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Oldest direct evidence of humans in the Americas?

PLOS Blogs Humans and Primates PLOS ONE

Featured Image Credit: Nick Poole and Thomas Spamberg, Liquid Junge Lab. CC-BY.

When did humans really arrive in the Americas? It has been a subject of a lot of debate by archaeologists and paleoanthropologists, For decades, it was generally accepted by most that around 13,000 years ago, the Bering Strait opened up enough (as in, became ice-free) for humans to traverse across from Asia into North America before spreading southwards. But as more and more pre-Clovis archaeological remains were discovered, paleoanthropologists came to a consensus that human arrival in the Americas could have been as early as 22,000 years ago. A more recent study published earlier this year in Nature gave a new oldest estimate that is 100,000 years older than all previous estimates placing human presence in the Americas, but is based solely on markings on mastodon bones and not on actual human bones. Critics of that study suggest that the markings cannot be definitively attributed to humans and could have been made naturally by rocks or other erosional processes.

Regardless, the oldest claims for humans in the Americas is based on tools, artifacts, scraps, and very little is based on osteological remains. When bones are found, they are often very fragmentary. A new paper, published today in PLOS ONE by Stinnesbeck et al., discusses some well-preserved human remains that were discovered in a submerged cave system in the Tulum area of southern Mexico.

The remains were first brought to the attention of the researchers in 2012 through social media. From the photographs of the site, it was clear that about 80% of the skeleton remained intact and partially articulated, including a skull and limbs. Unfortunately, about a month after first publicized on social media, the site was vandalized, and most of the bones were stolen, leaving only about 10% of the skeleton, mostly scrappy parts that were inaccessible to the vandals.

A) The skeleton in situ prior to its theft and vandalism; B and C) photographs showing bones before and after the theft, the orange arrow indicates the stalagmite growing out of the pelvis used for dating the skeleton. From Stinnesbeck et al. (2017). CC-BY.

The researchers were able to reconstruct the position of the skeleton based on interpretation of the photos. And visiting the site, they were able to collect many bones and bone fragments. Of particular note is a part of the pelvis that was firmly attached to the substrate due to the growth of a stalagmite.

It was this stalagmite that became important in dating this skeleton. Other human remains have been discovered previously in Tulum, and were carbon-dated to about 10,976±20 y BP to 11,670±60 y BP. However, this new study suggests that the bioapatite used for previous studies should be used with caution, as it is susceptible to contamination.

Different views of the Chan Hol II pelvis within the CH-7 stalagmite. Arrow in Fig A points to the CH-7 stalagmite prior to the robbery of the skeleton. From Stinnesbeck et al. (2017). CC-BY.

Instead, Stinnesbeck et al. (2017) used U-series techniques to date the skeleton. The U/Th samples were taken from the stalagmite that was growing through the pelvic bone, with samples taken adjacent to the bone both above and below. Using mass spectrometry, the data indicates that the skeleton from the Chan Hol underwater cave is approximately 13,000 years old, thus representing one of the oldest ages of a human present in the Americas, based on definitive direct osteological evidence.

At the time during the Late Pleistocene, this cave would have been dry and accessible, with the sea level more than 100 meters below its current level. This skeleton, along with other remains found in the Chan Hol cave system, could represent an early human settlement along the sea. The karst labyrinth of caves were later flooded, thus preserving the archaeological and paleoanthropoligcal remains inside.

Though it is unfortunate that the site was looted, the results of this study based on what was left behind give clear evidence and possibly a new understanding of the earliest settlements in Mesoamerica.

Reference:

Stinnesbeck W, Becker J, Hering F, Frey E, González AG, Fohlmeister J, et al. (2017) The earliest settlers of Mesoamerica date back to the late Pleistocene. PLoS ONE 12(8): e0183345. https://doi.org/10.1371/journal.pone.0183345

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