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GENETICS

The controversial conquest of the Americas

Data analysis suggests that humans reached the continent between 23,000 and 15,000 years ago and that some indigenous people in Brazil carry the DNA originating in peoples of Oceania

052-055_Colonização_234In late July, a controversial subject returned to the headlines: the arrival of human beings in the Americas. On July 21, 2015 two independent teams published their studies in rival scientific journals, Science and Nature, comparing the genetic characteristics of Native American populations with those of populations in other regions of the globe. The studies analyzed the broadest collection of genetic information available on peoples of the New World in an effort to reconstruct the story of the occupation of the last continent, not including Antarctica, in which Homo sapiens gained a foothold. The articles reached apparently different conclusions, but both suggested that a few present-day indigenous groups in Brazil are related in some way to the peoples of Oceania.

In the study published in Science, the group led by biologist Eske Willerslev of the University of Copenhagen in Denmark says that the first humans arrived in the Americas in a single migratory wave. They would have left East Asia at some time in the last 23,000 years and reached the New World after spending almost 8,000 years in Beringia, a vast land bridge (underwater today) that connected Siberia in Asia to Alaska in North America.

Willerslev and his colleagues – including archeologist Niède Guidon of the Museum of the American Man Foundation (FUMDHAM) in the Brazilian state of Piauí – reached this conclusion after sequencing the genomes of 31 individuals from 11 present-day native populations in the Americas, Siberia and Oceania and comparing these data with those of the genomes of 23 individuals from extinct populations in North and South America and with the genetic variations of another 28 populations.

The results suggest that, once this ancestral population arrived in the New World, it separated into two groups about 13,000 years ago. One group allegedly stayed in the northern part of the continent, giving rise to the Athabascan people in Alaska and the Chipewyan, Cree and Ojibwa indigenous groups in Canada. The other group spread to the southern part of North America and to the remainder of the continent, leading to the creation of most other ethnic groups there.

Even with more data available, Willerslev’s proposal is not entirely novel. In the last 15 years, other groups, including some from Brazil, had already suggested that the first humans to come to the Americas could have come from East Asia in a single movement, even if they stopped in Beringia (see Pesquisa FAPESP Issues nº 77 and nº 149). Both the proposal published in Science and its previous versions challenge earlier ideas, according to which two, three or even multiple waves would have been necessary to account for the genetic diversity and variety of skull features found in the Americas (see infographic).

Like most studies that refer to a single wave of migration to the American continent, Willerslev’s model provides a good explanation for the emergence in the Americas of native peoples whose genetic profiles are closer to those of present-day Asians, with whom they share some anatomical features, like a flatter face and a rounded skull. But the theory falls short in other respects. The idea of a single wave makes it hard to explain, for example, the genetic similarities between the Suruí Indians of the Brazilian Amazon, the Athabascans and natives of the Aleutian Islands in Alaska, and the native peoples of Oceania in the South Pacific.

Different, but how different?
Willerslev and his colleagues believe that these data might suggest that there was an exchange of genes subsequent to the initial peopling. A smaller wave of individuals related to the aborigines of Australia and Melanesia would have mixed with Asian populations that subsequently entered the Americas, possibly from the Aleutian Islands.

While Willerslev and his collaborators think in terms of one wave, possibly followed by a second, geneticist David Reich and his colleagues at Harvard University say they have evidence that two different populations had to have arrived at different times to explain the genetic diversity of Native Americans. Are both groups saying essentially the same thing? Well, yes and no.

In the article in Nature, Reich and his collaborators, four of whom are Brazilian, argue that only the arrival of two waves of people with distinct characteristics would help to explain why the Suruí and other indigenous groups in Brazil have a genetic similarity to peoples of the South Pacific. But that is not the whole story.

