If the sperm carries an X chromosome it will be a girl, if a Y chromosome a boy. Compared to other mammals, puberty in humans, the beginning of sexual maturation, takes place at a relatively advanced age. Sheep begin to show the onset of sexual maturity at about six months, cattle at 12 months and gorillas at eight years. It would seem to be an evolutionary benefit to reproduce and pass on any genetic advantages as soon as possible. But mankind has evolved as a social being in which a high degree of co-operative behaviour is necessary for survival.
Humans have to learn to deal with complex social relationships.
Thus the prolonged process of childhood, with its emphasis on play and learning, enables individuals to develop necessary social skills. The third programme concentrated on the brain. It included remarkable images of a single brain cell magnified 10,000 times firing off an electrical impulse during the thought process. The brain represents more than the sum of its parts. Its great ability, including abstract thought and sophisticated language, is a result of its enormous interconnectivity and complexity.