The mysterious and ancient ghost population known as "Y" Population mentioned in yesterday's post (and in several other posts over the past years) was explored in depth by Harvard geneticist, David Reich in his 2018 book Who We Are and How We Got Here: Ancient DNA and the New Science of the Human Past 🔓.
In Chapter 7 (starts on p. 155), dedicated to the original ancestors of the Native Americans, Reich explains the theories and timing of the peopling of America based on current archaeological and genetic data.
Reich tells how the one wave peopled South America mutated over the years, and now we believe that at least four separate migrations led to the current makeup of Native Americans. Of relevance is the section starting on p. 176 that looks into the Y population.
The basics are the following: Walter Neves was the first to propose (see post) base on skull morphology, that ancient Brazilian paleoindians, like those found in Lagoa Santa were very similar to those of Australian aboriginals and Melanesians from Papua New Guinea.
This was very controversial, and opposed by the mainstream anthropologists. However, Pontus Skoglund investigated this possibility and after looking into the genetics of different native people, "found two Native American populations, both from the Amazon region of Brazil, that are more closely related to Australasians than to other world populations... Skoglund found weaker signals of genetic affinity to Australasians, but still probably real, in other Native American populations ringing the Amazon basin. He estimated that the proportion of ancient ancestry in these populations was small—1 to 6 percent—with the rest being consistent with First American ancestry."
This pattern appeared not only in Skoglund's data (by the way, Skoglund was working with Reich as a postdoctoral student), but in other data sets collected by other scientists. Furthermore, they also showed that it wasn't a recent admixture with people from Australia or Papua New Guinea, nor the result of Polynesians carrying these alleles across the Pacific to America.
The remarkable findings are that:
- Similar yet not closely related: "while Amazonians had their strongest affinity to indigenous people from Australia, New Guinea, and the Andaman Islands (compared to East Asians as a baseline), they were not particularly close to any of them"
- It was highest among the Suruí, Karitiana, and Xavante groups, and almost absent elsewhere: "We found little or no Population Y ancestry in Mesoamerica or in South Americans to the west of the high Andes. We also did not detect Population Y ancestry in the almost thirteen-thousand-year old genome of the Clovis culture infant from the northern United States, or in present-day Algonquin speakers from Canada. The Population Y geographic distribution is largely limited to Amazonia, providing yet more evidence for an ancient origin" Reich attributes this limited distribution to an "original pioneering population that was once more broadly distributed and was then marginalized by the expansion of other groups."
- Two groups of Native Americans with skulls similar to those of Neves' Paleoindians were tested by other researchers, they were Pericués of Baja California, and Fuegians in Argentina, however, neither group carried the signal of Population Y alleles.
This evidence suggests different population possibilities to Reich: (1) Population Y reached America first, spread across the continent and later, when the "First Americans" (actually, the current ones, but not first, they were second) arrived they displaced and replaced the Population Y people "either completely or only partially, as in Amazonia. Population Y ancestry may have survived better in Amazonia than it did elsewhere because of the relative impenetrability of the Amazonian environment." So, in the Amazons the original Population Y was not razed, but admixed gradually with the newcomers.
The current levels of Population Y ancestry in the Amazonians may be small (~2%), Reich states that the impact of the ghost populations should not be dismissed. Reich proposes a second theory (2) in which he supposes that the Population Y people as they walked across Asia and Siberia into "northern North America where the ancestors of First Americans were also living. It is likely that Population Y was already mixed with large amounts of First American–related ancestry when it started expanding into South America. If so, then the ancestry derived from a lineage related to southern Asians is only a kind of “tracer dye” for Population Y ancestry—like the heavy metals injected into patients’ veins in hospitals to track the paths of their blood vessels in a CT scan. Our estimate of around 2 percent Population Y ancestry in the Suruí is based on the assumption that Population Y traversed the entirety of Northeast Asia and America without mixing with other people it encountered. If we allow for the likelihood that there was mixture with populations related to First Americans on the way, the proportion of Population Y in the Suruí could be as high as 85 percent and still produce the observed statistical evidence of relatedness to Australasians. If the true proportion is even a fraction of this, then the story of First Americans expanding into virgin territory is profoundly misleading. Instead, we need to think in terms of an expansion of a highly substructured founding population of the Americas. The history and timing of the arrival of Population Y in the Americas is likely to be resolved only with recovery of ancient DNA from skeletons with Population Y ancestry."
So, in this second scenario, the Y Population isn't the original one, instead it was part of the First People reaching America, in a very highly structured population which somehow (Reich does not explain the mechanism though!) had already admixed with other Native groups, yet did not leave any signal in North America or Central America, or anwyhere other than the Amazon region.
I agree that further studies are needed, and finding Population Y remains will be of capital intererst, but also, we need a theory proposing a clear mechanism, other than Bottlenecks and genetic drift, to explain how the Population Y signal was erased, and is absent among present day Amerindians all across America —except for these three Amazonian groups.
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