Wednesday, 29 July 2015

Peopling of the New World remains contentious

New genetic studies reach differing conclusions

It is generally accepted that the humans first reached the New World by crossing the land bridge between Siberia and Alaska during the last Ice Age. However, the number of migrations and their timing has been debated for many decades.

The Paleoamerican model states that the earliest Americans or Paleoamericans were replaced by a second, separate wave of migrants from which today’s Native Americans are descended. The model is based on apparent differences in craniofacial morphology between some early fossil remains and more recent Native American. Note that this hypothetical second migration is distinct from the much later migrations responsible for around half of Aleut-Eskimo ancestry, and a tenth of Na-Dene ancestry.

Two new studies, published respectively in the journals Science and Nature, have reached opposing conclusions. Publishing in Science, Raghavan and colleagues analysed whole genomes of 31 present-day people from the New World, Siberia and Oceania, 23 ancient New World genomes and single nucleotide polymorphism genotypes from 79 present-day people from the New World and Siberia. The ancient DNA included samples from a 4,000 year-old Saqqaq individual from Greenland and the 12,600 year-old Anzick-1 (Clovis culture) individual from Montana.

They found that the ancestors of all present-day Native Americans, including Athabascans and Amerindians, entered the New World in a single migration from Siberia no earlier than 23,000 years ago and after no more than 8,000 years of isolation in Beringia. Around 13,000 years ago, these ancestral Native Americans diversified into two basal genetic branches: one that is now dispersed across North and South America and another restricted to North America. Subsequent gene flow resulted in some Native Americans sharing ancestry with present-day East Asians, including Siberians and, more distantly, Australo-Melanesians. But populations believed to be relict Paleoamericans including the Pericúes from Mexico and the Fuego-Patagonians, are not directly related to modern Australo-Melanesians, contrary to the predictions of the Paleoamerican Model.

The second study, published by Skoglund and his colleagues in Nature, featured genomic data from 63 Native Americans, who belonged to 21 diferent populations, and showed no discernable evidence of European or African ancestry. Results showed that some Amazonian Native Americans descend partly from a founding population with an ancestry more closely related to Aboriginal Australians, New Guineans and Andaman Islanders than to any present-day Eurasians or Native Americans. This genetic signature is not seen in present-day Northern and Central Native Americans, or in the Anzick-1 genome. The source population for this Australasian-related ancestry was named ‘Population Y’ after Ypykue´ra, which means ‘ancestor’ in the Tupi language family spoken by the Suruı´ and Karitiana.

The researchers suggested that Population Y had already admixed with a lineage related to First Americans by the time it reached Amazonia, and that it was the explanation for the differing craniofacial morphology noted above. However, no ancient DNA directly extracted from remains with this morphology, so the results did not prove that these people were Population Y. The absence of linkage disequilibrium in Population Y suggests that it arrived in the New World a long time ago. Furthermore, while it shows a distant genetic affinity to Andamanese, Australian and New Guinean populations, it is not particularly closely related to any of them, suggesting that its ultimate source in Eurasia no longer exists.

It is to be hoped that future ancient DNA studies provide further insight into the results of the Skoglund study.

Skoglund, P. et al., Genetic evidence for two founding populations of the Americas. Nature 525, 104-108 (2015).
Raghavan, M. et al., Genomic evidence for the Pleistocene and recent population history of Native Americans. Science 349 (6250), 841, aab3884-1-10 (2015).x

Thursday, 23 July 2015

Ohalo II ‘proto-weeds’ indicate attempts to cultivate wild cereals 23,000 years ago

Evidence of low-level food production at Epipaleolithic site

Ohalo II is a well-studied sedentary hunter-gatherer settlement on the shores of the Sea of Galilee. Associated with the Kebaran culture, the site dates to the Early Epipaleolithic period and was occupied around 23,000 years ago. The partially-excavated site is believed to cover an area of around 2,000 sq. m. (21,500 sq. ft.), and excavations have revealed the remains of six huts. Faunal remains suggest that the Ohalo II people hunted gazelle and deer, trapped hare and birds, and caught fish. From preserved botanical remains, no fewer than 142 different plant species have been identified, including emmer wheat, barley, brome and other small-grained grasses, acorns, almonds, pistachios, olives, legumes, raspberries, figs and grapes. These were collected from a range of habitats, including the nearby Mount Tabor.

In a newly-published report, archaeologists report the identification of 13 plant species now classified as weeds, mixed with large quantities of wild cereal seeds, including emmer, barley and oats. The presence of such species among cereals is considered to be one of the key archaeological indications of food production – in this case some 11,000 years before the onset of full-blown agriculture in the region.

That the Ohalo II people were harvesting wild cereal stands is supported by a study of glossed flint blades found at the site. The pattern of use-wear ‘sickle gloss’ polish observed on the sharp edges of these blades is consistent with their use to harvest wild cereals before they fully ripen and scatter their grain. Such a practice known from the later Natufian culture, but has not previously been documented for the Kebaran.  The blade also bears traces of hafting on the opposite side to the cutting edge, indicating that it was possibly a part of a sickle. Again, such tools are very rare in a pre-Natufian context.

However, the report suggests that these techniques were not carried on in later times, and they evidently represent a failed attempt at low-level food production. Sickle-harvesting did not come into widespread use until the Early Natufian around 8,000 years later, or 15,000 years before the present.

Snir, A. et al., The Origin of Cultivation and Proto-Weeds, Long Before Neolithic Farming. PLoS One 10 (7), e0131422 (2015).

Wednesday, 22 July 2015

Earliest evidence of dentistry in Late Upper Palaeolithic

Stone tools used to treat dental caries 14,000 years ago

Humans have been practicing dentistry for a surprisingly long time. The earliest dental filling, made from beeswax and dating to 6,500 years ago, was reported from Slovenia in 2012 and a bow drill was apparently used to remove decay from molar teeth recovered from a 9,000-year-old Neolithic graveyard in Pakistan. The increase in carbohydrate consumption in the Neolithic was accompanied by an increase in dental caries, and a need for dentistry. The drilling, cleaning and filling dental cavities is documented in ancient Egyptian texts, which confirm that the practice was established by at least the fifth millennium BC.

Evidence of a much earlier origin for the treatment of dental caries has now emerged from Epigravettian site of Riparo Villabruna in northern Italy. Researchers studied a lower right third molar from a 25-year-old male, originally recovered in 1988 and radiocarbon dated to around 14,000 years ago. They noticed a dental cavity that had apparently been cleaned and on investigating with an electron microscope they found V-shaped striations that appeared to have been caused by scraping.

The researchers then attempted to replicate the striations on recently-extracted third molars, using pointed tools made from wood, bone and microlith. The use of such tools as toothpicks is well documented from the Palaeolithic. It was found that the microlith tool produced grooves and ridges matching those found on the Villabruna tooth, which thus provides the earliest known example of dental surgery. The cavity had been dug with a flint microlith to remove the dental decay and presumably relieve toothache.

The Villabruna specimen suggests that there was at least some knowledge of dental disease treatment well before the Neolithic. This study suggests that early forms of carious treatment entailed an adaptation of a toothpick to lever and scratch out decay rather than the drilling practices of later times.

Oxilia, G. et al., Earliest evidence of dental caries manipulation in the Late Upper Palaeolithic. Scientific Reports (2015).