Despite the lack of development in their bodies, babies are born with heads that are at an advanced stage of development. As the brain reaches half of its adult size by 9 months and nearly three-quarters by two years of age, a baby's head must be large and grow rapidly to accommodate the rest of the body's growth. While other factors may influence the relative sizes of a baby's head and body, a baby's large head is the result of millions of years of evolutionary pressures.
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In comparison with other primates, humans have disproportionately large brains. The great apes are the closest living relatives to humans, with brains weighing between 300 and 500 g. Despite similarities in body size between apes and humans, the average human brain weighs approximately 1300 g. As such, both adult and newborn humans need larger heads than other primates to house these enormous brains. Due to the imbalance between brain and body development during pregnancy, however, the heads of babies are much larger than those of adults relative to their body size.
The human lineage began to develop separately from other great apes approximately 5 million to 7 million years ago. While the brains of both branches continue to grow, the last 2 million years of human evolution have led to an explosion in brain growth. In contrast with the marginal increases in the great apes, the human brain has more than tripled in size over the past 2 million years. Despite such changes in the size of the brain, the human body has changed little in size over the same period.
Humans are the only primates that exclusively walk on two feet, a trait known as bipedalism. Similar to the explosion in brain size, the development of bipedalism occurred relatively recently in human evolutionary history. As walking on two feet requires relatively narrow hips, our ancestors' increasing reliance on bipedalism placed restrictions on the size of the human birth canal. As a result of the dual pressures of an increasingly large brain and a limit on the size of a newborn baby, fetal growth maximizes the size of a baby's head to ensure proper brain development and survival upon birth.
Fetal and Newborn Development
Due to the narrow limits of the birth canal, all humans are born at a premature stage of development. While their heads push the limits of the birth canal, the brains of human newborns are no more than 25 percent of their full size. However, as this small fraction of a human brain is equivalent in size to that of an adult great ape, human heads are as large as possible at birth. Coupled with a body that is approximately 5 percent of its full size, the large head required to house a newborn's enormous brain results in human babies having heads that appear very large in relation to their bodies.