The skeleton of a woman discovered in a flooded Mexico cave suggests our ideas about ancient people in America might need some rethinking. 

The skeleton is like few others seen in America at the time, only comparable to a handful of other skulls from the murky cave system. Was she part of a small and anomalous group? Or was the early migration into America a lot more diverse than previously assumed? 

Back in 2016, two explorers found a skeleton in the submerged Tulum cave system of Mexico's Yucatán Peninsula. Researchers led by Universität Heidelberg in Germany started to study the skeleton, dubbed Chan Hol 3, and discovered that it most likely belonged to a woman who died at around 30 years of age some 9,900 years ago.

We know a little bit about the earliest settlers of Yucatán thanks to nine other human skeletons found in other caves and sinkholes in Tulum. Curiously, the new skull – just like the others found in Tulum – has some notable features that separate them from other Paleoindians found in Mexico from the time. 

The new study, reported in the journal PLOS One today, used a non-invasive dating method and took craniometric measurements, then compared her skull to 452 skulls from across North, Central, and South America, including a number of other skulls found in the Tulum caves. 

Their analysis showed that the woman’s skull was much like the other skeletons found in Tulum, with broad cheekbones and a flat forehead. However, this is different from other skulls found from this time in America, which tend to have long and narrow features. Additionally, all Tulum cave skulls had cavities in their teeth, possibly hinting at a different diet with more sugar. On the other hand, skeletons from elsewhere in Mexico have worn teeth without cavities. 

"The Paleoindians in the Americas were not a single homogeneous group travelling North to South into the Americas," study author Professor Silvia González, from the Liverpool John Moores University, told IFLScience.

"We have found that there are two morphologically different human populations, with all the other Paleoindians from the Americas having long and narrow faces but the Yucatan individuals are different and are characterized by a flat forehead with wide cheekbones. These morphological traits have been detected in all pre-Mayan skeletons discovered in the Tulum cave system," continued Professor González.

"Sadly, Chan Hol 3 was poorly preserved with almost no organic matter left in the bones and teeth, making it impossible for any ancient DNA analysis," added study author Dr Samuel Rennie, a former PhD student at Liverpool John Moores University, UK.

The full story of how humans settled in America remains mysterious. However, combined with other recent research on the Tulum cave skulls, it’s starting to look like the early populations of Mexico were not uniform. Instead, the Chan Hol 3 woman further shows that at least two different groups, perhaps more, reached the Americas and settled here.

"The Tulúm skeletons indicate that either more than one group of people reached the American continent first, or that there was enough time for a small group of early settlers who lived isolated on the Yucatán peninsula to develop a different skull morphology," the authors conclude. "The early settlement history of America thus seems to be more complex and, moreover, to have occurred at an earlier time than previously assumed."