From Nature magazine.
Our ancestors bred with other species in the Homo genus, according to a study published today in the Proceedings of the National Academy of Sciences. The authors say that up to 2% of the genomes of some modern African populations may originally came from a closely related species.
Paleontologists have long wondered whether modern humans came from a single, genetically isolated population of hominins or whether we are a genetic mix of various hominin species.
Last year, an analysis comparing the Neanderthal genome sequence to that of modern H. sapiens showed that some interbreeding did take place between the two species in Europe some time between 80 and 30,000 years ago and that, to a certain extent, Neanderthals 'live on' in the genes of modern humans.
It has been a mystery whether similar genetic mixing took place among Homo species even earlier, before the populations that became modern humans left Africa.
To find out, evolutionary biologist Michael Hammer at the University of Arizona in Tucson and his colleagues studied DNA from two African hunter-gatherer groups, the Biaka Pygmies and the San, as well as from a West African agricultural population known as the Mandenka.
Each of these groups is descended from populations that are thought to have remained in Africa, meaning they would have avoided the genetic bottleneck effect that usually occurs with migration. This means the groups show particularly high genetic diversity, which makes their genomes more likely to have retained evidence of ancient genetic mixing.
To find signs of infiltration from other Homospecies, the researchers looked at 61 non-coding DNA regions in all three groups. Because direct comparison to archaic specimens wasn't possible, the authors used computer models to simulate how infiltration from different populations might have affected patterns of variation within modern genomes.
Then they looked for such patterns of variation in the DNA of the three African populations. On chromosomes 4, 13 and 18, the researchers found genetic regions that were more divergent on average than known modern sequences at the same locations, hinting at a different origin.
Mixing things up
Hammer and his colleagues argue that roughly 2% of the genetic material found in these modern African populations was inserted into the human genome some 35,000 years ago. They say these sequences must have come from a now-extinct member of the Homo genus that broke away from the modern human lineage around 700,000 years ago.
Hammer says this disproves the conventional view that we are descended from a single population that arose in Africa and replaced all other Homo species without interbreeding. "We need to modify the standard model of human origins," he says.
Geneticist Sarah Tishkoff, who studies population genetics and human evolution at the University of Pennsylvania in Philadelphia, is more cautious. "This raises the possibility that there may have been ancient admixture with archaic populations," she says.
Tishkoff would also like to see further work. "Analyses of whole genome sequences of these populations will be necessary to more definitively test this hypothesis," she says.
But some researchers will require yet more convincing. "The authors model differences in very small parameters, such as the difference between no admixture and 1-2% admixture with an archaic population," says anthropologist Brenna Henn, a graduate student at Stanford University in Palo Alto, California. "The ability to discern complex models of demographic history with such a small data set, when many of the basic features of African genomes and history remain unknown, concerns me."
This article is reprinted with permission from Nature magazine. It was first published on September 5, 2011.
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Add CommentInformation about this early interbreeding and other aspects of the relationships, can also be found in this article, which refers to information first published over three years ago:
Reply | Report Abuse | Link to thisNational Geographic Article, New Science Journal Study Confirm Billy Meier Information on Prehistoric Humans
“This hypothesis” (that, “we are a genetic mix of various hominin species”) does violate the relative comfort of the “the standard model of human origins”. After all, we have a natural wired fondness for simplicity (the search for a unified field theory, the notion that everything is beautiful in its own way, our irrational belief in near literal equality of the sexes and so on). But, face it, as the murky past clarifies its bound to get ugly.
Reply | Report Abuse | Link to thisUnpleasant thoughts abound, for example, these sexual unions were not likely the product of “consenting adults”. One possible scenario: male hunting parties raid neighboring “Homo genus”, raping some of the females then retreating. Next, females have the mixed babies, and, most importantly, protect them from the males of their group (basic female instinct 101). Such could best be accomplished in a societal structure of polygamy (a dominate male with a harem). Thus, the bonded group of females works together, with sufficient collective physical power to thwart the larger male, and protects the bastard babies from being killed by the male (much as it works in lion cultures). And, so it could be a combination of proto-feminism & brute male savagery brought us to our present marvelous diversity of genome. (Perhaps Margaret Mead would have liked this analysis.)