The study published this year fine-tunes a previous study. In 2012, Reich and researchers Maria Cátira Bortolini and Francisco Salzano at the Federal University of Rio Grande do Sul, Maria Luiza Petzl-Erler at the Federal University of Paraná, and Tábita Hünemeier at the University of São Paulo (USP), along with others, compared approximately 365,000 sections of the genomes of 493 individuals from 52 native populations in the Americas with those of 245 members of 17 groups in Siberia and those of 1,613 people from 52 populations from other parts of the globe. At the time, in an article also published in Nature, they concluded that the Americas were populated by three different migratory waves. The first and largest consisted of individuals with Asian genetic characteristics and features, who arrived in the Americas via Beringia at least 15,000 years ago and from whom most of the extinct and present-day populations in the Americas derive. The second wave, upon interbreeding with the first, led to the Eskimos in Greenland and the inhabitants of the Aleutian Islands in Alaska. Upon arrival, the third wave mixed with the first inhabitants of the continent, leading to the emergence of indigenous groups in Canada.

Now, after analyzing more sections of the genome (about 600,000) from more native populations (25) from Central and South America and comparing the data with those of 197 populations from other parts of the world, they found something similar to Willerslev’s observation and proposed one more migratory wave – a fourth, which would have reached South America more than 6,000 years ago – to explain the continent’s ethnic diversity.

In addition to the Suruí, who live in the Amazon Forest in Rondonia State, this more recent migration would have given rise to the Karitiana people of Rondonia and the Xavante Indians of the Cerrado in Mato Grosso State. These three groups share 1% to 2% of their genome with the peoples of Oceania. “This seems like a small number, but it’s significant,” says Hünemeier. “We have to imagine that it was much higher in the ancestral population that came to the Americas and then became diluted through hundreds of generations,” she explains.

Children of Ypykuéra
This most recent migratory wave – the fourth, according to the 2012 article or the second, according to the 2015 one – would not have consisted of individuals with genetic characteristics exclusive to the peoples of the South Pacific. These travelers were descendants of a mixed race population, resulting from the crossbreeding of native peoples from Oceania with those from Asia. Researchers called this group Population Y, from the first initial of the word Ypykuéra, which means ancestral in the Tupi language. Reich and his Brazilian collaborators believe that present-day Suruí, Karitiana and Xavante peoples are descendants of Population Y, but they do not yet know how they got to Brazil.

“The fact that the Suruí, Karitiana and Xavante share genetic characteristics with peoples of the South Pacific suggests that some miscegenation occurred in a larger area than originally believed,” says Hünemeier, co-author of the articles in Nature. Indeed, the first two groups live in Rondonia, in the Amazon Forest, hundreds of miles from the Xavante, who live in the savannahs of Mato Grosso. In addition to the physical distance, there are also cultural differences. The Karitiana and Suruí speak Tupi, while the Xavante speak Jê. “The miscegenation must have occurred earlier than 6,000 years ago, when these languages branched off from each other,” says Hünemeier.

The possible existence of a Population Y came as no surprise to some physical anthropologists who study the occupation of the Americas. “The results published in Nature indirectly support the idea I have been promoting for 25 years, which is that of two migratory waves to the Americas,” says bioanthropologist Walter Neves at USP. Based on an analysis of skull morphology from present-day and extinct populations from different regions of the Americas – including one that lived between 12,000 and 7,000 years ago in the region of Lagoa Santa in Minas Gerais  – Neves and Argentine archeologist Héctor Pucciarelli proposed that the Americas were colonized in two migratory waves: the first was 14,000 years ago and brought individuals with a morphology similar to that of the native peoples of Africa and Australia, followed by a second wave of people with Asian features who would have replaced the first group.

Rolando González-José, a physical anthropologist and director of the Patagonia National Center in Puerto Madryn, Argentina, believes the evidence presented in the Science and Nature articles is similar in some respects. “The connection to the Australian-Melanesians they found had been expected for years, and it points to the fact that the populations of the Americas share common ancestors,” says González-José. “The data are interesting, but the articles don’t really address them nor do they take into account all the possible explanations for them.” In 2008, González and his Brazilian colleagues put forth a theory that the Americas were colonized by individuals with a high degree of genetic diversity and differing skull morphologies, followed by a smaller group that gave rise to the Eskimos. According to this version, challenged about three years ago by Walter Neves, there would have been contact between the peoples of Asia and those in the Americas during the thousands of years in which Beringia existed.

Scientific Articles
SKOGLUND, P. et al. Genetic evidence for two founding populations of the Americas. Nature. July 21, 2015
RAGHAVAN, M. et al. Genomic evidence for the Pleistocene and recent population history of Native Americans. Science. July 21, 2015.

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