Sunday, January 04, 2004

Brain Size and Human Evolution

Via P.Z. Myers' Pharyngula blog, I've come across some interesting information about the relationship between brain and body size in hominids over the course of the last few million years. Suffice to say that it was not what I was expecting:

1. The hominid brain has increased approximately three times in size since the Pliocene, but so has the brain of equids. The tripling of hominid brain size has been considered as an indicator of increased mental abilities, as it coincided with the production of tools, weapons and other artefacts of increasing sophistication. No indicators of the increase in equid intelligence are known. Intraspecific correlation between brain size and variously measured 'intelligence' is, in modern humans, very weak if not completely absent. With the exception of size, there are no major differences between the anatomy of ape and human brains. 2. A study of 297 estimates of body height, 626 estimates of bodyweight and 276 estimates of the cranial capacity of hominids dated at various periods over the past 5 million years shows that the increase in hominid brain size was paralleled by an increase in body size. 3. In a sample of 45 variously dated fossil hominids, brain size correlates isometrically with body size. 4. Since the Late Pleistocene (approximately 30,000 years ago), human brain size decreased by approximately 10%; yet again, this decrease was paralleled by a decrease in body size. 5. Therefore, it may be concluded that the gross anatomy of the hominid brain is not related to its functional capabilities. The large human brain:body size ratio may be a result of the structural reduction of the size of the gastrointestinal tract and, consequently, its musculoskeletal supports. It is related to richer, meat-based diets and extra-oral food processing rather than the exceptional increase in the size of the cerebrum. The exceptional mental abilities of humans may be a result of functional rather than anatomical evolution.


What this suggests is that our big brains are a spandrel, a by-product of a developmental program that specifies that brains should grow at a certain fixed rate relative to body size, coupled to selection for a certain body size. Evolution has favored big brutes, and our brains have just fortuitously hitched a ride.

This does not mean that intelligence is just an accident, though. As the abstract points out, horse brains have also experienced a comparable explosion in growth over their evolutionary history, but there doesn't seem to have been any comparable expansion of their intelligence; the point is that within broad limits, varying brain size seems to be unlinked to wits. Other, subtler factors must be important, and we have to look deeper than just numbers of neurons.

Our brains have actually shrunk by a substantial 10% since the Late Pleistocene! Who would have thought such a thing would have been true? And yet, we are obviously none the worse off for it as a species; I dare anyone to argue with a straight face that we are mental midgets compared to the men of the stone age.