Endless Forms Most Beautiful - Denisovans and Neandertals

Denisovans & Neandertals

The 2010 discoveries of Denisovans, the 2012 findings of archaic African DNA, and the additional 2012 genetic sequencing of Denisovans are part of a longer trajectory dating to the discovery of fossil Neandertals. Ever since the fossil Neandertal discoveries in the 19th century, debates have raged about who they were. Were Neandertals direct ancestors to modern humans? A completely different species? Or a sub-species, like a race? And now what should we do with the Denisovans?

Anthropology can now confidently report that Neandertals, Denisovans, and others labelled archaic are in fact an interbreeding part of the modern human lineage. We are the same species. There has been extensive admixture across modern humans for tens of thousands of years, and at least some admixture across several archaic groups. Neandertals, Denisovans, and other archaics may be the best example of a true human race or sub-species. They are also fully part of the human lineage, with almost all contemporary humans showing genetic admixture with archaics in our genetic signatures.

Findings published in July 2017 seem to help resolve some of the puzzles that the 2010-2012 discoveries raised. As Carl Zimmer reports in the New York Times:

The common ancestors of Neanderthals and Denisovans spread across Europe and Asia over half a million years ago. Gradually the eastern and western populations parted ways, genetically speaking.
In the east, they became Denisovans. In the west, they became Neanderthals. The 430,000-year-old fossils at Sima de los Huesos — Neanderthals with Denisovanlike genes — capture the early stage of that split.
At some point before 270,000 years ago, African humans closely related to us moved into Europe and interbred with Neanderthals. Their DNA entered the Neanderthal gene pool. (In Neanderthal DNA, Signs of a Mysterious Human Migration; see also the finds of Homo sapiens in Morocco which helped set the stage for these earlier dates.)

Before the Replacement Hypothesis

The most recent findings can help return anthropology to the work done prior to the rise of the replacement hypothesis in 1987-2010. During these decades, researchers increasingly portrayed Neandertals as a completely separate species, an evolutionary dead-end with little or no interbreeding (see section on More Mothers than Mitochondrial Eve). This reached a crescendo in 2007, as prominent anthropologist and paleontologist Ian Tattersall pronounced in the Journal of Anthropological Sciences:

Interestingly, the new Neanderthal skeletal reconstruction . . . suggest[s] that differences in gait existed between Neanderthals and modern humans. In particular, the very broad and short waist would have imparted a “stiffness” to Neanderthal movement that would have made them cut a very distinctive figure on the landscape. The consequent distinctive behavioral signal further reduces the probability that the two kinds of hominid would have shared any elements of a specific mate recognition system, and that any biologically significant level of gene exchange ever occurred between them. (Neanderthals, Homo sapiens, and the question of species in paleoanthropology, 144)

I’ll admit to some childish laughter when we read this in class. The thought of modern humans being turned off by the stiff gait of the Neandertals is pretty funny stuff. It is also wrong. Current studies show genetically significant interbreeding with Neandertals, Denisovans, and possibly other archaics did occur. Milford Wolpoff’s idea that Neandertals should be considered a subspecies or race of humans seems closer to the truth (Wolpoff “How Neandertals inform human variation,” 2009). Neandertals are distinctive, so distinctive that many would say they were a separate species. Denisovans seem to be in a similar position. These are what races would really look like, not like the relatively minor differences observed in contemporary humans (see section Race Reconciled Re-Debunks Race).

Anthropology & Species Classifications

Tattersall’s article opens the larger question of species classification:

How to apportion the large mass of hominid fossils now known into biologically meaningful units has been debated endlessly, and seems set to splinter paleoanthropology for years to come. The negative consequences of this lack of consensus are severe . . . This is bad enough among colleagues . . . but it is nothing short of disastrous when it comes to communicating our science to the public that supports us. (2007:139)

Tattersall is correct about this potentially disastrous communication. But his answer–to say species are like “individuals” and knowing what a species entails is like how judges “claim to know pornography when they see it even if they cannot satisfactorily define it” (2007:140)–may perhaps be even more disastrous than splintered debate.

There is a better way. “Thus, we think formally of a species in terms of reproductive compatibility, with the trait-list as helpful identifiers, rather than as a set of organisms that share a particular suite of attributes” (Marks 2009:237-238).

Anthropology should go beyond the endless definitional debates to show porous species boundaries. There is really no harm in stressing interaction and admixture within and across species lines, welcoming Neandertals and Denisovans into the human family. The public is actually quite interested in these kinds of crossings.

