The genetic evolution of the indigenous South Americans in the course of the millenniums is unveiled in the model that geneticists Eduardo Tarazona, Sérgio Pena and Fabrício Santos, of the Federal University of Minas Gerais (UFMG), started to draw up in 1999. The model suggests that the indigenous populations of the west and the east of South America differentiate themselves by having followed opposite patterns in their genetic history. The researchers also concluded that the natives from the three Americas have the same well defined origin: two Siberian peoples whose lineage still survives.
They also confirmed the current theory, that the ancestors of the Amerindians arrived from Asia by the Bering strait, when there used to be a strip of firm ground there. And they showed that all the Amerindians have a great genetic similarity: which is why they believe that they came together, in a great migratory wave. A basic finding inspired the study: genetically isolated populations preserve the genetic identities that they had before the great migratory movements that took place after the great maritime expeditions of the 16th century. That is why the geneticists are interested in these isolated populations – like the Eskimos, the Yanomami from Brazil and Venezuela, and even the Berbers from the Sahara and the Finns – and are trying to trace their origins and migratory routes, which, in general, history was not able to record.
The study shows that the contributions from the Human Genome Project may serve not only the area of medicine, but also to discover aspects of our past: “It is the first more detailed study, from the genomic point of view, of the native populations of South America, suggesting a model that is coherent with the historical, archeological, linguistic and climatological issues”, explains Fabrício Santos, of the Institute of Biological Sciences at the UFMG, who is advising Tarazona guidance on his doctorate. To arrive at the evolutionary model proposed, the researchers opted for studying the molecular variability of the Y chromosome in South-Amerindians.
This chromosome is from the paternal lineage: transmitted only from the father to children of the male sex, it goes unchanged through the generations, until a mutation occurs (a variation in the DNA, the deoxyribonucleic acid that carries the genetic code). They wanted to use this to discover what had happened since settlement began. “We are doing a sort of molecular archeology”, says Tarazona, “which is possible because the demographic vicissitudes that a population undergoes leave a mark on the distribution of its genes. Through an analysis of the DNA, we can manage to identify these marks, which tell us what happened.”
Fabrício Santos explains that, to start with, they were looking for a genetic answer for the relationship between the peoples of the Andes and those of other regions of South America. Until then, the major part of the studies on molecular variability in native populations was directed in such a way as to answer only when and how the first peoples reached the American continent. This time, the researchers tried to go into greater depth.
To do so, they studied Andean groups from Peru and Ecuador. And they added the data from these groups to those of indigenous Brazilians – the Xavante, Wai-Wai, Karitiana, Ticuna, Gavião, Zoró and Suruí groups – previously studied by Denise Carvalho Silva (from the same group from UFMG), as well as from the Argentinean and Paraguayan tribes already studied by the group of Nestor Bianchi, of Buenos Aires. From the analysis of the samples of DNA of 192 individuals, from 18 indigenous groups from seven countries, they concluded that the populations of the east and of the west followed opposite patterns of demographic behavior, which is reflected in the genetic differentiation.
Accordingly, in the Andean region, the indigenous groups have large populations and have experienced greater levels of gene flow amongst themselves, the exchanges of genetic material through cross-breeding. On the one hand, this implies a trend towards homogenization at the general level, and, on the other hand, a greater genetic differentiation amongst individuals from the same population. The opposite happened in the east, with the groups from Amazon, the Central Brazilian Plateau and the Chaco. These have smaller populations and low levels of gene flow from one group to another. The result of this is tendencies towards many groups that are isolated and genetically differentiated, as well as to homogeneity within each group.
Therefore, the Andean populations, despite being numerous and geographically distant from each other, experienced an intense gene flow and kept a common cultural identity, sharing customs and a language, Quechua; other languages from the region, like Aimara and Araucan, are very similar to the dominant Quechua and belong to the same linguistic family. Within each tribe, there is a lot of genetic differentiation, and, overall, there are many similarities between groups that live up to over 3,000 kilometers from each other, from Peru to the north of Argentina.
The tribes from the Brazilian regions and the Chaco, though, show characteristics that are opposite to those from the Andes. They are physically closer than, for example, the Andeans from the north and those from the south. Given their mutual isolation, however, and in spite of the closer proximity, they are far from having a similar culture, they speak very different languages, and show little genetic differentiation amongst the individuals from each tribe.
Glaciers and forests
Paleoecological, linguistic and historical data blend well to provide the foundations for the proposed model. For example, in the last Ice Age – which lasted from 60,000 to 13,000 years ago -, the altitude of the glaciers in the Andes was a lot lower and the cold far more intense, which limited settlement. In the east, though, the open surroundings of the savanna predominated, practically without any closed forests, which for some time favored communication and gene flow.
