It’s pretty hard to catch single-celled organisms in the middle of sex.
“It’s sort of like if you put a male and a female together in the zoo. You can’t necessarily get them to do the thing,” John Logsdon, an early-eukaryotic-sex expert at the University of Iowa, told me. “If you were a Martian looking down on Earth and asking if humans were sexual or not, if you couldn’t look through the windows, you’d never see humans having sex. Well, rarely.”
Lots of single-celled creatures can reproduce both asexually (cloning themselves) and sexually (combining DNA with another organism to create offspring), and they generally prefer cloning.
Really, it’s very strange that anything would have sex at all. Having sex puts organisms in a vulnerable position. Plus, they spend tons of energy attracting mates and going through the complicated process of mixing their DNA with someone else’s.
Cloning is much simpler, and for a long time, many scientists thought the common ancestor of all eukaryotes—the branch of life that includes animals, plants, fungi, slime mold, and everything else besides bacteria and the small, strange archaea—took that route. But, Logsdon said, “our knowledge there was not super great.” Once biologists started examining different eukaryotic organisms to figure out how sex originated, they were surprised by what they found. Even those ancient eukaryotes did it, they now think. The basic elements of sex—attraction and the exchange of genetic material—are far, far older than the evolution of sexes and the familiar equation of man + woman = baby.