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One journey was enough

Study sustains that a single batch of hunters from Asia colonized America 21,000 years ago

The controversy over the moment when man arrived in America, a heated scientific debate, has just been rekindled by a genetic study with 30 Indians native to the continent, the majority belonging to tribes present in Brazil. An analysis of one part of the DNA (deoxyribonucleic acid) molecule of these descendants of the primordial peoples that occupied the New World strengthens the thesis that Homo sapiens reached Alaska, coming from Asia, via the Bering Strait, about some 21,000 years ago, and in just one migratory wave. Leaving Siberia around this time, a small population of hunter-gatherers with Mongoloid (oriental) traces may have crossed Beringia – a vast tract of land, covered today by the waters of the ocean, which used to link the two continents – in search of food, to set themselves up in America.

A team of researchers from nine centers in Brazil and one from Peru, coordinated by Marco Antônio Zago and Wilson Silva Jr., from the School of Medicine of Ribeirão Preto, of the University of São Paulo (USP), arrived at this conclusion after sequencing and studying the Amerindians for their profile of mutations and the diversity of the so called mitochondrial DNA – a kind of genetic material that, if properly worked on, is capable of opening up a window into the past and to provide close estimates of evolutionary processes. From the clinical point of view, it may also be potentially related to the occurrence of human diseases. For those who defend the classic theory of the colonization of America, today much questioned by recent archeological finds and genetic studies, there were three moves of Asian groups entering the continent, the first having occurred about 12,000 years ago.

The results of the new work suggest that all the members of the heterogeneous population of the Amerindians analyzed – 25 Brazilian Indians belonging to eight tribes (Guarani, Kaiapó, Katuena, Potururaja, Tirio, Waiampi, Arara and Yanomami) and five Quechuas from Peru – derived from a single ancestral group, possibly the first colonizers of America. “Our biological data supports the theory that the entry of man into the continent is older than is normally thought, and that this process took place in just one wave of migration”, explains Zago, whose team received funding from FAPESP and the National Council for Scientific and Technological Development (CNPq). “These populations of Amerindians probably had a common origin”.

For the purpose of comparison and the fine-tuning of the methodology employed, the researchers analyzed the mitochondrial DNA of ten non-Amerindian Brazilians, four of African origin (Blacks ), three of white forebears, and three of Asian (Japanese) descent. The work of Zago’s team was published in the July issue of the American magazine, The American Journal of Human Genetics, one of the most important in the area. Its main conclusions ratify the central ideas of an ample European study that, in 2000, made the pages of the Nature magazine, and was also based on the use of mitochondrial DNA, as if it were a molecular clock for human evolution.

In spite of having analyzed the genetic material of a relatively small number of Amerindians, present in only two South American countries, the Brazilians are convinced that their data is completely trustworthy. “A study with a larger and more diversified sample would arrive at substantially the same results”, comments geneticist Silva Jr. Also the fact of having sequenced and studied only 50% of the base pairs (chemical units) of the Amerindians’ Mitochondrial DNA – and not 100% – is not seen as a failure in the methodology of the work. On the contrary. “Our article is the second in the world to show that, with half of the Mitochondrial DNA mapped, it is possible to reach the same result as the complete sequencing of this molecule”, Zago assures.

Accordingly, as was expected, the genetic material of the 30 South American Indians studied fits into one of the four major lineages of Mitochondrial DNA that characterize the native peoples of America, called technically the A, B, C and D haplogroups. Amongst the ten Brazilians of non-Amerindian origin – people that obviously are not candidates for being descendants from the first populations that arrived on the continent -, only the mitochondrial DNA of a white individual belonged to one of the haplogroups typical of the American natives. Each lineage of Mitochondrial DNA shows a number and a pattern of mutations that give them their own identity, a characteristic genetic diversity. It is this that makes them different. The four haplogroups, however, show a lot of similarity amongst themselves – like brothers from the same family – and also with the Mitochondrial DNA of the current inhabitants of Asia, evidence that the primordial occupants of America came from that continent. In other words, the matrix of these four apparently kindred lineages is Asian.

That, by itself, is not concrete evidence that the four haplogroups originated simultaneously and from the same population. To demonstrate this kind of correlation, Zago’s group calculated when each one of the four haplogroups of Mitochondrial DNA differentiated itself from its mother sequence. It is something like discovering the birthday of each lineage. The results were very similar, suggesting the existence of an ancestor common to the four lineages, at some moment in the past. By the reckoning of the researchers, the A haplogroup arose 20,500 years ago; B, 18,100 years ago; C, 21,600 years ago; and D, 23,800 years ago. “This difference is not statistically relevant, in the kind of methodology that we used in the work”, says Silva Jr. “It is as if all the haplogroups had appeared at more or less the same time”.

