That’s what Timothy Weaver, Charles Roseman and Chris Stringer, asked in a Journal of Human Evolution paper titled, “Were neandertal and modern human cranial differences produced by natural selection or genetic drift?”
It is an interesting question to ask because if you’ve ever spent some time looking at a Neandertal skull and compared it to a modern human’s skull, you probably asked yourself, why the hell are most features in the Neandertal skull so robust? What made it that way? And, if you haven’t had the pleasure of holding and comparing these two heads, no worries… Please use the following image plucked from the article, to ask yourself those questions. On your left is a cast of a Neandertal from La Ferrassie, France and to your right is a modern human skull from Polynesia:
Now, Stringer, Weaver, and Roseman wanted to see if they can explain these differences due to some evolutionary method. They specifically wanted to see if the differences can be attributed due to genetic drift or due to natural selection. But you maybe asking yourself what is the difference between genetic drift and natural selection? I sometimes confuse the definitions, so forgive me if I’m being redundant by sharing with you some common evolutionary theory.
Genetic drift, in the most basic definition is just the probability an allele shows up in a population. The effect of the drift may cause an allele and the biological trait that it confers to become more common or more rare over successive generations. Ultimately, the drift may either remove the allele from the gene pool or remove all other alleles. So that being said, genetic drift is the fundamental tendency of any allele to vary randomly in frequency over time due to statistical variation alone.
It differs from natural selection, because natural selection in the most basic definition is when beneficial alleles become more common over time because they boost the survivability of the organisms and reciprocally detrimental alleles become less common.
But genetic drift and natural selection aren’t mutually exclusive. In other words, both forces are always at play in a population. However, the degree to which alleles are affected by drift and selection varies according to circumstances such as population size. In a large population, where genetic drift occurs very slowly, a weak selection on an allele will push its frequency upwards or downwards (depending on whether the allele is beneficial or harmful). However, if the population is very small, drift will predominate. In this case, weak selective effects may not be seen at all as the small changes in frequency they would produce are overshadowed by drift.
Before we get deeper into evolutionary theory, I want share with you the answer to the million dollar question asked in the title of the paper, ‘Were neandertal and modern human cranial differences produced by natural selection or genetic drift?’ No, Stringer and crew can’t explain human and Neandertal cranial differences with either evolutionary forces at this time… natural selection can not be supported nor can genetic drift by their measurements and calculations. Instead, what they conclude is that the differences can be explained as manifestations of only two outcomes pooled from a vast space of random evolutionary possibilities. Which leads me into what Razib brings up. He mentions that,
“[he does] know that human bone structure and teeth have become less robust over the last 10,000 years, perhaps due to agriculture. This might simply be relaxing the selection for more robust physiques.”
Agriculture and other aspects of culture and technology can and does skew defining whether natural selection or genetic drift played a role on our bodies. For example, if one calculated and concluded that Mayan skull modifications were actually due to genetic drift and totally ignored or glanced over modifications being a cultural preference, then a incorrect conclusion would be made. Many cultures select for certain traits in skulls and bodies, which throws a big stick in the wheels of determining the evolutionary force in play. I feel that culture has been a large determining factor in deciding whether or not modern humans had more gracile skulls than robust ones. Take a second and ask yourself how our modern day popular culture is selecting for bodies?
So if I’m right, and culture has a big role, then I don’t know of any statistical method that could discern the effect of culture on traits in the skull. But, I’m thinking that one way where Stringer et al. or someone else can reproduce this work but also integrate measurements from other members of Homo, like H. heidelbergensis or archaic H. sapiens, like Homo sapiens idaltu to see whether or not gracility was selected for in the H. sapiens lineage or if it was a random? It’s a matter of simply expanding the sample size and type… maybe more can be derived or maybe more will be confused.