Modern Humans and Neanderthals: Did they "do it?"

Or, to be less crude, did modern humans, having already evolved in Africa, interbreed with the local Europeans who were Neanderthals, and if so, did they produce fertile offspring ... and, did this happen in sufficient degree to have mattered at all to the genetics of later (but not necessarily living) people?

In my opinion, the answer is, of course they interbred. There are many reasons to believe this if almost no way to prove it. Indeed, the evidence of this interbreeding is virtually nil. With every additional test of the interbreeding hypothesis using DNA, the null hypothesis of no interbreeding is not falsified. Every morphological hybrid that is put forth is eventually, it seems, retracted.

I can't believe that they would not have interbred, yet there is not a shred of evidence that their DNA mixed, either in the form of the DNA itself or in overlapping or mixed phenotypes.

The latest study, that I'd like to report on here, is coming out shortly in the Journal of Human Evolution. It carefully looks at remains from Cioclovina Cave in Romania. This would be an excellent candidate, based on its age and location, for evidence of meaningful (as in produced offspring, not "oh, yes, our relationship is very meaningful") interbreeding between local Neanderthals and incoming Africans or Asian-Africans.

Here is the Cioclovina Calveria:

This is Figure 2 from the original paper, and the caption reads: "The Cioclovina partial cranium. a) Lateral and b) frontal views. c) Close up of the Cioclovina nuchal region. d) Posterior view of La Chapelle-aux-Saints, showing the occipital torus and suprainiac fossa."

The trick is to compare this skull with the skulls of the indigenous Neanderthals and the incoming Africans (or Asian/Africans) and see if this skull (Cioclovina) fits into one, the other, or is perhaps in between.

One could look at this skull and easily say "This is a modern human" or at least "this is not a Neanderthal." If that is acceptable as a starting point, then one could modify the above to the following question:

Is Cioclovina a full fledge modern human of the type that existed in those days, or does it have Neanderthal Traits, as though its parentage included Neanderthals?

One can formulate these statements into various hypotheses and null hypotheses, but in my view that is a misguided effort at this level. Hypothesis testing requires a much greater degree of control and understanding of the variables for the negative results of such a test to truly falsify something. What we are really looking at here is an exercise in "alpha-taxonomy" ... the primary classification of specimens into basic categories ... where we are trying to fit a particular specimen into a taxonomic category that would exist if Neanderthals and modern humans a) "did it"; b) produced offspring; and c) the offspring were intermediate or had a mixture of traits. This is not necessarily how the authors of this paper think of their comparison, but I think it is useful.

Of course, there is not really a "modern human" sample of skulls and a "Neanderthal" sample of skulls to make the comparison. There are some materials that are probably ancestral to Neanderthals (but maybe not), the "classic" Neanderthals, and several different groups of modern humans, including the living ones (like you and me) and various more out-of-date modern humans. Mixing samples together would likely cause the loss of important information, and keeping the samples more separated results in better behaved statistics and prettier graphics (these two results not entirely unrelated).

You will have to read the original paper for all the details, but these are the categories of material used in the analysis:

NEA

Classic Neanderthals from Europe and West Asia. This includes all the usual suspects such as Amud, La Chapelle-aux-Saints, La Ferrassie, Shanidar, and Tabun.

MPE

Middle Pleistocene Europeans ... These are early, dating to prior to European modern humans or Neanderthals. this includes P{etralona and Sima de los Huesos 5.

MLP

Middle and Late Pleistocene modern humans from Africa and the Levant. These are the "African morphology" modern humans. and are probably most like the "Modern Humans" but typically, we would expect, with increased robusticity and bone thickness (a general trend in all lineages, even non-primate, through the middle and late Pleistocene). This includes Qafzeh, Saldahna and Skhul.

UPE

Upper Paleolithic Europeans. These are the European skills which are most likely to either look like modern humans or show hybridization, depending on whether or not hybridization occurred. The total number of these remains has been reduced in recent years because of redating of some of the specimens. This sample includes, for instance, Cro Magnon.

Recent Humans

This sample includes a wide range of Africans, Asians, New World, European, Australian material.

The study is very detailed, uses several different statistical techniques, and is very thorough. The conclusion of the study is

Our description and analysis rejected all four tests of the hypothesis that Cioclovina represents a Neanderthal-early modern human hybrid. The specimen does not exhibit any of the morphological patterns predicted for a hybrid. ... it can be accommodated quite plainly within the variability of the Upper Paleolithic sample. Cioclovina is a fully modern human in all respects examined and analyzed and there is no hint in its anatomy that it may be a Neanderthal-early modern human hybrid.

The details are myriad and beyond the scope of a mere blog post such as this one. This paper would make an excellent exemplar for research in Physical Anthropology for a methods course. Here, I can only touch on a couple of the key results that relate to this conclusion.

The following shows the relationship between the shape of the skull based on all the available data for a "modern human-like" (blue) and a "Neanderthal-like" (red) template.

This is based on the first principal component of an extensive and intensive morphological study comparing these taxa. Print this out and bring it with you on your next field trip just in case you find a skull ... you'll be able to distinguish between Neanderthals and Modern Humans.

In all the comparisons and analyses presented in th is paper, the Cioclovina skull groups with the Modern Humans and not the Neanderthals. Within the modern humans, it groups especially among the Africans and somewhat less so among he New World and Asian material. It also does not group with the "MLP" set particularly well.

One of the better representations of this finding is seen in the following figure, which is a graphical representation of a set of Mahalanobis Squared Distances ... a measure of dissimilarity among metrics, essentially ... applied to the samples.

This shows Cioclovina among the moderns, very distant from the Neanderthals and their presumed ancestors. The MLPs are themselves intermediate between the the group that includes the UPEs, Recent humans, and Cioclovina.

Note that the MLP group includes individuals that in the past have been suggested as possible "hybrids" or at least intermediates. These specimens rest in both time and space between Classic Neanderthals and the earlier African Moderns. If Neanderthals and moderns of 40 thousand years ago shared a common ancestor just under a half million years in age (which they probably did) and had very little intermixing of genes during the previous 100 thousand years (i.e., since the rise of Neanderthals as a distinct morphology) then we would expect the MLP group to be where it is on this chart or in similar data sets.

In the end, Romania is a romantic place. This is obvious. Romance languages, Gypsy violinists playing the background music, Mysterious mountains and ancient, beautiful cities. But either this did not apply several tens of thousands of years ago, or sex among the Neanderthals and Moderns was not productive, in and near the vicinity of Cioclovina Cave. Or, most of the Neanderthal alleles that result in distinct phenotypes were recessive. (Which we know in the business as "Wolpoff's Worst Nightmare...")

HARVATI, K., GUNZ, P. & GRIGORESCU, D. (2007): Cioclovina (Romania): Affinities of an early modern European.. J Hum Evol, Advanced Online, .