[microgood]

There is strong evidence the first multicellar sexual critters were hermaphroditic (some simple multicellulars still are). Over generations, some began to specialize, as it were.

Interesting, but how would the male and female sexual organs that work together in perfect harmony mutate equally and at the same time. Would not one wrong mutation result in the extinction of the species since it can no longer reproduce? Like the eye and the leg (vice fin), this is hardest part of evolutionary theory to believe.

[Junior]

That's just it. The specialization didn't happen over night. Some of the animals spent more time as males than as females. Some decided to go the other route. Each initially had both sets of sex organs, so there was not need for them "to mutate equally and at the same time."