On the site for their new book, The 10,000 Year Explosion, Cochran and Harpending share some “deleted scenes” that didn’t make it into the book, including this piece, which asks, Why do we normalize brain size by body weight?
Like other early humans, Neanderthals were relatively uncreative; their tools changed very slowly and they show no signs of art, symbolism, or trade. Their brains were large and had grown larger over time, in parallel with humans in Africa, but we really have no idea what they did with them. Since brains are metabolically expensive, natural selection wouldn’t have favored an increase in brain size unless it increased fitness, but we don’t know what function that those big brains served. Usually people explain that those big brains are not as impressive as they seem, since the brain-to-body weight ratio is what’s really important, and Neanderthals were heavier than modern humans of the same height.You may wonder why we normalize brain size by body weight. We wonder as well.
Among less intelligent creatures, such as amphibians and reptiles, most of the brain is busy dealing with a flood of sensory data. You’d expect that brain size would have to increase with body size in some way in order to keep up. If you assume that the key is how much surface the animal has, in order to monitor what’s causing that nagging itch and control all the muscles needed for movement, brain size should scale as the 2/3rds power of weight. If an animal has a brain that’s bigger than predicted by that 2/3rds power scaling law, then maybe it’s smarter than average. That argument works reasonable well for a wide range of species, but it can’t make sense for animals with big brains. In particular it can’t make sense for primates, since in that case we know that most of the brain is used for purposes other than muscle control and immediate reaction to sensation. Look at this way – if dividing brain volume by weight is a valid approach, Nero Wolfe must be really, really stupid.
Fictional detective Nero Wolfe is frequently described by Archie Goodwin, the narrator of the stories, as weighing “a seventh of a ton” (about 286 pounds or 130 kg).
Anyway, they go on to address the larger problem of Neanderthal intelligence:
We think that Neanderthal brains really were large, definitely larger than those of people today. This doesn’t necessarily mean that they were smarter, at least not as a culture. The archaeological record certainly indicates that they were not, since their material culture was definitely simpler than that of their successors. In fact, they may have been relatively unintelligent, even with their big brains. Although brain size certainly is correlated with intelligence in modern humans, it is not the only factor that affects intelligence. By the way, you may have read somewhere (The Mismeasure of Man) that brain volume has no relationship to intelligence, but that’s just a lie.One paradoxical possibility is that Neanderthals lacked complex language and so had to be smart as individuals in order to learn their culture and technology, while that same lack severely limited their societal achievements. Complex language of the type we see in modern humans makes learning a lot easier: without it, learning to create even Mousterian tools may have been difficult. In that case, individuals would have to repeatedly re-invent the wheel (so to speak) while there would have been little societal progress.
It could also be that Neanderthal brains were less powerful than you’d expect because there just weren’t enough Neanderthals. That may sound obscure, but bear with us. The problem is that evolution is less efficient in small populations, in the same way that any statistical survey – polls, for example -becomes less accurate with fewer samples. Natural selection is pretty good at eliminating a defective gene when its disadvantage is significantly great than the inverse of the population size. When the disadvantage is smaller than that, the defective gene has a reasonable probability of reaching high frequency by drift. It can even become universal in that population. This tendency is insignificant in large populations, but it can lead to problems in small ones, as more and more slightly deleterious mutations accumulate. There is a countervailing tendency — the generation of favorable mutations, which are likely to spread — but that tendency becomes weaker and weaker as the population becomes smaller. Thus, over the long term, a population that is too small is likely to go extinct for purely genetic reasons, if some other disaster doesn’t strike first. This is an issue that concerns conservationists who are trying to maintain endangered species such as the whooping crane or Florida panther.