Human infants require more care than they should, if we form our expectations based on closely related species (apes, and more generally, Old World simian primates). It has been said that humans are born three months early. This is not accurate. It was thought that our body size predicted a 12, instead of 9 month gestation, and some suggested that Neanderthals would have had such, but this research conclusion has been set aside based on new analysis. But it is still true that developmentally, human children do not reach a stage of development that allows some degree of self care for a very long time compared to apes. The actual sequence of development is not directly comparable: It is not the case that after a certain amount of time humans reach a specific stage reached earlier in the lifecycle by Chimpanzees, as the differences are more complicated than that. For the present purposes, we can characterize the human condition for early development like this: Human babies are more helpless in more ways and for longer than comparable ape babies.
Later in life, humans have a longer period of what might be called pre-adolescence than they "should" based on comparisons with related species. Some have characterized this as the insertion of a number of extra years of development. It could also be characterized as a period of time-lengthened development. Neither is perfectly accurate. One way to characterize the human condition for this period is this: From some time several months after birth through about the age of five or six (or more) humans engage in developmental activities not seen (or not as extensive or intensive) in other apes, during which humans learn a number of important things and engage in a number of neural developmental processes.
It is during this period that humans develop their knowledge of the kinship systems they will live with for the rest of their lives. Western populations tend to have poorly developed kinship systems, so this is easy to overlook, but virtually all other human cultures have complex and pragmatically significant kinship systems, and it is easy to observe children becoming aware of them and learning how to engage in them during this time. It is during this time that human children develop certain aspects of their gender identity and gender roles appropriate to their society. They may learn class, caste, or ethnic roles as well. They start to learn the basics of the things they will need later in life, and what they learn is based entirely on what their society or culture requires: Being a blacksmith, a forager, a student, whatever.
Most significantly, it is during this period that the child learns to use human language, a trait that is absent from our most closely related species.
Years ago, Mel Konner suggested the use of the term "childhood" as the period of development in which humans engage that is absent from the apes. He made a documentary about the subject, and more recently, he wrote a book on the topic: The Evolution of Childhood: Relationships, Emotion, Mind. The word "childhood" existed previously, of course. The term was suggested for use as the technical term referring to the inserted or extended, and evolutionarily modified, extra five years or so of development. Primates have a juvenile stage followed by the transition to sexual maturity, but humans have a pre-juvenile stage as well. This model can be rather clumsy, but suffice it to say that human young are doing something quantitatively and qualitatively different than ape young.
Primates tend to learn much of their ultimate adult behavior from the other primates with whom they live and by interaction with their natural environment, and this allows for certain things to happen, such as the development of behaviors that would be difficult or impossible to program genetically. This is a trait found widely in mammals and birds, but more so in some groups, including primates. It is even more true of the apes than of other primates, and indeed, apes have long periods of parent (mother) - offspring association, and are observed to engage in long bouts of learning and, remarkably, occasional active teaching. Humans take this ape characteristic to a proverbial "order of magnitude" greater. One result of human hyper-extended and hyper-intensified child-age learning is the ability of human cultures to adapt (specialize) in a wider range of habitat exploitation strategies (lifeways) than otherwise possible. Indeed, the genetically coded behaviors that may well be present in primates (innate fear of certain things, certain aspects of territorial competition, sexual interaction, etc.) are often repressed or re-programmed in humans via culture. An interesting, though trivial, example is Heavy Metal. Heavy metal is a cultural manifestation (a "subculture"?) in which human participants revel in the instantiation of symbols almost all of which represent the repulsive, the dangerous, or the adaptively scary: Blood, predators, spiders, snakes, misplaced umlauts, and sharp things. That which we might reasonably guess would be genetically programmed into our beings as things to avoid is dragged out and made entertainment. This sort of thing proves that culture is capable of "overriding biology" (though that presumed relationship is often a falsehood) and suggests that human behavior in general may be primarily culturally coded rather than genetically coded. After all, culture is a powerful and rich source of information that can be passed on from generation to generation like genes, but altered along the way, in ways not possible with genes. One would expect selection to favor culturally mediated traits over genetically mediated traits.
And that may be our most important adaptation.
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