Sharing DNA

Defined broadly, the egg is a kind of cell that is specialized for the process of sexual reproduction, in which two parents contribute genes to the making of a new individual. The first living things were single cells and reproduced on their own, each cell simply making a copy of its DNA and then dividing itself into two cells. The first sexual organisms, probably single-celled algae, paired up and exchanged DNA with each other before dividing—a mixing that greatly facilitated genetic change. Specialized egg and sperm cells became necessary around a billion years ago, when many-celled organisms evolved and this simple transfer of DNA was no longer possible.