A lot has been said though little is known. But our human curiosity obliges us to try and reach conclusions. When I posted almost a month ago that notorious piece: "Skulls Add to "Out of Africa" Theory of Human Origins", inspired by an article in the Scientific American bearing the same title (actually I stole the title from them, BITE ME!), I concluded, basing on the decrease of variability of skull morphology (as the author of the article did before me ), that interbreeding just didn't happen. Now I did mention in my piece that, in theory, interbreeding could have happened without leading to any offspring, without leading to any viable offspring, or without leading to any fertile offspring. To be perfectly clear, I am 100% sure that Homo sapiens individuals and Homo neanderthalensis ones had sex, that's certain, but the fact that coming out of Africa Homo sapiens populations kept losing diversity means that the Homo neanderthalensis genome did not enrich ours, or at least it did not enrich ours in the sections monitoring and dictating the structure of skull bones, with that being said I stand corrected. I had failed in my previous analysis to anticipate the fact that the study focused primarily on skull remains. It is possible that Neanderthal genetic material would have integrated ours but didn't do so in the section concerning skull structure (or, as it has become obvious for you, those that did inherit of DNA chunks from Neanderthals coding for the skull were not viable/not fertile).
However, a few days after I posted that article another one was published in the Nat Geo online edition this time titled: "Odd Skull Boosts Human, Neandertal Interbreeding Theory", written by Brian Handwerk. The author of the study, reported by Mr Brian, talks about a certain skull with a groove in the bottom of the back of the skull, look at this picture (taken from the same Nat Geo page):
Do you see the horizontal groove at the bottom of the head? This is not a normal groove, this is not a structure that serves a function. Let me explain this a little, Bones are a living tissue, its metabolism is pretty slow but it's alive! It contains cells and the hard substance is constantly melted down and built up again. The role of bones is to protect soft tissues and to support the muscle thus enabling the body to actually move. We usually attribute movement to the dynamic of muscles and not the static bones. But in fact bones and muscles are both essential to produce movement. Complimentary in role they are also complementary in structure. The larger the muscles are, the thicker the bones will be, because stronger, more powerful muscles require stronger more resistant bones, that's how Anthropologists and Paleoanthropologists rebuild the whole body of a Neanderthal from some bone remains (no they don't just guess and let their artistic talent prevail!). Look at this picture for example it's a picture of an Australopithecus:
And compare it to this one:
Source: Encyclopædia Britannica, Inc.
Compare most of all the zygomatic bone (cheek bone). See how MASSIVE it was in the Australopithecus skull? This could be somehow facilitated or coded by the DNA but basically, it is a feature gained over the lifetime of the individual, as his diet (mainly roots and rough vegetal material) demands a powerful bite his muscles and concomitantly his bones (both the jaw and the zygomatic ones) grow, our diet (mainly cooked and soft food) doesn't require that much pressure, there fore the bones are smaller and the aspect is softer.
Now back to our groove, that groove is NOT like that, it is innate, Neanderthals have it, it does serve them a certain role, but it is not correlated to muscular activity. But this skull is not a Neanderthal's skull, it is in fact a Homo sapiens skull! But it has a Homo neanderthalensis' groove... And this is not the only metis ever found. There is a significant number of metis remains found throughout Europe in that period (20 000 - 40 000 years ago).
As I said before, these variations might not be innate but their presence can have an effect on the life of the individual bearing it, thus allowing the work of natural selection. The change of climate and the more effective life-style of modern man helped him, with his anatomical variations to dominate and out-compete Neanderthals.
For example, and to help illustrate that concept, consider the joint structure. Neanderthals had more massive bones in general, there fore their joints were less flexible. On the shoulder level, this means that Neanderthals (or Neanderthal/sapiens hybrids that had thick bones) were less agile and efficient in throwing spears for example, there fore less efficient in hunting from a distance (targeting much larger preys and allowing a better income). So maybe Neanderthals' bone structures were just not fit enough, and it is in the end the law of the survival of the fittest. Source: James Owen's Nat Geo article: Neandertals, Modern Humans Interbred, Bone Study Suggests
A little criticism to this theory:
- It could be an accidental aberration, or variation:
According to Eric Delson, this feature could have simply occurred naturally and not necessarily inherited from some Neanderthal parent. In fact, he believes it could be found in modern populations, who knows? This would need to be proven. This is also the same argument used by skeptics when the Homo florensiensis remains were found for the first time in that Indonesian Island of Flores. Skeptics back then said it could be a case of Microcephaly. However that argument didn't stand back then and it's unlikely to justify all the Neanderthal-like variations. You see, and as I always say, fossil remains are very very very rare to occur, very few individuals are fossilized. So an already very rare variant of skulls is unlikely to be fossilized (this is a simple statistical concept). Now of course, you could say: "well a 1/10000000000000 chance is still a possibility, who knows? Maybe this individual had that super rare morphological feature and that the super rare chance of getting fossilized, who knows?"
That is true, but that doesn't explain the recurrence of a number of these Neanderthal-like aberrations, compare the number of remains with to the number of remains without them, how often did these aberrations occur? It is obviously higher... Why?
- Earlier DNA studies revealed a rather early split between Homo neanderthalensis and Homo sapiens occurred as early as 400 000 years ago:
The study (done by James Noonan) even focused on Nuclear DNA (DNA contained inside the nucleus of the cell) and previous studies (source: Hillary Mayell's Nat Geo article: Neandertals Not Our Ancestors, DNA Study Suggests) focused on mitochondrial DNA and found similar resutls. Neanderthals don't seem to have contributed our DNA! It just doesn't seem to have happened.
So individually speaking, Neanderthals and modern humans seemed to be interbreeding but, on the long run they didn't contribute to the construction of our DNA... I vote for viable non-fertile individuals, what do you think?
P.S.: no don't get impressed with the large number of links, I just opened the links that were available in the initial page.