Although researchers are aware that most non-Africans possess at least a small amount of Neanderthal DNA, a new map of our ancient ancestors reveals that many bloodlines, especially those of South Asian descent, might contain more Denisovan DNA than Neanderthal. The study also suggests that modern humans likely interbred with Denisovans around 100 generations after they bred with Neanderthals.
Denisovans are a mysterious hominid population that lived during the same time period as Neanderthals. In addition to revealing these genetic presence in numerous bloodlines, the team used genetic information on both Denisovans and Neanderthals to predict where these genes are impacting modern human biology, revealing that Denisovan genes are likely linked to a subtle sense of smell in Papua New Guinea people and high-altitude adaptations in Tibetans. Conversely, Neanderthal genes are linked to stronger skin and hair in many types of people around the word.
"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," said David Reich, who participated in the research. "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 his team found evidence that ancestry from both the Denisovan and Neanderthal lines has been lost from the X chromosome, in addition to genes expressed in male tests. They believe that this loss is the likely cause of the reduced fertility in males that is typically seen in hybrids of two very different groups of the same species.
Using data from Neanderthal and Denisovan gene sequences across more than 250 genomes from 120 non-African populations, the team utilized a machine-learning algorithm to differentiate between both types of ancestral DNA.
The results revealed that while those from Oceania contain the highest percentage of ancient ancestry, south Asians possess more Denisovan ancestry than previously believed, suggesting previously unknown ancient interbreeding events connected to the mysterious hominid population. Conversely Western Eurasians are least likely to have Neanderthal or Denisovan genes.
"The interactions between modern humans and archaic humans are complex and perhaps involved multiple events," Reich said.
Although the results shed light on the complex nature of interactions between modern and ancient humans, the study's reliance on the current library of ancient genomes available makes it difficult to draw conclusions regarding ancestral traits and physical characteristics.
"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," said Sriram Sankararaman, first author on the paper. "We are still very far from understanding that."
The findings were published in March 28 issue of Current Biology.
