4 - Evolution of the Human Life Cycle

Summary

The previous chapter described the evolution of the human pattern of growth in terms of its mammalian and primate foundations. The emphasis was on form and function of anatomical structures such as the placenta, the brain, and the skeletal system and how these structures are developed during the stages of human growth. In this chapter the emphasis is placed on how the human life cycle evolved. The life cycle of any organism includes all the stages of growth, development, and maturation from conception to death. Major events in the evolution of the human life cycle influenced the prenatal stages of development. A brief review of some human prenatal growth and development was provided in previous chapters. For further information of discoveries made in the last century of research relating to primate fetal development readers may consult the review by Richtsmeier (2018). It is worth mentioning here one classic example of human differences in prenatal growth and development published by Schultz (1926). His sketches of the body proportions of hominoid fetuses are reproduced here as Figure 4.1. The human fetus “of the 4th month” has relatively shorter legs than the chimpanzee, orangutan, or gibbon. The accuracy of this difference assumes that Schultz estimated fetal development correctly for the nonhuman apes (see Figure 4.1 legend). Another difference in proportion, not noted by Schultz, is the size of the cranium relative to the face, which is larger in the human fetus than in the chimpanzee, orangutan, or gibbon.