Sex began approximately 385 million years ago with a fish that lived in what is now Scotland. In a recent study published by the journal Nature, scientists found that the fish Microbrachius dicki -- that name can't be a coincidence -- was the first animal to reproduce via internal fertilization. That's when fertilization happens inside, as opposed to outside, the female body.
"We have defined the very point in evolution where the origin of internal fertilisation in all animals began," Professor John Long, who led the study, told BBC News.
Long came to this realization while examining fish fossils. He saw that the male fish in question had L-shaped genitals that just happened to fit perfectly into an area near the female fish's rear end. Sounds familiar, right?
Unfortunately due to their anatomy, these fish didn't have many options when it came to, uh, positioning.
"They couldn't have done it in a 'missionary position'," Long said. "The very first act of copulation was done sideways, square-dance style."
Internal fertilization in fish wasn't a thing for long though. After Microbrachius dicki died out, fish went back to reproducing via spawning, a form of external fertilization. When fertilization happens outside the female's body, no penetration -- a.k.a. no sexytime -- is required. But as evolution continued, internal fertilization re-emerged millions of years later in ancestors of sharks, rays, and (obviously) humans.