Sex

As far as the human species is concerned, the most important task of an individual is to reproduce, to continue the species. While some animals just bud off an offspring in isolation, human reproduction needs two parents. This shuffles the pack of inherited characteristics, the genes, so the young are not identical to their parents, but spread a wide range of characteristics through the population. The woman has only a few hundred thousand egg cells, and they have been in her since birth. When her body matures, one egg cell is released from an ovary each month. The man's contribution comes from the testes, which produce millions of sperm each day. These tiny cells have whiplike tails the propel them at Olympic speeds and only one is successful. Mixing its genes with the genes in the egg, a new individual is created.