The incredible shrinking human brain
March 15, 2010Posted by on
The Times Online recently ran a story about a French team that have made an endocast by digitally scanning inside the skull of Cro-Magnon 1, perhaps the most famous of all Upper Palaeolithic skulls. The mould of Cro-Magnon highlights what has been long known about these European early modern humans since their first discovery in 1868 – they had bigger brains than us. In fact, average human brain size has been decreasing during the last 30,000 or so years. This revelation was rather troubling for nineteenth-century anthropologists who sought to link brain size with intelligence. Not only did these early modern humans have a larger brain volume than us, but so too did the Neanderthals, who were regarded by many at the time as “a barbarous and savage race” (Schaaffhausen 1858). To add injury to insult, the decrease in head size coincides with some of the greatest cultural innovations in human history.
The Times article forwards a number of the various hypotheses about why brain size has decreased. Antoine Balzeau reasons that “the cerebellum — a brain structure linked to language and concentration — appears to take up a larger proportion of the head now than in the time of Cro Magnon 1.” While it is true that the cerebellum is proportionally larger in modern humans, it is proportionally smaller than in apes, by around 20%. We still don’t know enough about brain function to be able to say what advantage, if any, a larger cerebellum would give us.
Second up, is the suggestion that big heads are somehow an adaptation to cold climate. There are a number of problems with this idea. If having a large skull is an adaptation to cold environments we would expect to see such traits peaking in the aftermath of the Last Glacial Maximum around 20,000 years ago, many millennia after Cro-Magnon walked the earth. As a general rule people living in Arctic regions tend to have more rounded heads, unlike the long headed Cro-Magnon. What’s more, the limbs of Cro-Magnon and their kin are quite long, contradicting Allen’s Rule which predicts that species will evolve smaller appendages as an adaptation to colder climes. Their body type also differs markedly from that of Neandertals, for whom there is a better case to made of being cold-adapted.
The article goes on to suggest diet as a driving force behind the decrease in head size. Cranial robusticity has indeed been shown to correlate with diet. It is important, however, to make the distinction between cranial size and robusticity. While the two are related they do not necessarily go hand in hand. While the Gravettian populations were undoubtedly more robust than most modern-day populations, they are not especially robust when compared to the Mesolithic populations of Téviec and Hoëdic or modern Aboriginal Australian or Fuegian populations. It is also unclear what dietary innovation could account for the decrease in head size. We have unambiguous evidence for the control of fire at around 250,000 years ago, while agriculture did not appear until around 10,000 years ago. The dates just don’t add up.
The article suggest one more hypothesis for the downsizing of the brain: “… with high infant mortality, only the toughest survived — and the toughest tended to have big heads.” Infant mortality is an ever-present problem for humans because bipedalism has constrained the size of the birth canal. If anything, giving birth to a larger headed children is going to lead to increased mortality for both the mother and child. Indeed, natural selection has restricted in utero brain growth in humans, with a large proportion of brain development occurring outside of the womb. In most non-human primates, the brain is close to adult size by the first year of life. In humans, on the other hand, near-adult brain size is not reached until about ten years of age.
Perhaps, the best explanation for the larger head size of our ancestors is one that the authors failed to mention – allometry. Bigger animals have bigger brains. While the cranial capacity for modern humans is large for a primate of our size, it is still only about a quarter of the size of that of an elephant. The decrease in brain size during the late Pleistocene was also accompanied by a decrease in body size. In other primates that show a decrease in brain size, there is an accompanying decrease in body size. Having a larger brain comes at a cost. The brain is a greedy glucose-guzzling tissue. The is possible that our smaller brain has allowed us to reallocate energy for other bodily functions.
References and further reading
Henneberg M. Evolution of the human brain: is bigger better?. Clin Exp Pharmacol Physiol 1998, 25:745-749.
Schaaffhausen H. On the crania of the most Ancient Races of Man. Müllers Archiv 1858:453.