Anthropology should work to undefine species.

Anthropology needs to stress how it is impossible to define species characteristics outside of an environmental context. “There is no formal, species-specific ground-plan hovering in the background, immune from time and change” (Ingold 2006:263). Or, “If evolution has taught us anything, one might think, it is that there is no essence of humanity, no fixed or final form” (Proctor 2003:220).

There are several hopeful signs:

a) Multispecies

A 2010 special issue of Cultural Anthropology called The Emergence of Multispecies Ethnography (Kirksey and Helmreich 2010). As a follow-up, the 2010 American Anthropological Association Meetings featured a “Multispecies Salon.” Other anthropologists have stressed interspecies interaction, such as Insectopedia by Hugh Raffles (2011), Friction: An Ethnography of Global Connection by Anna Tsing (2005), and the edited volume by Rebecca Cassidy and Molly Mullin, Where the Wild Things Are Now (2007). Such multispecies interactions set a framework for fuller inclusion of Neandertals and Denisovans.

b) Hybridization and Admixture

A 2010 article on species hybridization reports:

It turns out that hybridization among distinct species is not so rare. Some biologists estimate that as many as 10 percent of animal species and up to 25 percent of plant species may occasionally breed with another species. . . .
The discovery of hybrid species and the detection of past hybridizations are forcing biologists to reshape their picture of species as independent units. The barriers between species are not necessarily vast, unbridgeable chasms; sometimes they get crossed with marvelous results. (Carroll 2010, Hybrids May Thrive Where Parents Fear to Tread)

The author, Sean B. Carroll is at the forefront of the Evo-Devo biology discussed in the section Human Nature and Anthropology.

c) Canis soupus

There is the recognition that dogs, coyotes, and wolves are all part of one big species. “No wonder, then, that interactions among these species have led to a genetic mess that researchers sometimes refer to as ‘Canis soupus’” (Yoon 2010). That one of the most beloved household pets is species undefined can help foster a practical appreciation for why we need to undefine species.

d) Paleoanthropologists too

Discoveries of Denisovans and Neandertal admixture may be causing movement within the paleoanthropological community, such as discussed in Is the Biological Species Concept a “minority view”? (Hawks 2011).

Denisovans and Their Discontents

Still, there have been some problematic aspects to welcoming Neandertals and Denisovans into the human family. Several very wrongheaded interpretations of Denisovans, Neandertals, and archaic admixture have emerged. Because the dominant paradigm has been the replacement hypothesis rather than gene flow across a wide range, contributions from Neandertals and Denisovans are seen as affecting only specific peoples. Since most analyses are carried out in genetic language within a society convinced of genetic determinism, the Neandertal contribution is seen as adding a set of genetic resources other groups do not share.

Typically, these wrongheaded interpretations come from the reporting of Nicholas Wade. Wade’s prominent 2010 reporting put in place a view that “Neanderthals interbred only with non-Africans, the people who left Africa, which would mean that non-Africans drew from a second gene pool not available to Africans” (Signs of Neanderthals Mating With Humans). And in 2012, Wade still misrepresents admixture as “interracial marriage“!

I asked Keith Hunley, one of the editors of Race Reconciled for insight. I am grateful for his clarifications.

First, Hunley notes that even with Neandertal admixture for non-Africans–and admixture evidence with Denisovans–the greatest genetic diversity is still found within sub-Saharan Africa. This is similar to what John Hawks describes on his blog: “Africans are a lot more diverse than other populations, and this diversity itself does reflect the dynamics of the ancient African population. The Neandertals aren’t so different from that pattern that now still exists within Africa–they’re extending the notion that ‘modern’ is something that’s been evolving for a long time” (2010). And it seems the 2012 findings are a confirmation of complex population structures, migrations, back-migrations, and admixture as a rule, not an isolated event.

Second, only a tiny portion of this variation is unique to non-Africans. It has taken a lot of effort and extremely sophisticated analysis to even find this non-African variation. At least some of these analyses are of “neutral genetic variation,” which does not contribute to phenotype (Hunley, personal communication 2010).

Third, people migrated back to Africa, carrying the diversity with them, and it would also exist among contemporary peoples of African descent living in Europe or the Americas. This is also supported by Hawks: “Do living Africans have Neandertal ancestry, too? . . . We know that the answer is nonzero, because Africa has received immigrants from other parts of the world during historic times. The same genetic patterns that reflect population contacts up and down the East African coast, and across the Sahara into West Africa, show the possible conduits for the flow of Neandertal-derived genes into African populations” (2010).

