Genetic ghosts seem to be lurking around the evolution of Homo sapiens. They are ancestral members of the human race whose fossils have not yet been identified, but who would have bequeathed stretches of their DNA during crossover episodes with the ancestors of today's people.
If the hypothesis is correct, the evolution of human beings would have been marked by successive miscegenation processes on a global scale, as different waves of hominids (ancestors and close relatives of man) left Africa and met “cousins” who had already reached other continents.
One of the new studies on the subject, published in this week's edition of the specialized magazine Science Advances, proposes that quite primitive hominids, dubbed “supercaicos”, would have participated in the process about 700 thousand years ago, during one of the waves of African expansion towards European and Asian territory.
Alan Rogers and his colleagues at the University of Utah's Department of Anthropology (southwestern USA) came to that conclusion after using statistical methods to sift data from the genomes of modern humans in Africa and Europe and compare it with the DNA of two extinct hominids.
Those missing species that entered the analysis, the Neanderthals and the denisovanos, are far from being “ghosts”. Scientists have already found their fossils (in the case of Denisovans, only a few fragments, for now) and have the relatively complete sequence of chemical "letters" that make up their DNA. Both lived in Eurasia (the Neanderthals to the west, the Denisovans to the east) and disappeared at the end of the Ice Age, 30,000 years ago. And both miscegenated with the Homo sapiens before they disappear. All non-African people carry a small percentage of Neanderthal DNA, and Asians and natives in Oceania also have some Denisovan genes.
Essentially, what Rogers and company did in the new study was to compare the patterns of exchanging individual “letters” in DNA across all of these strains. At each location in the genome, it is possible to classify these letters as “ancestors” (that is, maintaining the pattern existing in the hominid's remote past, before different species appeared) or “derivatives” (that is, that have undergone mutations and were exchanged ).
The analysis showed that the pattern of mutations in the various branches of human evolution is such that both Neanderthals and Denisovans carry DNA letters “derived” differently from other branches – and with such a high frequency, throughout the genome, that it would be unlikely that these mutations had happened spontaneously in the DNA of both species, Rogers explained to Folha.
It is more logical to imagine that at least some of them were transmitted by the crossing with supercaic hominids, which would carry part of these divergent regions of the genome. This would have happened, at first, before Neanderthals and Denisovans became separate lineages (the genome, in fact, indicates that they were more related to each other than to the direct ancestors of modern humans).
But who are the super-caicos? It is difficult to say with certainty, although fossils and stone tools indicate that very primitive hominids had already arrived in the Caucasus (between Europe and Asia), in present-day Georgia, 1.8 million years ago.
"They could be members of the species Homo erectus or another hominid that has not yet been identified," says Rogers. The Utah researcher and his colleagues calculate, using genetic data, that the lineage of the supercaicos had separated from the others about 2 million years ago, which matches the archaeological data about the first colonization of Eurasia by hominids.
In another study, also recently published in the same scientific journal, the pair formed by Arun Durvasula and Sriram Sankararaman, from the University of California at Los Angeles, used similar genomic and statistical methods to identify the genetic contribution of ghost hominids within the African continent itself.
Analyzing the DNA of West African populations, such as the Yoruba people in Nigeria (related to many of the enslaved people who came to Brazil) and the Sierra Leone mandes, they propose that somewhere between 2% and 19% of the genetic material of these groups could come from archaic hominids, which diverged from other human lineages just before the appearance of Neanderthals (about half a million years ago, therefore).
The situation in Africa is more complicated than in Eurasia because, due to the warmer climate, the African fossil record is more sparse, making it difficult to identify the “ghosts” and extract DNA from the fossils that have already been found. In any case, the researchers' bet is that the scenario of human evolution must become even more complex.
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