(CN) – In the depths of the underground caves in Tulum, Mexico, flooded by bodies of water and layered in limestone rock, scientists have discovered ancient skulls that hold evidence of North America’s earliest populations. These mysterious skulls have provided remarkable data suggesting the early humans may have already had a high level of biological diversity, both internally and externally.
In a study published Wednesday in the open-access journal PLOS ONE, Mark Hubbe from Ohio State University, Alejandro Terrazas Mata from Universidad Nacional Autónoma de México and their colleagues discuss the high level of morphological diversity.
The debate about the origins of North America’s earliest human populations has raged for years, and has been ever-evolving. As new information, theories, and materials come forward, the consensus changes. However, because early human remains are a rare find in North America, the debate relies on relatively little evidence.
The coastal cave systems that held these ancient skulls sit along the Yucatan peninsula, a great big limestone shelf that was a cradle for Mayan culture. North of this peninsula would have been nearly uninhabitable with its infertile soil and lack of natural resources, forcing the Mayan people to quickly adapt.
Mostly flooded with freshwater reserves underneath the city of Tulum in the Mexican state of Quintana Roo, the caves contain at least eight different dig sites with discoveries of ancient human remains from approximately 8,000 to 13,000 years ago.
Tulum’s caves hold great historical significance. The entrance to these caves, called cenotes, were the main water source for many Mayans and a sacred site for rituals and sacrifices to pay tribute to their gods. The caves are home to hundreds of archaeological sites holding Mayan artifacts, remains of long-extinct animals, and secrets to North America’s first inhabitants.
The submerged cave system in question is comprised of the two most prominent systems in the region, the Sac Actun system and the Dos Ojos system. They were previously thought to have been separate until a diver discovered a narrow passageway connecting the two in 2018. Since then, it has been renowned as the longest underwater cave system in the world reaching about 216 miles long.
The four relatively well-preserved skulls were retrieved from different sites within this cave network, and after dating and scanning them, Hubbe and his colleagues used craniofacial morphology to compare these skulls with a reference dataset of worldwide modern human populations. This is a technique that examines the different parts of the head that enclose the brain and face and compares the data with the other morphological forms and structures.
The study authors found the skulls were unexpectedly diverse. The oldest skull showed close morphological similarities with modern arctic North Americans from Greenland and Alaska, but the second-oldest skull showed strong connections with modern European populations. Of the two remaining skulls, one seemed to show associations with Asian and Native American groups, while the second showed associations to populations in the Arctic as well as some modern day South American features. These astonishing findings are new for early American remains in terms of this type of reference comparison.
"Four ancient skulls discovered in the submerged caves of Quintana Roo, Mexico, show that early Americans had high biological diversity since the initial occupation of the continent," the authors add for clarification.
This study’s results are incredibly surprising, considering that there have been no preceding research projects or evidence that show this level of diversity. On the contrary, earlier work on South American remains has instead found consistent associations with modern Australo-Melanesian and African groups, as well as with Late Pleistocene specimens found in Europe and Asia.
The authors note that although these early North American residents may have been highly diverse, that diversity would have gradually reduced when some populations dispersed into South America. The results of this study further emphasize the need to pursue new archaeological evidence across the continent to piece together more complete models of early diversity, migration and dispersal across the Americas.
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