settlers may have been living in isolation on the Yucatán Peninsula and developed a different skull morphology over a short period of time. Prof. Dr Silvia Gonzalez and Dr Sam Rennie, both from Liverpool John Moores University (Great Britain), suggest that the early settlement history of the Americas is therefore more complicated and may date back earlier than commonly believed.
The woman's remains were recovered by Mexican divers Vicente Fito and Iván Hernández and then documented. She was approximately 30 years old at the time of her death. Her skull had multiple injuries, but they may not have been the cause of death. The researchers also discovered signs of a potential treponemal bacterial infection that caused severe alteration of the cranial bones. Like the other Tulúm skeletons, the woman's teeth had cavities, possibly due to a diet high in sugar. In contrast, the teeth of most Paleoindian skeletons from Central Mexico and North American are worn down and cavity-free, suggesting they ate hard food.
To precisely date the find, the researchers used a dating method from physics based on the radioactive decay of uranium and its conversion into thorium. The researchers dated the uranium-thorium isotopes of a lime crust that had grown on the finger bones in the originally dry Chan Hol cave. Prof. Dr Norbert Franck and his team from the Institute of Environmental Physics of Heidelberg University were able to give the skeleton a minimum age of 9,900 years. However, the body was then already skeletonised and the prehistoric find may be older.
In 2017, Wolfgang Stinnesbeck and his team of researchers had already documented another human skeleton from Chan Hol cave, which was then considered to be 13,000 years old based on a stalagmite that had grown on its hip bone. For the researchers, these bone finds prove the unexpectedly early settlement of southern Mexico. Scientists from Germany, Great Britain, and Mexico took part in the research, which was funded by the German Research Foundation (DFG) and the German Federal Ministry of Education and Research (BMBF). The results of the research were published in the journal "PLOS ONE".