Phylogenetically, humans represent an evolutionary puzzle. Walking on two legs free the hands to do new things, like chip stones to make modified tools — the first artefacts, dating to 2.7 million years ago — but it also narrows the pelvis and dramatically limits the size of possible fetal cranium. Thus the brain expansion that began after 2 million years ago should not have happened.
But imagine that, alongside chipped stone tools, one genus of hominin appropriates the looped entrails of a dead animal, or learns to tie a simple knot, and invents a sling (chimpanzees are known to carry water in leaves and gorillas to measure water depth with sticks, so the practical and abstract thinking required here can be safely assumed for our human ancestors by this point).
In its sling, the hominin child can now hip ride with little impairment to its parent's hands-free movement. This has the unexpected and certainly unplanned consequence that it is no longer important for it to be able to hang on as chimps do. Although, due to the bio-mechanical constraints of a bipedal pelvis, the hominin child cannot be born with a big head (thus large initial brain capacity) it can now be born underdeveloped. That is to say, the sling frees fetuses to be born in an ever more ontogenically retarded state. This trend, which humans do indeed display, is called neoteny. The retention of earlier features for longer means that the total developmental sequence is extended in time far beyond the nine months of natural gestation. Hominin children, born underdeveloped, could grow their crania outside the womb in the pseudo-marsupial pouch of an infant-carrying sling.
From this point onwards it is not hard to see how a distinctively human culture emerges through the extra-uterine formation of higher cognitive capacities — the phylogenetic and ontogenic icing on the cake of primate brain function. The child, carried by the parent into social situations, watches vocalization. Parental selection for smart features such as an ability to babble early may well, as others have suggested, have driven the brain size increases until 250,000 years ago — a point when the final bio-mechanical limits of big-headed mammals with narrow pelvises were reached by two species: Neanderthals and us.
This is the phylogeny side of the case. In terms of ontogeny the obvious applies — it recapitulates phylogeny. The underdeveloped brains of hominin infants were culture-prone, and in this sense, I do not dissent from Dan Sperber's dangerous idea that ‘culture is natural'. But human culture, unlike the basic culture of learned routines and tool-using observed in various mammals, is a system of signs — essentially the association of words with things and the ascription and recognition of value in relation to this.
As Ernest Gellner once pointed out, taken cross-culturally, as a species, humans exhibit by far the greatest range of behavioural variation of any animal. However, within any on-going community of people, with language, ideology and a culturally-inherited and developed technology, conformity has usually been a paramount value, with death often the price for dissent. My belief is that, due to the malleability of the neotenic brain, cultural systems are physically built into the developing tissue of the mind.
Instead of seeing the brain as the genetic hardware into which the cultural software is loaded, and then arguing about the relative determining influences of each in areas such as, say, sexual orientation or mathematical ability (the old nature-nurture debate), we can conclude that culture (as Richard Dawkins long ago noted in respect of contraception) acts to subvert genes, but is also enabled by them. Ontogenic retardation allowed both environment and the developing milieu of cultural routines to act on brain hardware construction alongside the working through of the genetic blueprint. Just because the modern human brain is coded for by genes does not mean that the critical self-consciousness for which it (within its own community of brains) is famous is non-cultural any more than a barbed-and-tanged arrowhead is non-cultural just because it is made of flint.
The human brain has a capacity to go not just beyond nature, but beyond culture too, by dissenting from old norms and establishing others. The emergence of the high arts and science is part of this process of the human brain, with its instrumental extra-somatic adaptations and memory stores (books, laboratories, computers), and is underpinned by the most critical thing that has been brought into being in the encultured human brain: free will.
However, not all humans, or all human communities, seem capable of equal levels of free-will. In extreme cases they appear to display none at all. Reasons include genetic incapacity, but it is also possible for a lack of mental freedom to be culturally engendered, and sometimes even encouraged. Archaeologically, the evidence is there from the first farming societies in Europe: the Neolithic massacre at Talheim, where an entire community was genocidally wiped out except for the youngest children, has been taken as evidence (supported by anthropological analogies) of the re-enculturation of still flexible minds within the community of the victors, to serve and live out their orphaned lives as slaves. In the future, one might surmise that the dark side of the development of virtual reality machines (described by Clifford Pickover) will be the infinitely more subtle cultural programming of impressionable individuals as sophisticated conformists.
The interplay of genes and culture has produced in us potential for a formidable range of abilities and intelligences. It is critical that in the future we both fulfil and extend this potential in the realm of judgment, choice and understanding in both sciences and arts. But the idea of the brain as a cultural artefact is dangerous. Those with an interest in social engineering — tyrants and authoritarian regimes — will almost certainly attempt to develop it to their advantage. Free-will is threatening to the powerful who, by understanding its formation, will act to undermine it in sophisticated ways. The usefulness of cultural artefacts that have the degree of complexity of human brains makes our own species the most obvious candidate for the enhanced super-robot of the future, not just smart factory operatives and docile consumers, but cunning weapons-delivery systems (suicide bombers) and conformity-enforcers. At worst, the very special qualities of human life that have been enabled by our remarkable natural history, the confluence of genes and culture, could end up as a realm of freedom for an elite few.