Furthermore, the cost of bipedalism and encephalization was an increase in the difficulty of human birth. 1
The long post-menopausal period, another life cycle characteristic unique to humans, may have evolved as a way for human females to cease the high-risk reproduction early, and help daughters with giving birth, or care for children or grandchildren. This “grandmother hypothesis” of the grandmother’s participation in childcare would also be important for human society to evolve family-based cooperative breeding. 2
■References (Books, papers, Web articles, etc.)
『サルの生涯、ヒトの生涯——人生設計の生物学』 デビッド・Ｓ・スプレイグ （京都大学学術出版会、2004）
『家族進化論』 第4章・第11節 老年期の進化 山極寿一 （東京大学出版会、2012）
Hawkes, K. (2003). “Grandmothers and the evolution of human longevity”. American Journal of Human Biology 15: 380–400.
1With the introduction of upright bipedal walking, the human pelvis transformed into a dish-like shape to support the weight of the internal organs and upper body. Moreover, as the legs move back and forth in parallel, the pelvis is unable widen laterally. As a result, the birth canal is narrower than the size of the baby’s head. It therefore takes time for mothers to deliver, and there is even a risk that the mother may die due to labor difficulties.
2The American anthropologist Christian Hawks proposed the so-called “grandmother hypothesis” to explain why human females, unlike other primates and human males, are not capable of breeding until the very end of their life. Although humans have higher fecundity than other great apes, our children take time to grow. So, after the birth, the still-healthy grandmother can assist her daughter with raising her children and providing food to increase their survival rate. In addition to the difficulty and risk of human childbirth, this may be a reason for the early human menopause and resulting extension of post-menopausal lifespan.
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