There are usually two key indicators said to provide an accurate measure of smartness: encephalisation quotient and absolute brain size. Encephalisation quotient or EQ is simply the ratio of the mass of the brain to the mass of the body. Overall size is seen as critical due to the number of neural connections required for complex thought processes; you can only squeeze so many neurons into any given volume. Having said that, there is some considerably flexibility around this, thanks to variation in neuron density. The reason that some birds, especially the crow and parrot families are highly intelligent despite the small absolute size of their brains is due to their higher neural density compared to mammals.
Analysis of data from the examination of thousands of human fossil remains suggests that our species reached a peak in EQ around 70,000 years ago, followed by a gradual decline. The reduction in brain size appears to be due to a loss of the archetypal grey matter itself, rather than the white matter that provides support to the neural architecture. However, one key issue is lack of agreement as to a definitive start date for this decline, with 20,000 to 40,000 years ago being the most commonly cited origin. With such basic points remaining unsettled, it's perhaps not surprising that there is a plethora of opinions as to the cause. Here are some of the more popular hypotheses for the decline in human brain size:
1. Change to body size
The first and perhaps most obvious - but easily refuted idea - is that human body size has been steadily declining and so cranial capacity has kept in step with this. While it is true that archaic sapiens may have had a higher mass and even stature than modern humans, the reduction in brain size is greater than would be expected when compared to the overall shrinkage. The assumption is that the development of material culture, from clothing to weapons, has given humans a less calorie-demanding lifestyle.This would allow - although not dictate - natural selection to trend towards a smaller body size. This doesn't appear to offer any help for the comparatively greater reduction in brain mass, although we should remember that an overall reduction in body size means a smaller birth canal. This in turn requires a smaller skull at birth; as is well known, the human gestation period is three months' less than for similar-size mammals, but our seemingly premature delivery is necessary for the pelvis to maintain efficient bipedalism.
2. Self-domestication
Another idea is that humanity has become domesticated via the impact of culture upon natural selection. Following the population bottleneck of 70,000 years ago - the cause of which is not yet confirmed, despite attempts to correlate it with the Toba super-volcano - there has been continual growth of the human population.Just as all our domesticated animal species have brain sizes some 10-15% smaller than their wild cousins and ancestors, so the move to larger groups sizes may have led to a more docile humanity, with associated traits such as a smaller cranial capacity being carried along with it.
There are several issues with this hypothesis, ranging from a lack of data on the size of gatherer-hunter bands to the biological mechanisms involved. As regards the latter, there has been some speculation concerning neoteny, in which a species no longer grows to the final stage of maturity. The idea is that if adults are more aggressive than juveniles but peaceful collaboration can lead to larger groups, mutual aid and longer lifespans, then unintentional selective breeding for the retention of juvenile characteristics, including smaller brains, may cause a shift away from the fully mature but more aggressive individuals.
Research in recent years has suggested our brains may continuing to grow into our early thirties rather than cease growing in our teens, so it's possible there could be some truth to this; it would interesting to seek evidence as to whether the brains of archaic sapiens continued growing longer than ours do.
3. The impact of culture
Taking this a step further, increased population density allows a more rapid development and transmission of new ideas, including those that lead to better health, longer lifespans and so to an increased birth rate. Culture and sophisticated language may have reduced the need for most people to gain a wide range of skills - courtesy of a higher intellectual capability - as tasks could be shared and specialisation take hold. In effect, larger societies provide a safety net for those who would be less able to cope in smaller groups.If ideas could be handed down, then individuals wouldn't have to continually 'reinvent the wheel' in each generation, allowing survival despite a smaller brain size and decreased level of intelligence. The problem with this scenario is that we have no proof the 10-17% reduction has led to an associated drop in intellect; it may well be that the size of certain lobes, used in specialist thought processes such as formulating complex speech, far outweigh any decline in less critical areas.
4. The expensive big brain
One possibility that has a clear cause-and-effect concerns the energy demands of having larger brains. Although they consume a quarter of our daily calories, the human brain is less than five per cent of our body weight. Therefore, there could be a case for arguing the existence of an evolutionary competition between smaller-brained individuals who can survive on less food with those who use their larger brains to improve food collecting strategies. Unfortunately, there are so many variables that it's difficult to judge whether the former would continually trend against the latter and - considering it clearly occurred - why the larger brain managed to evolve in the first place?5. The more efficient brain
Although a smaller brain might have fewer neurons than a larger version with similar architecture, it has been suggested that its shorter pathways would lead to more rapid thought processing than in a larger counterpart. In addition, there might be fewer neural pathways, again increasing the efficiency. This 'nimble thinking' approach certainly seems logical, although again it doesn't explain the evolution of larger EQ in archaic sapiens.This is certainly a subject ripe for much more research. I've often concluded with a statement along the lines that it wouldn't be surprising if some or all these factors were involved, since nature rarely conforms to the nice, neat patterns we would like to lay upon it. There is an even possibility that brain size - like so many other aspects of all animal species - fluctuates around a mean value, so that what goes up may come down again, only to later go up again.
At least one anthropological study on both Afro-Americans and US citizens of European descent proposes that over the past few hundred years there may have been an upward drift towards larger brains. Assuming the research is accurate, one possibility is that the superior nutrition available since the Industrial Revolution is allowing such development, thanks to the comparative ease with which its energy demands can be fulfilled.
It would certainly be interesting to investigate this hypothesis on a global scale, considering the wide differences between the clinically obese nations and those still subject to frequent famine. Whatever the results, they are unlikely to be the simple 'just-so' stories often passed-off as to the public in lieu of accurate but understandable science communication. The answers may be out there somewhere...I'd certainly love to know what's been happening to the most sophisticated object in the known universe!
