In four million years of human evolution, brain size roughly tripled from 500 cm3 in Australopithecenes to 800 cm3 in Homo habilis to 1000 cm3 in Homo erectus to over 1300 cm3 in modern Homo sapiens and over 1500 cm3 in late ice age Europeans. However about 10,000 years ago, the human brain began to shrink quite precipitously only to rebound since the industrial revolution. Here’s my best attempt to show the historical trend in sex-combined Europeans, though it may need some revising:
The simplest explanation for this is the development of agriculture, as scholar Richard Lynn explains:
The period of around 25-10,000 yr ago was the last ice-age and at this time the Caucasoid peoples of Europe and the Near East had brains of approximately the same size as today (Henneberg, Budnik, Pezacka & Puch, 1985). These peoples lived largely by hunting because plant foods were unavailable for much of the year and their meat diet evidently provided them with a high standard of nutrition and enabled them to develop their large heads and brains.
Following the recession of the ice age people evolved a new life style living in permanent village settlements with domestic animals and cereal agriculture. But although the new life style was more convenient the quality of nutrition fell and many skeletal remains show signs of rickets and other malformations caused by suboptimal nutrition (Festinger, 1983).
For the last 2000 yr data on heights of adult males in Britain have been collected by Kunitz (1987) and by Floud, Gregory and Wachter (1988). The broad trend is that height has been constant at a mean of approx. 172 cm up to the cohort born around 1930. From this date onwards height has increased. It seems reasonable to infer that brain size and intelligence were approximately stable for about 2000 yr up to around 1930 and it is only in the last half century that the increases have occurred. In the economically advanced nations the improvements in nutrition have enabled us to recover the brain size and intelligence levels of our ancestors of 25,000 yr ago.
Of course this analysis ignores long term secular changes which may have taken place in the genetic and cognitive stimulation determinants of intelligence, but is nevertheless offered as an approach to the intriguing question of long term historical trends in intelligence.
As usual, Lynn provides the most parsimonious explanation. Of course not everyone agrees that agriculture was the cause of falling brain size, because brains shrunk even in sub-Saharan Africa and Australia where agriculture arrived late. While it’s true Australian aboriginals had much bigger brains 10,000 years ago than they do today, there’s seemingly no evidence that their brains shrunk prior to European colonization and the resulting change in diet and life style. Meanwhile the chart below shows little evidence that brain size in Africa shrunk prior to the arrival of agriculture there, several thousand years ago.
Thus I think brains shrunk because of malnutrition and disease (agriculture) and not because of backwards evolution. Of course with the end of the ice age, intelligence was not as crucial to survival (even in the tropics where it caused drought) so that may have relaxed selection for higher IQs, creating a dysgenic effect. Also, a warming climate selects against brain size because a big head overheats like a 100 watt light bulb. However ice ages have ended before and brains did not shrink. What was different the last time? Agriculture.