An article published in the journal “Nature” reports a genetic study carried out using data obtained from 27,566 Icelanders to understand which parts of modern humans’ genome contain genes inherited from Neanderthals following interbreedings, and what role it plays in modern humans. A team of researchers performed a comparative analysis with hominin genomes sequenced at the Max Planck Institute for Evolutionary Anthropology (MPI-EVA) in Leipzig, Germany, finding many different genes scattered in the Icelandic genomes. By putting them together, they reconstructed at least 38% of the Neanderthal genome using 14 million fragments. A surprise came from the discovery of Denisovans genes, usually present in Asian populations, but it’s possible that they too were inherited from Neanderthals who descended from interbreedings with Denisova.
The history of humanity is increasingly becoming a story of interbreedings between different populations which in certain cases also means between different species. Most modern non-African humans inherited Neanderthal genes from interbreedings of this type, and thanks to the success in DNA sequencing of various Neanderthals it’s been possible to start tracing those genes. That’s useful among other things because these genes are also present in different ways in different people, therefore examining many genomes of living people allows to trace many different Neanderthal genes. At MPI-EVA various hominin DNA sequencings have been performed since the institute is at the forefront of paleogenetics, the discipline that studies the DNA of extinct species.
Laurits Skov, a postdoc student at the Bioinformatics Research Center (BiRC) at Aarhus University, Denmark, the first author of this research, developed a method to trace Neanderthal DNA fragments in that of modern humans. He has applied it together with other colleagues to a large number of Icelanders whose genetic data have been accumulated by the deCODE genetic firm. Skov stated that the combination of his method and deCODE data allowed to analyze 27,566 genomes making this study ten times larger than previous studies of Neanderthal genes inherited from modern humans.
This research led to the discovery of several genetic fragments inherited from another species of hominins now extinct, the Denisovans. Only a few of their bones are known today but luckily they were discovered in very cold places such as Siberia, ensuring their conservation to the point of being able to sequence their DNA with good results. Denisovan genes have been discovered in various Asian populations and Australian Aborigines, but it’s surprising to find them in Icelanders.
The Neanderthal genes found in Icelanders show a relationship with the populations of the Altai mountains in Siberia and the Vindija cave in Croatia. In the Altai mountains there’s also the Denisova cave, so it’s possible that the Neanderthals from which the Icelanders descended had already interbred with the Denisovans. Another possibility is that Homo sapiens met Denisova before the Neanderthals.
In any case, genetic analyzes indicate various examples of introgression. In essence, interbreedings between species produced hybrids, who in turn interbred again with at least one of their two parent species forming other hybrids. As the generations passed, these interbreedings introduced various gene combinations of the other species into the species.
The image (Courtesy Laurits Skov et. Al. All rights reserved) shows the phylogenetic relationships of the archaic fragments sequenced. The possible scenarios are illustrated in graphs a and b.
Various studies attempted to establish if the genes inherited from Neanderthals have a relevance in Homo sapiens. Certain disease risks may be linked to some of the genes, but studies of the impact of genes on disease predisposition are still ongoing with results that vary widely from disease to disease.
What emerged from this research is an important spread of Neanderthal genes among Icelanders with some genes inherited from Denisovans. There are still genes whose origin is unknown, but that might be due to the limited availability of genomes of these hominins. It remains a confirmation that the history of humanity is also made up of migrations and interbreedings between different populations, even of different species.
