Genetics

The tribes of the world

Team from Minas Gerais uses news markers to group the populations of the planet

Imagem: HÉLIO DE ALMEIDAEven when they arrive at the same results of others, some pieces of research gain value when they reveal unexpected paths. It was like this with a work from a team from the Federal University of Minas Gerais (UFMG) coordinated by Sérgio Danilo Pena, which examined the DNA of 1,064 persons from 52 of the world’s populations. The conclusion is the same one reached by an American research group: the world population can be grouped into five large blocs, which correspond roughly to the major world geographical groups: America, Sub-Saharan Africa, East Asia, and a bloc made up of Europe, North Africa, Middle East and Central Asia. These subsets can be seen as the present-day representatives of the groups that lived in isolation in the primordial days of human civilization.

The team from Belo Horizonte, the capital of the state of Minas Gerais,  used a different strategy from the one used by the Americans to detect the differences between the long stretches of DNA. In the work that originated Pena’s work, Noah Rosenberg, from the University of Michigan, United States, examined DNA samples from 1.056 persons from 52 populations by means of genetic markers called microsatellites, defined as repeated sequences of nucleotides, the basic elements of DNA – adenine, guanine, cytosine and thymine. People are differentiated by the number of repetitions: each one can have, for example, 10, 11, 12 or 13 repetitions of guanine-adenine-thymine-adenine. Presented in Science in 2002, this classification of the world population in five groups, including the linguistic and cultural similarities, has exhibited a far greater precision than that obtained by Richard Lewontin in 1972, using proteins from the blood. But there were criticisms: microsatellites were said to be unstable and capable of causing statistical distortions.

The team from Minas Gerais, though, has availed itself of 40 markers called indels, an abbreviation that joins together two words, insertions and deletions of adenine, guanine, cytosine or thymine. They are harmless mutations transmitted from one generation to another; each variation works as a marker, because it has propagated itself from a mutation occurring in just one person. Pena and his students Luciana Bastos-Rodrigues and Juliana Pimenta, who sign with him the study published in March in the Annals of Human Genetics with these results, believe that indels, more stable than microsatellites, may perhaps be used to complement the studies on the Brazilian population and to define with greater precision the susceptibility of diseases associated with ancestral factors.

Partial vision
The genetic history of the Brazilian population has been analyzed mainly by means of two kinds of markers: those of the Y chromosomes, found in the nucleus of cells in men, which helps to reconstruct the paternal lineage; and those of the mitochondrial DNA, contained in another cell compartment, the mitochondria, and used to study the maternal lineage. The replies obtained depend a lot on the types of markers: none of them offers a complete or irrefutable vision, since other markers may lead to other results.

Pena recognizes that there may be what he calls “apparent correspondence” of the five geographical groups and the five races defined in the 18th century by the German anthropologist Johann Friedrich Blumenbach based on skull profiles and the color of the skin: the Caucasoid, the Mongoloid, the Ethiopian, the American and the Malayan. “But this similarity is merely superficial and deceptive”, Pena says. Rosenberg himself, after redoing his 2002 work using 993 markers instead of the 377 used on the first time, shows in an article published in December 2005 in PLoS Genetics that these groups are really consistent – or, as the specialists would say, the clusters are robust. Rosenberg had found a value of up to 5% for the genetic variation that occurs between the continental groups. For the team from Minas, the difference may reach 12%.

Miscegenation
In 2000, with the Molecular Portrait of Brazil, Pena demonstrated that miscegenation in the country was so intense that it made it impossible to attempt to form distinct groups. The analysis of markers of the Y chromosomes and of the mitochondrial DNA indicated that men self-denominated as white were not always descendents of whites. The majority were descendents of European countries, but the probability of the mother also being European was only 39%: one in every three of the 250 men that took part in the study represented an Amerindian maternal lineage, and a little more than one in every four (28%) brought in their blood the indelible heritage of an African mother.

“We cannot generalize”, comments Francisco Mauro Salzano, a researcher from the Federal University of Rio Grande do Sul (UFRGS), who has been working in this area for 50 years. “For us to have a more precise idea of these contributions, we have to ascertain the history of each community.” Pockets of extremely low miscegenation then emerge. This is the case of Veranópolis, a town of the Serra Gaúcha almost fully inhabited by descendents of Europeans, by the maternal and paternal lineages, according to a study from UFRGS, with the participation of Salzano, published in 2005 in the American Journal of Human Biology. In another 13 towns from Rio Grande do Sul, the researchers found in the maternal lineage 36% of the heritage of the ancient indigenous people that lived in the region – the Guaranis – and 16% of African blood.

And miscegenation tends to increase, in Brazil and in the world, points out Marco Antonio Zago, a professor of the University of São Paulo (USP) in Ribeirão Preto. In one of the most recent studies from his team, published in Human Biology, the focus is blacks that live in isolated communities – the quilombos (former runaway slaves communities) – in the states of Pará and in Maranhão, studied by means of the mitochondrial chromosome. Zago and Wilson Araújo Silva, also from USP of Ribeirão Preto, with biologists from Rio Grande do Sul and from Pará, concluded that there was a remarkable contribution from Amerindian women in the formation of the quilombos. The historians had already concluded that the number of men that fled to form the quilombos was much greater than the number of women, but they had no way of arriving at this level of detail.

This study also brought fresh information about the slave traffic to Brazil. The historical records sustain that the majority of the Africans that arrived in Brazil between the 16th and 18th centuries were of the Benin race, from the west of Africa. However, the geneticists found predominant signs of representatives from another race, the Bantus, who came from the center-west and from the west of Africa. So from the laboratories there comes, little by little, a clear picture of the consequences of the mixture of races furthered by the sellers of slaves in the warehouses of the port of Salvador as a way of preventing rebellions and by internal migrations, after their arrival in Brazil. “Because of this miscegenation amongst races”, Zago comments, “Brazil is home today to a very specific population of blacks, different from that of any other place in the world”.