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also meant that care and nurture came gradually to count for more than large litters in ensuring the survival of the species. This in turn implied further and sharper differentiation in the roles of the sexes. Females were being pinned down much more by maternity at a time when food-gathering techniques seem to have become more elaborate and to demand arduous and prolonged cooperative action by males – perhaps because bigger creatures needed more and better food. Psychologically, too, the change may be significant. A new emphasis on the individual is one concomitant of prolonged infancy. Perhaps it was intensified by a social situation in which the importance of learning and memory was becoming more and more important and skills more complex. About this point the mechanics of what is going forward begin to slip from our grasp (if, indeed, they were ever in it). We are somewhere near the area in which the genetic programming of the hominids is infringed by learning. This is the beginning of the great change from the natural physical endowment to tradition and culture – and eventually to conscious control – as evolutionary selectors, though we may never be able to say where precisely this change occurs.
    Another important physiological change is the loss of oestrus by the female hominid. We do not know when this happened, but after it had been completed her sexual rhythm was importantly differentiated from that of other animals. Man is the only animal in which the mechanism of the oestrus (the restriction of the female’s sexual attractiveness and receptivity to the limited periods in which she is on heat) has entirely disappeared. It is easy to see the evolutionary connection between this and the prolongation of infancy: if female hominids had undergone the violent disruption of their ordinary routine which the oestrus imposes, their offspring would have been periodically exposed to a neglect which would have made their survival impossible. The selection of a genetic strain which dispensed with oestrus, therefore, was essential to the survival of the species; such a strain must have been available, though the process in which it emerged may have taken a million or a million and a half years because it cannot have been effected consciously.
    Such a change has radical implications. The increasing attractiveness and receptivity of females to males make individual choice much more significant in mating. The selection of a partner is less shaped by the rhythm of nature; we are at the start of a very long and obscure road which leads to the idea of sexual love. Together with prolonged infant dependency, the new possibilities of individual selection point ahead also to the stable and enduring family unit of father, mother and offspring, an institution unique to mankind. Some have even speculated that incest taboos (which are in practice well-nigh universal, however much the precise identification of the prohibited relationships may vary) originate in the recognition of the dangers presented by socially immature but sexually adult young males for long periods in close association with females who are always potentially sexually receptive.

    In such matters it is best to be cautious. The evidence can take us only a very little way. Moreover, it is drawn from a very long span of time, a huge period which would have given time for considerable physical, psychological and technological evolution. The earliest forms of Homo erectus may not have been much like the last, some of whom have been classified by some scientists as archaic forms of the next evolutionary stage of the hominid line. Yet all reflections support the general hypothesis that the changes in hominids observable while Homo erectus occupies the centre of our stage were especially important in defining the arcs within which humanity was to evolve. He had unprecedented capacity to manipulate his environment, feeble though his handhold on it may seem to us. Besides the