SYDNEY: As humans migrated out of Africa 100,000 to 50,000 years ago, some individuals interbred with Neanderthal and as a result some genetic sequences can be found in all non-African humans, according to an international team of scientists.
“It’s cool to think that some of us have a little Neanderthal DNA in us,” said Svante Pääbo from the Max-Planck Institute for Evolutionary Anthropology in Leipzig, Germany, lead author of the study published in Science.
Non-Africans are more closely related to Neanderthal. This, they suggest, is because the inter-species liaisons most likely occurred in the Middle East 100,000 to 50,000 years ago, after non-Africans migrated out of Africa.
Neanderthals went extinct 30,000 years ago
Neanderthal, a hominid species that became extinct around 30,000 years ago stood about 1.62 m high and had heads large enough to house human-sized brains.
Neanderthal were once thought to be sub-species of humans (Homo sapiens neanderthalensis), but in this study, which looked at 1.1 billion DNA fragments from the Neanderthal genome, researchers confirm they were a separate species ( Homo neanderthalensis ).
Because humans and Neanderthal are our closest relatives evolutionarily, the Neanderthal genome has helped to identify some of the genetic material that has arisen since the two species split.
Uniquely human genes discovered
According to the paper, some of those exclusively human genes are those that code for cognitive development (mutations of which cause diseases such as autism and schizophrenia) and energy metabolism.
There’s also genes that code for skeletal development – parts of the skeleton like the cranium, clavicle and ribcage, that make us morphologically different from Neanderthals and apes.
The researchers used small amounts of bone powder from three 40,000-year-old Neanderthal individuals found in a cave in Croatia to build 60% of the genome, enough to announce that the genome is complete.
DNA used from five humans
Correcting for a sample bias in the genetic databases of modern humans, the researchers sequenced DNA from five different individuals from different regions around the world: Southern Africa, West Africa, Papua New Guinea, China and France.
When the researchers compared the genomes of the five modern day humans to the Neanderthal, they found that the non-African genomes were more similar to Neanderthals than the African genomes.
Because these similarities are present in the genomes of the individuals from China, Papua New Guinea and France, this suggests that human and Neanderthal interbreeding took place when they shared a common ancestor – after the migration out of Africa.
Interbreeding was infrequent
Still, interbreeding was probably infrequent, as only 1-4% of the modern non-African human genome appears to be from Neanderthals.
Though this study points to inbreeding as the most likely explanation for the overlapping genomes, there is a chance this did not occur.
“We cannot currently rule out a scenario in which the ancestral population of present-day non-Africans was more closely related to Neanderthals than the ancestral population of present-day Africans due to ancient substructure within Africa,” the researchers wrote in the paper.
No mix up in DNA
Previous Neanderthal genetic studies have been plagued with contamination from the DNA of bacteria found on the bones, as well as the DNA of modern humans.
In this study, the Neanderthal bones sequenced were not well preserved – as much as 95% of the genetic material gleaned from the bones was actually from bacteria that colonised the bone. But the researchers are sure they got the right DNA to include in the analysis.
This study showcases new methods used to avoid genetic contamination of the test material, and suggests ways to identify and eliminate problems of contamination in future studies involving particularly degraded specimens of prehistoric animals.
An interesting history
David Reich, member of the research team at the Broad Institute of MIT & Harvard Medical School, said, “our paper shows the power of ancient genomes for learning novel things about human inferences about history.”
Scientists including Reich are concerned that their findings, which highlight genetic differences in modern humans, particularly relatedness to Neanderthals, may spark racial questions.
“One thing that is important to keep in mind is that the Neanderthal ancestry in modern non-Africans occurs at a very low percentage- 1-4%- and hence only explains a tiny proportion of the genetic heritage of non-Africans,” said Reich.
“There is no evidence at all that the Neanderthal ancestry present in non-Africans confers any disadvantage or susceptibility to disease that marks them out as different from people who have not inherited African ancestry.”
“[This research] is kind of like opening the gate to information about early modern humans […] it’s a huge technological step forward, but lends itself to more research,” said Alan Cooper, director of the Australian Centre for Ancient DNA in Adelaide, Australia, who did not work on the paper.