One particular effect of the depleted resources and high predatory threat faced by early humans in the savannah would have been increased infant mortality. Increased fertility must have been the most effective counter to that issue. In order to increase the breeding rate, it is necessary to shorten lactation intervals by ending lactation early and resuming estrus, resulting in high fertility. 1 In fact, the weaning period of modern humans is much faster than it would be if estimated from the general rules that apply to other primates. 2
■References (Books, papers, Web articles, etc.)
『人類進化論——霊長類学からの展開——』 第3章・第3節 人類の生活史の特徴 山極寿一 （裳華房、2008）
『家族進化論』 第4章・第9節 人類の生活史と進化 山極寿一 （東京大学出版会、2012）
1It is a common misunderstanding that evolution occurs in a way which reflects the intentions of groups or individuals. In that sense, it is not correct to say that “our human ancestors weaned their infants off quickly in order to increase fertility as a counter to the high threat of infant mortality,” because the will of individual humans is irrelevant to evolution. It is more accurate to say that “individuals who happened to have the genetic characteristics for a shorter weaning interval and shorter inter-birth intervals were more suited to the savannah environment, with its high rate of infant mortality. Such individuals left more offspring than others. Therefore, the genes responsible for the short weaning period gradually spread to the entire population of our human ancestors.” The first explanation is merely a simplification of the second explanation.
2In primates in general, weaning coincides with the time when the molars begin to grow. From this general rule, the weaning period of human beings would be estimated at around five to seven years old. In reality, however, human weaning occurs earlier in many cultures.
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