Brief thoughts on Upper Paleolithic 'hybrids'

Science News has a long feature article by Bruce Bower that recounts the new wave of examining hybridization in human origins: “Animal hybrids may hold clues to Neandertal-human interbreeding”. He features the symposium at the AAPA meetings this spring organized by Rebecca Ackermann, which involved many scientists researching hybridization in primates and other mammals.

The big idea is that the biological consequences of hybridization are often recognizable in the skeleton. Fossil human populations may include individuals with ancestry from diverse populations, like Neandertals and Denisovans, much more genetically different than any living populations are. Paleontologists might use insights from hybridization in other mammals to understand which fossils show evidence of such population mixture. The big success story is the Oase 1 mandible, which has a high proportion of Neandertal ancestry:

Since the fossil’s discovery in 2002, paleoanthropologist Erik Trinkaus of Washington University in St. Louis has argued that it displays signs of Neandertal influence, including a wide jaw and large teeth that get bigger toward the back of the mouth. In other ways, such as a distinct chin and narrow, high-set nose, a skull later found in Oase Cave looks more like that of a late Stone Age human than a Neandertal.
Roughly 6 to 9 percent of DNA extracted from the Romanian jaw comes from Neandertals, the team found.
“That study gave me great happiness,” Ackermann says. Genetic evidence of hybridization finally appeared in a fossil that had already been proposed as an example of what happened when humans dallied with Neandertals.

Not only does the Oase mandible have a high fraction of Neandertal ancestry, roughly 3 times higher than any living people today, but also that Neandertal-like DNA occurs in large chunks, indicating a recent Neandertal ancestry. This individual may have had a Neandertal great-great grandparent.

Such a recent potential Neandertal ancestor means that the population mixture in this real-life case is not too far from what occurs in some natural and laboratory experiments on hybridization. The models from the present day have great relevance to this past case.

I will add, the Oase individual is not a singular case. Many early Upper Paleolithic skeletal remains from Europe have comparable evidence of anatomical features that are common in Neandertals but very rare or absent in other populations. David Frayer has documented many of these in detail, joined by other scientists, especially Milford Wolpoff.

It just happens that Oase 1 is the first specimen with ancient DNA evidence showing substantially elevated Neandertal ancestry beyond that observed in later populations. The ‘Ust-Ishim femur does not have more Neandertal ancestry than recent humans, and although the value for Kostenki 14 is high, it is not an outlier. Both those specimens have Neandertal DNA in relatively longer linkage tracts than today’s people, allowing us to see that mixture had preceded the birth of these individuals by only 10,000–15,000 years or so. But Oase 1 was nearly on the scene; where this individual lived, population mixture was probably still underway.

We know from ancient DNA evidence of later specimens. The earlier Upper Paleolithic populations had more genetic influence from Neandertals than populations from the Mesolithic and later. In part, this reduction of Neandertal ancestry reflects natural selection against functional parts of the Neandertal genome; in part it probably reflects repeated immigration into Europe of people from West Asia or other parts of the world. A pattern that appears in morphological traits, with Neandertal resemblance declining over time in European Upper Paleolithic and later populations, is paralleled by genetics.