My fondness for Neanderthals is documented, and one thing that has always been on the teeter-totter of scientific opinion was the mechanism of their extinction. Did they just get out-competed by Homo sapiens? Were they murdered, driven to extinction? Did they just die out, unrelatedly? Or were they absorbed, swallowed up through inter-breeding? For a while it seemed that there might not have been any gene-mingling—which seems odd, given humanity’s fairly active sexual appetite—which sort of stood against the notion of hybridization. Since the completion of the Neanderthal Genome Project in 2010, that tide has shifted, and Scientific American’s May issue has the most recent broadside in that volley, in an article by Michael F. Hammer called “Human Hybrids.”
I think it is smart to announce biases up front, and in this case in particular, it bears mentioning. The Out of Africa model’s ascendency over Multiregionalism—the idea that humans evolved in pockets, then intermingled, as opposed to Out of Africa’s single origin—was how I learned things in the first place, back in college. Recently, Out of Africa’s champion, Christopher Stringer, published Lone Survivors, which is the other thread in the genome of my bias, if you’ll forgive such an outrageous metaphor.
In Lone Survivors, Stringer talks about modifications to the Out of Africa model, so I was already fairly up to date on the armchair scholarship front for my read of the Scientific American article. Specifically, regarding the African Replacement model, which has been the standard for the past few decade: the idea that Homo sapiens evolved in Africa, while other Homo breeds like Neanderthal were already extant in Europe. Humans left Africa, then…well, killed or out-competed the other hominins.
In contrast, voices like Günter Bräuer and Erik Trinkaus have argued for a more flexible version of this theory, the Hybridization model, which is largely and broadly sort of a version of the Out of Africa model that incorporates the possibilities of hybrids—of the archaic Homo sapiens leaving Africa and not just “replacing” the existing populations but also, well…mating with them, at least a little. The question then becomes—did Homo neanderthalis just breed into modern “Humans” completely, disappearing—the Assimilation model—or was the gene flow more sporadic, only occasionally bringing new traits into the sapiens pool?
For a while, the conversation was largely being held in an echo chamber; with fragmentary archeological evidence, it was hard to make a convincing argument, though it managed to be enough to put Multiregionalism into decline. Increasingly, however, genetic analysis has been transforming the debate. The first volley was from mitochondrial DNA—you know all about the tiny little guys swimming in our cells, I’m sure—and from the start, it was a mixed bag. When UC Berkeley’s “Mitochondrial Eve” analysis came out in ’87, it confirmed the Replacement model; there was a clear line back into Africa, across the board. It took another decade for the Max Planck Institute to put out its findings that there didn’t appear to be any interbreeding between sapiens and neanderthalis.
Ah, but then things started changing as the picture got crisper, as more information (not just mtDNA) started flooding in. Different bits and pieces of human DNA started showing outliers, pieces of the genome that seemed to come from Asia rather than Africa, with an age dating to before the Homo sapiens exodus from Africa. From there, the evidence took a decided turn toward the models with at least some elements of hybrization. Svante Pääbo at the Planck Institute’s Neanderthal Genome Project estimated that a full 1% to 4% of people of non-African descent’s genome is Neanderthal.
Upsetting the primacy of the Replacement model even further was the fact that people of Oceanic descent—Aboriginal Australian, Melanesian, Polynesian, etc—shared 1% to 6% of their genome with Denisovians, while people of Eurasian and African decent did not. What exactly the Denisovians are is a bit of a mystery, for the moment—the evidence points to them as another species of human, like sapiens or neanderthalis, to my eyes, though there are voices that posit them as a sub-species or a break-away hybrid group—but the fact that humans of various species interbred with each other in a meaningful way is now becoming the consensus.
Michael Hammer, the author of the Scientific American article, with his team at the University of Arizona and Jeffrey D. Wall at UC San Francisco, dug deeper into the genetic soup, focusing on Africa. Though Neanderthal (and Denisovian) is the popular focus, Africa would have provided many opportunities for mating between groups, both on the basis of Africa as the home of the most hominin diversity and the longest periods of species existing along side each other. They concluded—based on computer analysis of three populations—that 2% of those groups’ genome came from interspecies sources. A mystery regarding a South Carolina man of African descent’s Y chromosome and fossils at Iwo Eleru in Nigeria and Ishango in the Congo (where the famous primitive lunar calendar is from) offer further evidence for admixture.
Did this mingling provide evolutionary advantages? Analysis of a few key regions seems to suggest that it does; some of the inherited sequences relate directly to the immune system, for instance, and the evidence seems to suggest that they spread at a higher rate because of pressures from natural selection. The picture painted is a complicated one: it supports the Out of Africa model…but. It appears that Homo sapiens didn’t evolve in a vacuum; rather, relatively isolated but significant interspecies breeding happened. Hammer suggests that the African Multiregional Evolution model, in which hominins of various species shared DNA as humans moved from archaic to modern and then tapered off, in combination with Hybridization. What the complete picture is, however, is elusive, but at least our understanding of it is deepening.