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A reconstructed Neanderthal skeleton, right, and a modern human skeleton, on display at the Museum of Natural History in New York. (Frank Franklin II/AP)

A new study adds more evidence to the theory that humans and Neanderthals interbred thousands of years ago. The study found that many humans outside of Africa share DNA with the long-extinct species. 

An international team of researchers has found that a small part of the human X chromosome, which originates from Neanderthals, is present in about nine per cent of individuals from outside of Africa. 

"Somewhere very early on, there was an encounter between Neanderthals and humans," Dr. Damian Labuda of the pediatrics department of the University of Montreal said.  

Labuda led a team of international scientists who compared chromosomal DNA from 6,000 individuals from around the world to the Neanderthal genome. 

They found that many people from across all continents except for sub-Saharan Africa shared a piece of DNA called a haplotype with Neanderthals.

Labuda says this haplotype was most likely introduced to the human genome when modern humans were making their way out of Africa and settling in other parts of the planet. 

He estimates "intimate contact" between humans and Neanderthals took place in the Middle East 50,000 to 70,000 years ago. 

Neanderthals first appeared around 400,000 years ago and went extinct about 30,000 years ago. Scientists have always wondered whether the physically stronger Neanderthals ever intermixed with humans.

But Labuda say thanks to recent advances in understanding the Neanderthal genome, we now know the answer.

Recent breakthroughs 

Back in 1999 Labuda's team identified a haplotype in the human X chromosome that seemed different.

"We had this piece of DNA which from the beginning looked bizarre," Labuda said.

He speculated that this haplotype dubbed the "Neanderthal signature" was evidence of the two species mixing. 

"It was just a finding observation," he said, "we couldn't prove much about it."

It wasn't until another team of scientists mapped the Neanderthal genome that he was able to test his theory.

In 2010 a team of scientists led by Ed Green, of the University of California, Santa Cruz, was able to extract enough DNA from fossils to have one copy of 60 per cent of the entire Neanderthal genome.

Soon after, Labuda and his team got to work.

More research

More research and analysis is needed to fully understand the role of Neanderthal genes in human evolution. But Labuda says intermixing may have provided an evolutionary advantage to humans by adding to our genetic variability.

"More variability means more chances to survive and adapt," he says.