Assessment Questions 4.8

The reconfiguration of the human pelvis, necessary for bipedal locomotion, posed a challenge for childbirth by narrowing the pelvic outlet in females. This made it difficult for a baby's body to pass through the birth canal. Natural selection addressed this challenge through two strategies. First, it fine-tuned the configuration of the pelvis, making male and female human pelvises readily distinguishable due to sexual dimorphism. Second, natural selection timed the birth of human babies at an earlier stage in fetal development when the head, though large, was still small enough to pass through the birth canal. This timing allowed for a smaller head, proportional to the ultimate adult size, in human newborns compared to other primates, facilitating childbirth despite the constraints of the pelvic outlet size necessitated by upright walking.

Human babies are considered secondarily altricial because, although born at an advanced state in terms of sensory and brain development, they are physically immature and dependent for an extended period. This condition resulted from the reconfiguration of the human pelvis, which narrowed the pelvic outlet, making childbirth more challenging. The advantages of being secondarily altricial include earlier stimulation and learning, stronger bonding between parents and children, and an extended childhood. In a species heavily dependent on learned behavior and social relations like humans, being born at an immature stage of development with an extended childhood allows for a steeper and longer learning curve.

The pelvis of a female Homo erectus recovered from Gona, Ethiopia, dating between 0.9 and 1.4 million years ago, provides evidence. The pelvis has a relatively large birth canal, and researchers calculated that a Homo erectus baby with a cranial size of about 315 cc could have passed through the birth canal. This is about 33% of the size of an adult Homo erectus cranium. Compared to other primates, a newborn human baby's brain is only about 25% of its adult size, indicating that Homo erectus babies were born at an earlier stage of development than chimpanzees but not as immature as Neandertals or modern humans.