While I am in the midst of an ongoing series on Adam, Eve and population genetics, a recently published analysis of an archaic human genome is too rich to ignore until my current series ends – and indeed, this new evidence will be useful as that series continues. As such, we will take a brief hiatus and explore these new and exciting findings.
This new study shows that a modern human living in Europe between 42,000 and 37,000 years ago had a very recent Neanderthal ancestor, within six generations back. These results give us more information about how human and Neanderthal populations interacted over time and geographic space, provide a clear example of how present-day humans are not the only individuals we need to account for in determining the population size of our ancestral lineage, and close a loophole that one Christian apologetics organization hoped would explain away the evidence for interbreeding.
Assembling all of this information reveals the following tale: the common ancestor of Neanderthals and Denisovans migrated from Africa to the Middle East between 500,000 and 300,000 years ago, leaving a population behind that would eventually become modern humans (at around 200,000 years ago). In the Middle East, the populations destined to become Neanderthals and Denisovans part ways, with their differences accumulating over the next several hundred thousand years to make them distinct species. When a population of modern humans leave Africa around 50,000 years ago, they encounter, and breed with, Neanderthals shortly after. This genetic exchange is small, since there are partial reproductive barriers in place, but a small fraction of Neanderthal DNA becomes established in this lineage. Groups from this population then part ways, with some migrating into Europe and others into Asia. This latter group then encounters the Denisovan hominids, interbreeds with them, and a fraction of Denisovan DNA takes hold as a result. This population goes on to colonize southeast Asia, Oceania and Australia, where we see this variation today in Melanesians. Modern humans thus have different evolutionary trajectories: Melanesians have both Neanderthals and Denisovans in their lineage, Europeans have Neanderthals, and Africans have neither.
Given the rapid technical improvements we are seeing in paleogenomics (the sequencing of DNA from ancient remains), it was really only a matter of time before new information would be added to this overall picture. Last week, such a study was published: it analyzed the DNA of a European, modern human (named Oase 1 for the location he was found) who lived between 42,000 and 37,000 years ago. The results of the analysis were both straightforward and intriguing: this individual had much more Neanderthal DNA than present-day humans do (6-9%), and a large fraction of his Neanderthal DNA is present in continuous blocks – in other words, in several locations in his genome, he has runs of Neanderthal DNA that are far, far longer than those seen in any present-day human. Taken together, these results indicate that not only did this individual have Neanderthal ancestry, but he had very recent Neanderthal ancestry, within the six generations preceding him. At the point of hybridization, his human/Neanderthal hybrid ancestor would have two chromosome sets: 23 from the human parent, and 23 from the Neanderthal parent, with one chromosome from each making up each pair. This hybrid individual would thus have runs of Neanderthal DNA as long as each chromosome. In the successive generations as this individual bred with a human, their offspring bred with a human, and so on, these long stretches of Neanderthal DNA would be broken into smaller and smaller stretches by recombination with human chromosomes. The fact that the subject of the study still has a several very long runs of Neanderthal DNA in his genome allows us to estimate, based on known human recombination rates for various locations in the genome, just how many generations exist between the subject of the study and his hybrid ancestor. The results indicate that not more than six generations separate them, or less than 200 years.
Scientifically, these results are significant because they are the first genetic evidence that humans and Neanderthals interbred in Europe, long after the initial interbreeding events that occurred in the Middle East shortly after humans left Africa. For Evangelicals interested in the evolutionary history of our species, these results are a “smoking gun” for human/Neanderthal interbreeding. Moreover, these results close a particular scientific loophole that one Old Earth Creationist (OEC) organization, Reasons to Believe (RTB) has promoted as an alternative interpretation of the Neanderthal/human interbreeding evidence. Let’s explore how this new evidence creates significant problems for RTB’s model.
Neanderthals and Reasons to Believe: new evidence, new problems
When the Neanderthal genome was first sequenced and analyzed in 2010, RTB came out strongly against the notion that humans and Neanderthals interbred, since this notion stands in sharp contrast to their stance that humans are separate creations that share ancestry with no other species. As such, RTB views Neanderthals as animals, and human/Neanderthal interbreeding as humans committing bestiality (for those following my current ongoing series, we will soon see that William Lane Craig holds a similar stance).
In 2012, RTB became aware of a minority scientific hypothesis – a proposal that the Neanderthal DNA seen in non sub-Saharan humans is the result of an ancient population structure in Africa prior to humans (or Neanderthals) leaving, not later interbreeding with Neanderthals in the Middle East. In this model there are at least two populations of hominins in Africa with limited genetic exchange between them, and Neanderthals and non-African humans emigrate from the same sub-population. In other words, the 1-3% Neanderthal DNA we see in non-Africans is because these two groups are, for a portion of their genomes, closer relatives to each other than either is to the population that gave rise to African humans. RTB has recently promoted the aspect of the model that suggested humans did not interbreed with Neanderthals as they left Africa, without discussing how the model requires some humans to share a more recent common ancestor with Neanderthals than with other human populations. While this minority hypothesis has not stood up to further experimental scrutiny, the point is now null and void for RTB in any case: we now have evidence that modern humans and Neanderthals interbred that cannot be explained by ancient African population structure. Neanderthal DNA is found in modern humans in only tiny segments, due to 60,000 years of recombination breaking them into ever-smaller stretches. In the case of Oase 1, however, we have the DNA sequence of a human with large continuous swaths of Neanderthal DNA from a very recent hybridization event. Such large swaths of DNA could not have been transmitted for hundreds of thousands of years from an ancient African sub-population without recombination: they are recent, and indicate that humans interbred with Neanderthals within the last 42,000 years.
As our technological ability to recover and sequence DNA from ever more ancient and poorly preserved samples improves, the trajectory of the evidence is likely to provide increasing challenges to RTB’s model regarding Neanderthal/human interbreeding.