The proposal outlined in the previous section presumes a dialogue between brain size and culture. But what we now mean by "culture" is incomparably more elaborate than anything experienced by our remote ancestors. Does the dialogue still continue?
As far as we know, H. sapiens has been the only extant species of Homo for at least 20,000 years. Cultural change during this period has been radical. Refinements in stone tools, and probably in social organisation among nomadic groups, led some 9-10,000 years ago to the first hints of a settled way of life and the beginnings of civilisation. Traces of einkorn wheat48 from that time have been found in the Karacadag Mountains of south-eastern Turkey. Its original cultivation has been attributed to a single tribe. Settled agriculture led to increases of human group size and division of labour. In due course it led to cities, kings, scribes, craftsmen and professional soldiers, and to written language.
In the Euphrates valley, some way downstream from the Karacadag range, writing was allegedly invented by the Sumerians around 5,300 years ago. This momentous event might have been part of a steady cultural progression rather than a sudden novelty, as is often supposed. Oval stones with pictogram carvings dating from about 10,000 years ago have been found near the Euphrates in Syria. What these carvings denote is a mystery,
48 Einkorn wheat was one of the "founder crops" of Neolithic agriculture. It still grows wild over much of the Middle East and the Southern Balkans.
but they imply the use of abstract symbols, which is the essence of writing. We might therefore infer that the first glimmerings of written culture coincided with the first glimmerings of settled life and civilisation. Thus, Sumerian writing could have been the culmination of a four or five millenium cultural progression in symbolic expression, not a radical innovation; just as the Sumerian cities were the culmination of a progressive development in settled community structure.
A written culture in a large, settled, differentiated, civilised community placed greater demands on learning and memory than life in a small, undifferentiated, preliterate nomadic tribe. Today, people from nomadic tribes can adjust to modern city life within a generation or two, showing that their brains have all the requisite capacity. The simplest inference is that civilised culture has evolved during the past 5000 years without any further significant changes in the human brain. However, there is probably no one alive today whose ancestors have, without exception, avoided all significant contact with civilisation. Perhaps many Homo sapiens 10,000 years ago could not have coped with the learning and memory demands of even rudimentary civilisation, let alone the modern city; but natural selection has eliminated them. Therefore, it is possible that the progress of H. sapiens towards civilisation has entailed continuing expansion of the brain.
Evolutionary psychologists reject this view. They assert that humans have not evolved since the Pleistocene. Therefore, modern people are endowed with "innate Pleistocene dispositions". This causes us to kill one another and practise infanticide, males to rape, and females to fall for rich men. This assessment purports to be Darwinian, but it depends heavily on an analogy between humans and scorpion flies, which seems tenuous. Its proponents account for inconsistencies between their hypothesis and observed fact by invoking "free will", a notion that Darwin rejected. The writings of evolutionary psychologists always seem to exude a "fall-of-mankind" pessimism, which may explain why they are fashionable. Why should the evolutionary dialogue between brain and culture have ceased? Evolutionary psychologists do not consider this question.
Unfortunately, the evidence on this point is not decisive. Adult human/hominid brain volume has increased by about 300% over a period of roughly six million years, which amounts on average to 1.5% every 30,000 years. The earliest H. sapiens whose cranial capacities can be reliably estimated are in the order of 30,000 years old; so all other things being equal, we would expect a 1.5% difference between their brain volume and ours. But 1.5% of 1400 cc is only 21 cc, which is less than the measurement error and less than the standard deviation in modern humans. Thus, the evidence does not support the claim that modern human brains are bigger than those of the earliest sapiens; but nor does it refute it.
On the other hand, we keep people alive today who in the past would have died in infancy because of genetic defects. For example, colour blindness has probably doubled in frequency since the earliest sapiens. Perhaps this "weakening of the gene pool" militates against further increase in complexity of our species and therefore against further increase in brain size. In any case, the limits of our mutation-correction machinery might have been reached in an organism as complicated as the modern human (see chapter 13); modern humans might be close to the theoretical limit of organism complexity. So there are plausible arguments that the human brain will stop increasing in size and complexity - but there is no compelling reason to suppose that the limit has been reached yet.
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