Richard Carlson
"We need to modify the standard model of human origins," Hammer says. I think we need to do more than just modify the standard model of human origins and get our minds off this politically correct path we seem to got stuck in. Why don't Hammer and his team check the DNA of the Aborigines in central Australia? And why hasn't anyone done that so far? He may find more Neanderthal and African DNA there than he cares to report on.
Reply | Report Abuse | Link to thisThere is a big difference in 35,000 years and 700,000 years. Why can't he find ancient African and Neanderthal DNA in all of the humans north of the equator? According to Hammer's findings, there should be at least 2% of ancient African or Neanderthal DNA in every human north of the equator...but there isn't. Where did the humans come from that has no ancient African or Neanderthal DNA in them?
We do not have a single human fossil that looked like people today until about 10,000+ years ago. Instead of indiscriminately calling different fossils before then “species,” it is more likely that, after Lucy, Nariokotome Boy belonged to the beginning of a single species that, in divergent ways, with different parts evolving at different rates of speed, produced the high diversity that Mother Nature prefers. Admixture between members of the single species, over 2 million years, would produce humongous diversity. Especially now that we know that about 175 minor mutations occur at every human birth (cf Sean Carroll). There is no need to consider warlike behavior, genocide and rape. That occurred at a far greater rate per capita in civilization than among primitives. There is not a single Neanderthal among our 500 Neanderthal fossil remains who has a war wound except for one in Shanidar Cave in Iraq, with a spear head in his ribs, who may have received it from a cave-in as he was polishing the point, as cave-ins were more frequent among them than invaders probably ever were. We cannot assume that early sapiens were as badass mean as we are today.
Reply | Report Abuse | Link to thisModern studies of fossil DNA are seriously flawed by inability to define a criteria for sample. Because a bone looks like it came from a Neanderthal does not mean that its DNA represents all Neanderthals over 200,000+ years. This is like taking a Caucasoid off the New York City subway and sampling his DNA and saying,” Now we know all about Caucasoid DNA.” This is not how science is supposed to work. More on this, including a look inside the Late European Neanderthal brain, can be found on my blog www.paleoepica.com.
It seems that somebody recently published that the DNA replication machinery of african people differs from that of europeans and of other ethnicities. How do you handle or explain this ?
Reply | Report Abuse | Link to thisI am not surprised as even today 10 percent of us regularly stick their wil.....s in sheeps, cows, dogs whatever. The fact is sexual urge is so over powering and strong no mammal can control it. And people would stick it in any hole.
Reply | Report Abuse | Link to thisIt is now known (except to those who believe that the earth and the heavens are only 6,000 years old) that modern humans have up to 4% of Neandertal DNA. That the interbreeding was not voluntary on both sides has always seemed likely to me. so I must agree with rgcorrgk's comment from early this morning.
Reply | Report Abuse | Link to thisYour brutish analysis is completely based on individual imagination, it totally ignores most animal behavior patterns, except for grasping at a fantasy speculation relying upon some reported male lion behaviors when the alpha male changes.
Reply | Report Abuse | Link to thisYou misunderstand the reference. The genetic mixing occured 35,000 years ago; the species with which the genetic mixing occurred separated from our species 700,00 years ago, i.e. 650,000 years before the mixing. That is not a long time as species separations go.
Reply | Report Abuse | Link to thisMale lions frequently do kill cubs, particularly when they have just taken over a harem.
Reply | Report Abuse | Link to thisBut that aside, we know virtually nothing about the social behavior in these populations. Spinning rape fantasies about them does nothing to advance our knowledge in any way, shape or form. However, it does reveal a great deal about those who spin such fantasies.
Considering that a basic definition of "species" precludes the possibility of fertile offspring, why would we call these two populations different species? Now that we see a reasonable likelihood of interbreeding in the same range, should we not start referring to these groups as variant populations of the same species?
Reply | Report Abuse | Link to thisThis is bad news in the sense that the standard model allowed us to believe in a human genetic unity, racists will use this new insight to claim the the Neandertal hybrid had some special facility for creativity or some other nonsense.
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