12,000 years ago, however, the transition between the Pleistocene period and the Holocene took place, and this brought radical change. In the Andes, the level of the glaciers rose considerably, allowing wide scale human colonization, which favored the homogeneous cultural development that is evident up until now. “The glaciers”, says Tarazona, “set the Andes free, allowing human populations to settle and develop in common a cultural complex – and a biological one, according to the study -, which facilitated the migrations.”
In the east, though, the climatic changes caused the expansion of the isolated refuges of the tropical forest, which went on occupying and closing the open spaces of the savanna, until the immense Amazon Forest was formed – dense forest that, unlike the savanna, came to limit the gene flow. This resulted in fragmentation in the population and isolation in culture. The Pleistocene/Holocene transition was, then, like a kind of evolutionary interrupter, determining divergent patterns of genetic variability. That is why the Andean peoples from far distant areas are more alike than, for example, the Ticunas from the Amazon and the Suruís from Rondônia.
The group is expanding the studies and testing its model, based on the variations of the Y chromosome. The next step will be to compare this data with that of the mitochondrial DNA, which is transmitted by the maternal lineage, from the mother to children of both sexes. The studies of mitochondrial DNA are receiving the assistance of Silvia Fuselli and Davide Pettener, of the University of Bologna, Italy, where Tarazona did a doctorate in anthropology. According to Tarazona, the results are highly consistent with the model.
In fact, the model of the variability of the Y chromosome is the third stage of the study that Fabrício Santos started in 1993, when studying for a doctorate at UFMG, under the guidance of Sérgio Pena. Santos tells how the group has important contributions by means of the study of the paternal lineages: “Our first publications, in 1995 and 1996, revealed a genetic identity between the native peoples of the three Americas, as if they were all descendants of a single father – the American Adam -, who had lived between 12,000 and 25,000 years ago. In 1999, another publication revealed the genetic portrait of the ancestors of the native Americans who inhabited Asia between 20,000 and 30,000 years ago. It is as if we were doing a series of paternity tests involving thousands of past generations.”
The initial studies have confirmed the current theory, that the ancestors of the native Americans arrived at the continent between 40,000 and 13,000 years ago – there is much disagreement over the dates. But they have limited the period of the arrival to a narrower range, between 15,000 and 30,000 years ago. They have also confirmed that the Amerindians came through Beringia, the strip of firm ground that at the time of the ice age linked Asia to Alaska, where the Bering Strait is today (Hence the name Beringia). But they went further: they showed that the natives of the three Americas are very similar genetically, despite the strong linguistic and cultural diversity, which suggests that they all came together from Asia in one main migratory wave.
Going further still, they concluded in 1999 that the ancestors of the native Americans were Siberian peoples from the Keti and Altai linguistic groups. The Altais are from the great group of Turkish languages, the same one as that of the Mongolian and Japanese peoples. The Ketis, though, consist of an isolated linguistic group, with no similar one in the world, and whose original language is practically extinct. In some stages, the group researched with teams from other universities in Brazil (in particular, Francisco Salzano’s group, from the Federal University of Rio Grande do Sul) and from other South American countries, from Mongolia and from England.
Then in 1995, Santos and Sérgio Pena published in Nature Genetics the work entitled Main Founding Effect in the Indigenous American Populations. In the following year, a group from Stanford University (USA) confirmed the Brazilians’ conclusions and used a new variation of DNA, DYS199, which has two different bases: allele C, present in all Europeans, Asians and Africans, and T, which is characteristic of the Amerindians.
Afterwards, other Americans, from Tucson University, also confirmed the Brazilians’ data, which in 1999 indicated the Ketis and the Altais, among various Siberian groups examined, as those with the greatest degree of parentage with the principal Y chromosome of the native Americans. Drawn up by Tarazona and Fabrício Santos, The Model of Evolution for the Native Populations of South America was presented in April 2001 at the Portugalia Genetica event, in September at the 10th International Congress of Genetics in Vienna, and in October, at the Brazilian Congress of Genetics at Águas de Lindóia.
It also merited an article last June in the American Journal of Human Genetics. Now, the geneticists from Minas intend to research other mysteries, such as the specific origin of the Tupi peoples that populated the coast at the time of the Discovery and had a decisive influence in the formation of the Brazilian people. They also want to confront the genetic data collected with other theories on the origin of the peoples of Amazon and Brazil’s Central Plateau.Republish