First Pioneers: Hartley, NM and the mysterious 'Population Y'

Last week, a particularly spectacular discovery was published. Archaeologists working in the arid sticks of New Mexico had explored what was thought to be a Clovis-period mammoth-butchering site, only to reveal something else: that the lowest layers of mammoth bones, bearing traces of butchery marks and other modification, dated to circa 37,000 BP. To put this in context, the other oldest confirmed site in the Americas is only 22,000 years old, also in New Mexico, in the form of ghost footprints.

Although it is not clear-cut evidence for direct human exploitation - the bones may have been scavenged well after the mammoths met their end - the presence of bone micro-flakes, localised burnt sediments and percussion-marked lithics all but definitely shows that people were around in New Mexico at the time. Given the sun-drenched state is thousands of miles from the coast, this raises the question: when and where did this mysterious group first originate? 

Ancient and modern genomes from Australasia, south China and the Americas make clear that an enigmatic 'Population Y' arrived in the New World long before the classic First Americas, i.e. the bearers of the Clovis Culture and its Siberian predecessors, crossed the Bering land bridge. The latest genomic models suggest this took place in or around 55,000 years ago, as a scattering of isolated family groups hailing from southeast Asia. To the authors of the new research, the Hartley mammoth site backs up claims of cultural occupation for sites like Monte Verde I in southern Chile (~33,000 BP), Chiquihuite Cave in Mexico (~26,000 BP), and Bluefish Caves in the Yukon (~24,000 BP) depicting them all as markers of Population Y. 

How did the First Pioneers arrive in the New World? The Bering Land Bridge, or Beringia, a stretch of steppe grassland between Alaska and eastern Siberia, was submerged for most of the time between 60 and 30ka. It was, however, around for a period of ten thousand years before that. This could indicate that Population Y crossed into modern Alaska ~60ka. As we can assume that they were modern Homo Sapiens, as their genetic legacy is found across the Pacific and preserves few 'archaic' genetic features, that limits their arrival date between 60 and 50,000 years ago. Realistically speaking, unless Population Y were making a beeline for new lands, it would have taken them tens of thousands of years to get from south Asia to the Americas. There were and are plenty of rich habitats to exploit in the areas between, so there would not have been a hurry to find greener pastures.

Beringia, during the Last Glacial Maximum (33,000 - 16,000 BP)

On the other hand, while they must have been the first human Pioneers, we can't be sure that they were the first hominid to reach the New World. Theories about Bigfoot aside, two sites in the Americas have unbelievable old and seemingly reliable dates. The oldest is the Hueyatlaco site of the Valsequillo Basin, another simple butchery site which has been dated by an array of scientific methods to 250,000 BP, or according to others younger than 220,000 BP. The second is the cut-marked mastodon fragment from Cerutti, California, unearthed during road construction, which was subsequently dated to ~130,700 BP. These are both very rudimentary sites, preserving only butchery marks which did not require a huge amount of brainpower to make. Regardless, both sites are hard to explain as the result of rodent or small mammal action. 

While Hueyatlaco and Cerutti can't be compared directly, as one is a single bone while the other is a jumble of bones and dubiously-deposited stone tools, their shared deep antiquity makes it possible, if they are considered genuine, that they were created by the same species of Homo, which was definitely not Homo Sapiens. The two sites are also both close to the coast. So who where these early adventurers? 

Monte Verde under excavation by Tom Dillehay and colleagues in the 1980s, which discovered burnt sediments and worked bones dating to ~33,000 BP.

A prime candidate is the Denisovans. The Moyjil midden site in Victoria state, Australia dates similarly to 125,000 BP, and is fairly credible. Genomic sequencing lies on the potential that Denisovans had reached Papua New Guinea before this, based on the high percentage of Denisovan DNA (5%) in modern Papuans. Reaching Pleistocene Australia, or Sahul, would have required substantial watercraft and navigation knowledge; the same that would have been essential in crossing the watery Bering land bridge around 200,000 years ago. Denisovans are not really associated with any landmark lithic technologies (the exception being in Denisova Cave itself), and certainty did not need a Clovis-like toolkit to survive out in these early frontiers. Their genetic adaptation to low-oxygen environments, which was inherited by modern Tibetans, would have helped them out considerably.

To sum up, we could be dealing with two lost population groups: Population Y, arriving around 60ka and surviving in the Americas until being assimilated by Clovis peoples, and 'Population X', a ghostly lineage of Denisovans that probably died out tens of thousands of years before modern Homo Sapiens arrived in the New World. It is also possible that the very crude stone tools found at Pedra Furada, in the thick of the Brazillian rainforest, were made by an earlier hominim descended from Denisovans. 

Tools from Pedra Furada, Brazil. Image Credit: Boëda et al (2014)

To say anything definitive would require dozens of more sites from the Population X and Y categories. After all, extraordinary claims require extraordinary evidence. Probably the main reason only a handful of sites exist so far is that the vast majority of activity happened in now submerged landscapes, particularly near to Beringia and the Pacific northwest. That being said though, the Hartley bones were found by chance, sticking out of a sandstone ledge out in the wilderness, so a systematic survey of the region might tease out several more. 

References:

Bennett, M. R., Bustos, D., Pigati, J. S., Springer, K. B., Urban, T. M., Holliday, V. T., et al. (2021). Evidence of humans in North America during the Last Glacial Maximum. Science 373, 1528–1531.

Boëda, E., Ramos, M., Pérez, A., Hatté, C., Lahaye, C., Pino, M., et al. (2021). 24.0 kyr cal BP stone artefact from Vale da Pedra Furada, Piauí, Brazil: techno-functional analysis. PLoS One 16:e0247965. doi: 10.1371/journal.pone.0247965

Chatters, J. C., Potter, B. A., Prentiss, A. M., Fiedel, S. J., and Haynes, G. (2021). Evaluating Claims of Early Human Occupation at Chiquihuite Cave, Mexico. PaleoAmerica 8, 1–16.

Holen, S. R., Deméré, T. A., Fisher, D. C., Fullagar, R., Paces, J. B., Jefferson, G. T., et al. (2017). A 130,000- year-old archaeological site in southern California, USA. Nature 544, 479–483. doi: 10.1038/nature22065

Metcalfe, S. E., Leng, M. J., Kirby, J. R., Huddart, D., Vane, C. H., & Gonzalez, S. (2016). Early–mid Pleistocene environments in the Valsequillo Basin, Central Mexico: a reassessment. Journal of Quaternary Science, 31(4), 325-336.

Rowe, T. B., Stafford Jr, T. W., Fisher, D. C., Enghild, J. J., Quigg, J. M., Ketcham, R. A., ... & Colbert, M. W. (2022). Human Occupation of the North American Colorado Plateau∼ 37,000 Years Ago. Frontiers in Ecology and Evolution, 534.