But researchers cannot yet say how these archaic sequences affect people today, much less the humans who acquired them some 50,—55, years ago. That will be exciting when someone does. Some , or more years ago, the group of hominins that would evolve to become Neanderthals and Denisovans left Africa for Eurasia.
A few hundred millennia later, about 60, to 70, years ago, the ancestors of modern non-Africans followed a similar path out of Africa and began interbreeding with these other hominin groups. Researchers estimate that much of the Neanderthal DNA in modern human genomes came from interbreeding events that took place around 50, to 55, years ago in the Middle East.
Thousands of years later, humans moving into East Asia interbred with Denisovans. Most Neanderthal variants exist in only around 2 percent of modern people of Eurasian descent. But some archaic DNA is much more common, an indication that it was beneficial to ancient humans as they moved from Africa into Eurasia, which Neanderthals had called home for more than , years. In their study, Vernot and Akey found several sequences of Neanderthal origin that were present in more than half of the genomes from living humans they studied.
The regions that contained high frequencies of Neanderthal sequences included genes that could yield clues to their functional effect. Base-pair differences between Neanderthal and human variants rarely fall in protein-coding sequences, but rather in regulatory ones, suggesting the archaic sequences affect gene expression.
A number of segments harbor genes that relate to skin biology, such as a transcription factor that regulates the development of epidermal cells called keratinocytes. These variants may underlie traits that were adaptive in the different climatic conditions and lower levels of ultraviolet light exposure at more northern latitudes.
No one has actually shown yet in culture that a human and Neanderthal allele have a different physiological function. It was unclear, however, what specific effect the Neanderthal variants had on phenotype. For that, researchers needed phenotypic data on many different kinds of traits, paired with genetic information, for thousands of people.
Right around the time the scientific community was beginning to map Neanderthal DNA in the genomes of living people, eMERGE organizers were compiling electronic health records and associated genetic data for tens of thousands of patients from nine health-care centers across the US.
They reported in that Neanderthal DNA at various sites in the genome influences a range of immune and autoimmune traits, and there was some association with obesity and malnutrition, pointing to potential metabolic effects. The researchers also saw an association between Neanderthal ancestry and two types of noncancerous skin growths associated with dysfunctional keratinocyte biology—supporting the idea that the Neanderthal DNA was at one point selected for its effects on skin.
At the same time, Kelso and her postdoc Michael Dannemann were taking a similar approach with a relatively new database called the UK Biobank UKB , which includes data from around half a million British volunteers who filled out questionnaires about themselves, underwent medical exams, and gave blood samples for genotyping. Formally launched in , the UKB published its ,person-strong resource in , and Kelso and Dannemann decided to see what information they could extract.
Among the many links Kelso and Dannemann identified as they dug into data from more than , individuals in the UKB was, once again, an association between certain Neander-thal variants and aspects of skin biology. People who carried Neanderthal DNA there tended to have pale skin that burned instead of tanned, Kelso says. And the stretch that included BNC2 was just one of many, she adds: around 50 percent of Neanderthal variants linked with phenotype in her study have something to do with skin or hair color.
Neanderthals thrived in Eurasia as a dominant hominin group for hundreds of thousands of years and have long been a focus of scientific inquiry. But less than a decade ago, researchers discovered that there was another group of archaic hominins that coexisted with Neanderthals and the ancestors of modern humans. DNA collected from a single finger bone and two teeth appeared to be neither Neanderthal nor human, and scientists named a new group, the Denisovans, after the Siberian cave in which the remains were found in Once researchers reconstructed the entire high-quality Denisovan genome in Science , —26, , it became clear that, like Neanderthals, Denisovans had interbred with modern humans during the time that they coinhabited Eurasia, with analyses suggesting that the introgressed DNA likely came from multiple Denisovan populations within the last 50, years, sometime after mixing occurred between Neanderthals and human ancestors Cell , P53— E9, ; Cell , P— E32, Their findings are the first to show human gene flow into the Neanderthal genome as opposed to Neanderthal DNA into the human genome.
This data tells us that not only were human-Neanderthal interbreeding events more frequent than previously thought, but also that an early migration of humans did in fact leave Africa before the population that survived and gave rise to all contemporary non-African modern humans. We previously mentioned the lack of genetic contributions by Neanderthals into the modern human mtDNA gene pool. There are several potential explanations for this. It is possible that there were at one point modern humans who possessed the Neanderthal mtDNA, but that their lineages died out.
It is also highly possible that Neanderthals did not contribute to the mtDNA genome by virtue of the nature of human-Neanderthal admixture. While we know that humans and Neanderthals bred, we have no way of knowing what the possible social or cultural contexts for such breeding would have been. Because mtDNA is passed down exclusively from mother to offspring, if Neanderthal males were the only ones contributing to the human genome, their contributions would not be present in the mtDNA line.
It is also possible that while interbreeding between Neanderthal males and human females could have produced fertile offspring, interbreeding between Neanderthal females and modern human males might not have produced fertile offspring, which would mean that the Neanderthal mtDNA could not be passed down.
While exciting, she adds, it also presents an analytical challenge. Yet acknowledging the winding roots of humanity and developing methods that can map out these twists and turns is the only way forward. In the last several decades, however, the driving question turned to mixing with modern humans.
Did these two hominins interbreed. In , with the first publication of a Neanderthal whole genome , scientists finally had an answer: Yes. Comparison of Neanderthal DNA to five living humans revealed that Europeans and Asians—but not Africans —carried traces of interbreeding. Studies since have hinted at some limited Neanderthal ancestry in Africa, but no one has fully traced these tangled branches of our family tree. Read more about the many lines of mysterious ancient humans that interbred with us.
For a fresh look at this genetic mixing, Akey and his team developed a new way to study the scattering of ancient hominin DNA in modern genomes. All models tackling this question must not only identify shared genetic sequences, but they also have to figure out what makes it similar because not all shared genetic code is the result of interbreeding.
Some DNA could be similar thanks to a common hominin ancestor. By setting up a model in this way, these analyses hide potential Neanderthal ancestry for people of African descent. Instead, Akey and his lab used large datasets to examine the probability that a particular site in the genome was inherited from Neanderthals or not. They tested the method with the genomes of 2, individuals from around the world—East Asians, Europeans, South Asians, Americans, and largely northern Africans—collected as part of the Genomes project.
They then compared this DNA with a Neanderthal genome. The results suggest that modern Africans carry an average of 17 million Neanderthal base pairs, which is about a third of the amount the team found in Europeans and Asians. The result suggests an order of magnitude or more Neanderthal ancestry in Africa than most past estimates. The straightforward answer would be that Neanderthals ventured into the continent.
Instead, the data reveals a clue to a different source: African populations share the vast majority of their Neanderthal DNA with non-Africans, particularly Europeans. Modeling suggests that just a tiny trickle over the last 20, years could account for its current distribution, Akey notes. Pinning down the timing is tough—a sliver of the genetic contribution also likely comes from more recent invasions of Africa, including the Roman empire and the slave trade, over the last few millennia, he says.
Some might have set out more than , years ago. These early wanderers likely interbred with Neanderthals more than , years ago, leaving their own genetic fingerprints in the Neanderthal genome. Thus a part of the Neanderthal DNA in African populations may actually be traces of this shared past.
Read more about what may be the oldest modern human yet found outside of Africa. Intriguingly, the new method also reveals slightly more Neanderthal DNA in modern Europeans that was previously overlooked, narrowing the baffling 20 percent gap once thought to exist between Neanderthal ancestry in Europeans and East Asians.
0コメント