Researchers have found evidence that ancient humans have interbred with an extinct hominin species, the Denisovan, not just once but at least twice.

Study researcher Sharon Browning, from the University of Washington School of Public Health, and colleagues came to this conclusion after examining over 5,500 genomes of modern humans.

"At least two distinct instances of Denisovan admixture into modern humans occurred, involving Denisovan populations that had different levels of relatedness to the sequenced Altai Denisovan," the researchers reported in their study published in the journal Cell on March 15.

Effects Of Interbreeding

A 2015 study suggests that sexual contact with the Neanderthals and the Denisovans helped human ancestors to explore the world outside Africa thousands of years ago.

The interbreeding of ancient humans and other hominin species, however, had lasting effects. Some of the gene variants derived from the Denisovans influenced some traits in modern humans.

Enhanced Sense Of Smell

People from Papua New Guinea have about five percent Denisovan ancestry and the natives of the country are believed to have inherited Denisovan genes responsible for their enhanced sense of smell.

High-Altitude Living

Another research revealed that the ability of the Tibetans to thrive in high-altitude places, unaffected by low oxygen levels, has originated from the Denisovans.

Most Han Chinese and other groups lost the Denisovans' version of the so-called EPAS1 gene, which regulates the body's production of hemoglobin because it was not particularly beneficial to them. The Tibetans, who lived on the high-altitude plateau, however, retained this gene because it helped them adapt to their way of life.

"Our findings illustrate that admixture with other hominin species has provided genetic variation that helped humans to adapt to new environments," study researcher Rasmus Nielsen, from University of California, Berkeley, and colleagues wrote in their 2014 study.

Increased Infertility

Breeding with the Denisovans may have also caused increased modern human male infertility. In a 2016 study, researchers searched for Denisovan genes that are expressed predominantly on the X chromosome. They found evidence that the Denisovan and Neanderthal ancestry disappeared from the X chromosomes and genes expressed in the male testes.

"What they would reflect is that the males who happened to carry Denisovan or Neanderthal DNA in these sections were not as successful in terms of producing offspring as others, and because of that those sections were removed in that first handful of generations after the mixture occurred," said study researcher David Reich, from Harvard Medical School.

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