A world map of Neanderthal and Denisovan ancestry in modern humans
"There are certain classes of genes that modern humans inherited from the archaic humans with whom they interbred, which may have helped the modern humans to adapt to the new environments in which they arrived," says senior author David Reich, a geneticist at Harvard Medical School and the Broad Institute. "On the flip side, there was negative selection to systematically remove ancestry that may have been problematic from modern humans. We can document this removal over the 40,000 years since these admixtures occurred."
Reich and lab members, Swapan Mallick and Nick Patterson, teamed up with previous laboratory member Sriram Sankararaman, now an Assistant Professor of computer science at the University of California, Los Angeles, on the project, which found evidence that both Denisovan and Neanderthal ancestry has been lost from the X chromosome, as well as genes expressed in the male testes. They theorize that this has contributed to reduced fertility in males, which is commonly observed in other hybrids between two highly divergent groups of the same species.
The researchers collected their data by comparing known Neanderthal and Denisovan gene sequences across more than 250 genomes from 120 non-African populations publically available through the Simons Genome Diversity Project (there is little evidence for Neanderthal and Denisovan ancestry in Africans). The analysis was carried out by a machine-learning algorithm that could differentiate between components of both kinds of ancestral DNA, which are more similar to one another than to modern humans.
The results showed that individuals from Oceania possess the highest percentage of archaic ancestry and south Asians possess more Denisovan ancestry than previously believed. This reveals previously unknown interbreeding events, particularly in relation to Denisovans. In contrast, Western Eurasians are the non-Africans least likely to have Neanderthal or Denisovan genes. "The interactions between modern humans and archaic humans are complex and perhaps involved multiple events," Reich says.
The study's main limitation is that it relies on the current library of ancient genomes available. The researchers caution against drawing any conclusions about our extinct human ancestors based on the genetics and possible traits that they left behind. "We can't use this data to make claims about what the Denisovans or Neanderthals looked like, what they ate, or what kind of diseases they were susceptible to," says Sankararaman, first author on the paper. "We are still very far from understanding that."
Source: Cell Press [March 28, 2016]
Labels Anthropology, ArchaeoHeritage, Archaeology, Breakingnews, Early Humans, Genetics, Human Evolution