There can be no doubt that the way in which humans consciously build ‘shields’ of many kinds between themselves and their surroundings placed our species, and those leading up to it, in an increasingly different relationship to the environment than those of other organisms. Fire, habitations, tools, weapons and clothing emerged far back in our evolutionary ‘bush’, to be followed more recently by artificial means of feeding ourselves in a vast range of climatic conditions. In the last century these ‘shields’ have been added to by medical protection against pathogens.
Many of the physical traits of the modern human frame would not be ‘fit’ in a purely Darwinian sense for life unprotected by myriads of cultural devices: they arose from genetic potential largely because growing human culture allowed them to be fit for purposes other than survival at its simplest level. The range of basic physiognomies among modern humans does seem to reflect natural selection to suit various climatic regions, such as the differences between cold- and heat adapted peoples. That perhaps began during the great expansion out of Africa some 70 ka ago. But the much greater range of facial characteristics among all populations (a really human characteristic compared with other primates) is probably a result of genetic drift at random, rather than any kind of evolutionary selection. There are also differences that have arisen since the widespread adoption of agriculturally produced foods since about 10 ka ago, as in jaw shapes and those of the skull, probably linked to easier mastication. That can be explained most easily by the manner in which the use of muscle tends to sculpt the bone to which it is attached: it arises during the life of the individual.
With what appears to be the start of a global unification of cultures, and greater security for the more fortunate one third of humanity at least, it might be expected that natural selection is on the wane for humans. A mere 10 thousand years since the rise of agriculture and far less since modern cultures arose, it is perhaps too soon to conclude that we have cut loose from Darwinian processes. Indeed, recent genetic research has come up with several developments that must be recent results of natural selection. One is the split between adults who can metabolise cows’ milk and those who cannot. The first group, a minority, cluster around the Near East (most Europeans) and in a few parts of Africa where cattle domestication arose. A large block of the human genome, about a million base pairs of nucleotides, includes the gene that produces the necessary enzyme lactase, and its persistence in those adults able to digest milk. The large size of the whole haplotype is typical of recent genetic developments, and the researchers are certain that it resulted from selective pressure where dairy farming began at between 5-10 ka.
Genes that confer resistance to infectious diseases that can cut life short before successful reproduction are good candidates for showing the effects of natural selection, especially in those areas where medical care and drugs are not available. For a long while natural resistance among some west Africans to malaria parasites was linked with heritable sickle-cell anaemia, but recent research has shown a more complex reason that involves several genes. Interestingly, ‘dating’ of the associated genetic changes gives recent ages between 3 and 6 ka, perhaps linked to the rise of farming practices. Clearing land and ponding of water on fields would have encouraged the malaria-carrying Anopheles mosquitoes, which are not forest species: a cultural change presaged a genetic one. Similar results have emerged from studies of inherited protection against HIV/AIDS, yet that only appeared in pandemic form very recently (unless misidentified earlier). An explanation may centre on selective pressure on mutation to form the protecting gene as a result of the appearance of previous epidemics, such as plague and smallpox among early Europeans, who seem to have the highest resistance to HIV/AIDS.
So it is hard to say if selective pressures will work in future on the human genome, as culture convergence continues, and (hopefully) equitably shared living standards. Since the limit on human brain size is the skull, and that is limited by the near-maximum pathway through the human female pelvis, it is very difficult to imagine our evolution into big-heads.
Source: Balter, M. 2005. Are humans still evolving? Science, v. 309, p. 234-237.