A brief history of sex – with no pictures

There's a classic Gary Larson cartoon showing two amoebas sitting at home in their living room chairs, with one, holding a beer, telling the other, "Hey, I got news for you, sweetheart! … I am the lowest form of life on earth!"
Part of the humor of this is that these single-celled critters are known to reproduce asexually. They come from a family of single-celled organisms that were around on this planet long before sex. And indeed, from what we know of the history of life, the invention of sex was comparatively late in the game and occurred for multiple reasons (none of them very romantic).