" Life managed without males for its first billion years, much of which was passed as single cells in a series of warm ponds. Then, in some ancient and neutral Eden, the fruit of the tree of sexual knowledge - a new mutation - persuaded members of a particular clone to fuse with cells from another, and then to divide. That ingenious idea is good news for the novel gene, as it doubles its rate of spread, but is a lot less so for those who receive it, who are obliged to copy the extra DNA. At once, two factions emerge, one keen to force itself upon the other. Thus sex was invented.
Soon one contestant began to cheat. Large cells are expensive, but are better at dividing because they have more food reserves. Small cells are cheaper to make, but cannot afford to split. Their sole chance of success hence lies in fusion with a large cell. The first males had appeared on the scene. "

― Steve Jones , Y: The Descent of Men