And when could this have been? About 21,000 years ago, since this result is equivalent to the average age of all the lineages. Accordingly, if all the kinds of Mitochondrial DNA found today in over 90% of the native peoples of America go back to one and the same time, this probably means that all the members of these groups descend from one single migratory wave coming from Asia.In this kind of genome study, one of the great challenges is to find markers that show when this differentiation of the Mitochondrial DNA occurred that led to the massive dissemination of the A, B, C and D haplogroups just in America.

What evidence is there that this process happened here and not in Asia itself? After all, the four lineages could have arisen before the occupation of the NewWorld and have been brought over here by another migratory movement, simultaneous or not. “The presence of the A, B, C and D lineages is a minority in Asia, where other varieties of Mitochondrial DNA predominate”, comments Sandro Bonatto, a researcher from the Pontifical Catholic University of Rio Grande do Sul (PUC-RS), who also took part in the study. If this differentiation had occurred before the arrival in America, there ought to be today more representatives of these lineages in that continent”.

Bottleneck effect
For the authors of the work with the Amerindians, the origin of these haplogroups is clearly linked to a process of occupation of the continent by a single and small migratory wave possibly carried out by a few thousands of individuals. If other migrations had occurred, they argue, there would be more lineages of Mitochondrial DNA around here, older or newer than the four haplogroups. As the Amerindians show less genetic diversity than the Asians, the researchers believe that the natives of America are the result of an evolutionary process called a bottleneck: starting from a few individuals – call it low genetic diversity – an enormous population arises that is going to colonize a large area – the three Americas, in this case.

Those who do not work with genomics have difficulty in understanding how the researchers are able to build up their theoretical hypotheses on the occupation of America, based on an analysis of a few thousand base pairs of Mitochondrial DNA. How can this kind of genetic material throw light on the process of colonization of the New World, which took place thousands of years ago? Before anything else, it is necessary to understand the importance – and the specificity – of this kind of genetic material, in relation to the whole human genome. Each human cell houses DNA in two structures: the nucleus, which contains more than 99.9% of all the genetic material of the species, and the organelle responsible for the production of energy, the mitochondria. While the DNA of the nucleus of the cell shows more than 3 billion base pairs, the mitochondrial DNA holds only 16,500 base pairs and shows some peculiarities that turn it into a good biological marker.

The mitochondrial DNA is independent of the nuclear DNA and is passed to future generations only by the female line, without suffering any recombination with the genetic material coming from the paternal heritage. But this does not mean that the mitochondrial DNA is immutable and exactly the same in any person. Because of random mutations and mistakes in the copying process, the base pairs that form its sequence undergo alterations from time to time. Many scientists believe that the mutations of the mitochondrial DNA occur at a more or less constant pace, and are creating mathematical models to try to establish, in an approximate way, when two distinct populations had a common ancestor. “This kind of analysis does not bring ready replies, but points to strong tendencies”, Silva Jr. points out. Roughly speaking, this is what the Brazilian and Peruvian researchers achieved with their work.

After sequencing a continuous region of 8,800 base pairs of the mitochondrial DNA of the 30 Amerindians (and also of ten Brazilians of non-indigenous origin), the team started the task of trying to decipher the origins, in time and space, of this genetic material, and infer when and in how many waves man reached America. This is not a job that is starting from nothing, from scratch, but rather from some ideas assumed and disseminated in other scientific works. As backing for their mathematical calculations and the genome comparisons that would lead them to estimate the moment of the arrival of the first colonizers in the New World, the researchers had, logically, to adopt a starting point for the existence of the original forms of hominids on Earth: they assumed, as have other works also, that hominids differentiated themselves from chimpanzees some 5 million years ago.

Next, they compared the mitochondrial DNA of these animals with that of the Amerindians, to establish how frequently alterations (mutations) occur in this region of the genome in humans. After making the calculations, they showed that there are, every year, 2.4 x 10-8 replacements (mutations) per base pair in the length of the mitochondrial DNA of the Amerindians studied by them. “This is the same as saying that a mutation occurs in this region every 5,000 years”, Sandro Bonatto comments. If this rate is correct, and the occupation of the New World took place 21,000 years ago, today’s Amerindians ought to show at least four mutations more in their mitochondrial DNA than their Asiatic ancestors.

