(CNN) When human ancestors left Africa and met Neanderthals in Europe 50,000 years ago, they had children together and clues were left behind in modern human DNA.
Now, a study of genomes from more than 27,000 people in Iceland has revealed new insights about Neanderthals -- and a mysterious ancient human species called Denisovans -- that could potentially change the narrative of our evolution. The research published last week in the journal Nature.
Previous studies have established that humans today, outside of Africa, have 2% of Neanderthal genes in their DNA. This new study confirmed that percentage and adds more detail.
But the actual 2% of genes isn't the same for everyone. Different people received different pieces of Neanderthal genes, so a wide study analyzing the genomes of modern humans can reveal different aspects.
Based on this study, the researchers were able to pick out different traits of Neanderthals.
For example, female Neanderthals became mothers later in life between 100,000 to 500,000 years ago compared with female Homo sapiens living in Africa. And male Neanderthals were fathers at younger ages when compared with male Homo sapiens in Africa.
But someone else appeared in the genomes who the researchers weren't expecting: Denisovans. These mysterious ancient humans, a sister species of Neanderthals, are only known from a few fossil fragments.
Today, their genes can be traced in Aboriginal Australians, East Asians and people in Papua New Guinea. During this study, their genes also appeared in the DNA of Icelanders.
The researchers believe that two possible explanations account for this.
Neanderthals and Denisovans may have met and had children together before they encountered Homo sapiens. In that case, those offspring likely would have met humans, mated with them and passed on genes from both species.
The relatively recent discovery of the fossil of a Denisovan-Neanderthal hybrid child lends some evidence to this theory.
The other possibility is that Denisovans and Homo sapiens met first, long before Neanderthals entered the scene.
If this is true, that challenges the current belief that humans and Neanderthals intermingled first before humans encountered Denisovans tens of thousands of years later.
"Up to now, we believed that the Neanderthals modern people have had children with were 'pure' Neanderthals," said Mikkel Heide Schierup, study author and professor in the Bioinformatics Research Centre at Aarhus University in Denmark, in a statement.
"It's true that researchers have found the remnants of a hybrid between Denisovans and Neanderthals in a cave in East Asia, but we have not known whether there were more of these hybrids and whether, thousands of years later, they had children with modern humans."
The researchers believe that both scenarios are likely.
Another aspect of the study revealed more information about the potential risk of disease that Neanderthal genes pose in modern human DNA. It turns out that Neanderthal DNA is only a drop in the bucket compared with the rest of our genes.
This study suggested that Neanderthal DNA contributes a lower risk of prostate cancer, faster blood plasma clotting, lower levels of hemoglobin and shorter body length by one millimeter.
"We have previously thought that many of the Neanderthal variants previously found in modern human DNA were associated with an increased risk of diseases," Schierup said. "However, our study shows that the human gene variants located directly beside the Neanderthal genes are better explanations for the risk."
The researchers from universities in Denmark, Germany and Iceland worked with deCODE Genetics in Iceland, a subsidiary of the biotechnology company Amgen. The study disclosed that many of the authors on the paper work for deCODE and Amgen.
Researchers at deCODE analyze human genomes to seek out risk factors for diseases and determine methods of diagnosing, treating and preventing them.
"We spent several months at deCODE in Reykjavik on what can be called field studies for a computational biologist," said Laurits Skov, study author and a postdoctoral student in the Bioinformatics Research Centre at Aarhus University, in a statement. "By combining my method with deCODE's data and expertise, we have analysed 27,566 genomes, and this makes our study 10 times larger than previous studies of Neanderthal genes in human DNA."
Skov developed a method to trace Neanderthal genes in human DNA.
This collaboration enabled the researchers to use genetic data from more than half of the population in Iceland.
The genes from Icelanders, along with genome sequences from 286 sub-Saharan Africans, were gathered during the 1000 Genomes project. This project, which ran between 2008 and 2015, created the largest public catalog of human variation and genotype data, according to the project's site.
"What we know is that in the 50,000 years from their time to this, our adaptability and diversity have enabled us to mix and move, settle and thrive in every corner of the planet as [ancient humans] did not," said Kári Stefánsson, senior study author and CEO of deCODE, in a statement. "In these dark days we would do well to remember that our differences are literally the mark of our success, and so to help each other as best we can."