Finally, it is worth noting how much the human genome has been changing since the time of Neandertals and Denisovans. This is a little-known feature that is only now starting to receive more attention. As Hawks ends his “Neandertals Live!” post: “In adaptive terms, it is really true–we’re more different from early ‘modern’ humans today, than they were from Neandertals. Possibly many times more different” (2010).

Sadly, racist interpretations of Neandertals, Denisovans, and archaic admixture are already circulating. The claim is the Neandertal admixture helped non-Africans become more “advanced” whereas Denisovans were a step toward lower IQs. This kind of thinking can derive from statements like the one in the Sean B. Carroll article on hybridization: “It now appears that 1 percent to 4 percent of the DNA sequence of Europeans and Asians, but not Africans, was contributed by Neanderthals mixing with Homo sapiens, perhaps in the Middle East 50,000 to 80,000 years ago. It is possible that some Neanderthal versions of genes enabled modern humans to adapt to new climates and habitats” (2010). This statement had innocent intentions, but commenters need to understand the extent of entrenched racism, combined with a public steeped in genetic determinism–for a 2012 follow up, see Denisovan Brains, Science, Media, Anthropology.

Hunley’s comments, with back-up from Hawks, are a very smart and necessary rejoinder to some press portrayals of Neandertals and Denisovans. They provide a guideline for what the public needs to hear when presenting these studies. It is important for anthropologists to remember the press is not a place to play out debates between competing scientific hypotheses. Most people do not know the details of any competing hypotheses–what we need are statements about what happened and implications for contemporary human populations.

Words matter, and it is important to use them carefully. Consider, for example, the multiply co-authored article “Why not the Neandertals?” This 2004 article was certainly an interesting and important statement at a time when the out-of-Africa replacement hypothesis had multiregionalism on the run. However, the final two sentences are very strange: “For us Europeans, the Neandertal debate is nearing resolution and the conclusion is that they are one of us. Recognizing this is a key step in the process of understanding how and why we became different” (Wolpoff et al. 2004:538).

First, who are “us Europeans” exactly? There are nine co-authors listed in the paper, all affiliated with universities in the U.S. Are they saying all the co-authors are a European us? Or targeting European readers? And while becoming “different” is not necessarily a good thing, they must understand–especially if they are all European–that the context of European difference in world history has been one of asserting superiority and claiming the mantle of civilization. Such sentences undermine the goodwill the multiregional hypothesis might have generated.

Fortunately, at least in the latest portrayal of Neandertal interbreeding, “the Leipzig scientists assert that the interbreeding did not occur in Europe but in the Middle East and at a much earlier period, some 100,000 to 60,000 years ago, before the modern human populations of Europe and East Asia split” (Wade 2010). Unlike Wade, I would not emphasize a European and East Asia split–see It’s admixture all the way down–but at least this means we can stop looking to Neandertals as the source of exclusively European difference.

So, let us accept Neandertals, Denisovans, and other so-called archaics as members of the human species, perhaps as a true sub-species or race. But let’s retain the beauty of the original multiregional model proposed by Franz Weidenreich, with human populations “being interconnected by nearly continuous gene flow throughout the Pleistocene, with the gene flow being of sufficient magnitude such that the human continental populations define an intertwined trellis. There is no tree of human populations of any sort in Weidenreich’s figure” (Templeton, Genetics and Recent Human Evolution, 2007:1509).

This last point brings up the idea of metaphors and how we should represent human evolution–see blog-post The Tangled Bank and the blog-post by Jonathan Marks, Clades versus Rhizomes: “I think human evolution is strongly rhizotic.” Indeed, the rhizome model is strikingly similar to the Weidenreich’s intertwined trellis. I was interested to see Dienekes Anthropology blog take up the same issue in the 2012 blog-post Admixture matters: “It’s time to give up trees and embrace networks!” It seems when everyone is dropping the tree taxonomies and embracing networks (or maybe even rhizomes?), we are in a new place to appreciate the complexity of human becoming, and More Mothers than Mitochondrial Eve.

Next: 1.12 – More Mothers than Mitochondrial Eve
Previous: 1.10 – Stone Tools for 2.5 million years

To cite: Antrosio, Jason, 2012. Denisovans, Neandertals, Archaics as Human Races. Living Anthropologically, http://www.livinganthropologically.com/biological-anthropology/denisovans-neandertals-human-races/. Last updated 5 July 2017.