There are several theories that try to explain the arrival of man in America, using the analysis of paleo-archeological material and, more recently, with the help of studies of human DNA. The work by the researchers from USP in Ribeirão Preto and their collaborators is one more to be included in this growing tendency for using genetic information to try and understand a process of colonization, the understanding of which until a very short time ago depended basically on recovering bones of humans and animals, on analyzing artifacts and drawings made by forebears of our species, besides work on the linguistic evolution of the Amerindians and of the climate on the continent. The study does not intend to exhaust this controversial and exciting subject, nor is it exempt from criticisms, above all from the supporters of other views about the arrival of Homo sapiens in the New World. “But our analysis is objective and based on reliable data”, Zago points out.

Until a few decades ago, the predominant view about colonization was dictated by the more traditionalist Americans, who always spread out the thesis that there were three migratory movements of Mongoloid populations coming from Asia. The first batch of colonizers was said to have arrived about 12,000 years ago. This line of scholars says that the first culture to be established here was the one at Clovis, New Mexico, United States, where, however, human bones were never found, though many spearheads, proving the existence of this primordial civilization.

Alternative theories
Other currents postulate that the initial landfall of the wave of hunters and gatherers coming from Asia in the direction of the New World took place before this date. A recent study, carried out by researchers from the Federal University of Minas Gerais (UFMG) who analyzed the Y chromosome – transmitted only by the father to children of the male sex – of 18 groups of Amerindians, maintain that there was only one wave of migration, between 15,000 and 30,000 years ago – a conclusion along the lines of the articles now produced by their colleagues from USP in Ribeirão Preto on mitochondrial DNA.

There are also those, like archeologist Niède Guidon, who defend the thesis that human presence in America is far older, going back some 50,000 years. She maintains this idea based on dating carried out on the remains of a campfire (said to have been made by humans) found in São Raimundo Nonato, in Piauí, where rock paintings of over 10,000 years old were also located. The Monte Verde archeological site in Chile has also provided evidence that man has been in South America for over 12,000 years.

Loss of lineage
In another line of work, bioarcheologist Walter Neves, from USP’s Institute of Biosciences, advocates a different hypothesis. Based on the anatomy of human skeletons found at points of South America, including Luzia, the most completeHomo sapiens cranium found on the continent (recovered in the Lagoa Santa region of Minas Gerais and dated at 11,000 years), Neves says that the first occupants of the New World arrived here between 13,000 and 14,000 years ago, and that they were not Mongoloids, but rather members of an Asian population similar to today’s Australian aborigines and African Blacks. The problem is that this supposed primordial colonizer may have become extinct and have been superseded by the Mongoloids.

It may not be possible, therefore, to study the mitochondrial DNA of Luzia’s descendants, simply because they may not exist. This, however, does not bother Neves. “This model of occupation of America, advocated by geneticists using an analysis of the mitochondrial DNA of today’s Native American groups does not tally with what the fossils show”, the bioarcheologist explains. “They do not take into account that there may have been a loss of lineages of DNA with the passing of time, both in Asia and in America.” The geneticists admit that, if Luzia and her contemporaries have left no descendants, it is not possible to deny or to prove their existence from the analysis of the current populations. “But our data shows that the occupation of America by Mongoloids started at least 20,000 years ago, long before the period put forward by Neves”, explains Zago.

In search of the ills of mitochondria

Besides being a sort of molecular archives of the past, mitochondrial DNA may also provide important clues about mutations potentially associated with the development of some diseases, like Parkinson disease, Alzheimer’s, and type 2 diabetes mellitus. In a study published in the May issue of The American Journal of Human Genetics, the same American magazine that accepted the Brazilians’ study with the Amerindians, researchers from a biotechnology company from California, MitoKor, revealed that they have sequenced the mitochondrial genome of 560 persons of Asian, European and African origins. His effort made by the company has possibly produced the largest database with sequences of this region of human DNA, made up of only 16,500 base pairs (chemical units).

The scientists’ great challenge today is to map all the mutations of the mitochondrial genome and to discover which of these alterations may be factors of risk for the appearance of the diseases mentioned above. To reach this objective, which would open up the way for the development of new therapies against these disorders, the genome sequences of healthy individuals and victims of Alzheimer’s, Parkinson’s and diabetes are to be compared in detail. The task is not an easy one, nor should it bring results in the short term, but this does not discourage the researchers. “The central issue is that there are no simple answers”, says Neil Howell, the principal author of the study. “We are going to have to characterize the mitochondrial genome of even larger populations to find the answers”.