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Fish invented sex 385 million years ago. Here's what it looked like.

The fish that we know today tend to spawn in order to reproduce — releasing zillions of eggs and sperm into the water and hoping they'll meet. But a new paper published October 19 in Nature posits that fish were actually having sex 385 million years ago.

That's the oldest known instance of copulation on Earth. And, for some reason, fish then gave up the habit.

"When these ancient fish called antiarchs first evolved, they were the first creatures to have jaws, the first creatures to have hind-paired limbs," said Flinders University paleontologist John Long, who led the project, at a press conference. "Now we know that [fish] also invented copulation."

(A quick side note on the term "sex": Technically, fish are engaging in sexual reproduction, whether the sperm and egg are meeting outside the body, as in spawning, or inside the body, as in copulation. But for the purposes of this story, let's use the term "sex" as it's commonly used — to mean copulation or sexual intercourse.)

The Nature paper looked at fossils from a group called antiarch placoderms — generally thought to be the oldest types of fish with jaws. The researchers found structures called copulatory claspers on males and corresponding dermal plates on females. They posit that a male clasper would be inserted between the female's plates to hold on to her during sex and transmit sperm inside her while the mating couple was arranged side by side:

Fish sex 2 Brian Choo, Flinders University

This is the proposed method for how these ancient fish copulated: side by side. (Brian Choo, Flinders University)

The practice of fish copulation using claspers still exists today. Some sharks, for example, have been observed copulating, although shark claspers seem to have evolved separately. (If you'd like to see an actual photograph of a shark's copulatory claspers, click here.)

But by and large, most fish today don't use this method. Instead, they release sperm and eggs into the water, conceiving outside the body. Why this changed is a real mystery.

So why did fish stop having sex?

"We don't know," Long said. "It could be a change in environment or a special adaptation that happened at the time."

Over the years, scientists have definitely thought about the relative benefits of copulation versus spawning. Species that copulate — and join sperm and egg together inside the body — tend to invest more energy into fewer offspring. These offspring are generally better protected from the environment and predators until they're born.

By contrast, species that release sperm and eggs into the water, like a lot of fish do, take the opposite approach. They produce far more young, but many of them don't make it to adulthood. In essence, these species are playing a numbers game.

Whatever the merits of each, it's clear that spawning works well for fish — they've been doing it for a very long time, and they're still around. And one factor that may have encouraged fish to reproduce externally is that they live in water, which helps ensure that eggs, sperm, and embryos won't dry out. That's much harder to do on land.

Still, it's not yet clear why fish evolved to have sex and then shifted back to external fertilization.

But the ability to copulate may have been passed down from fish all the way to mammals, including us. Long suggested that the ancient antiarch fish probably developed a gene for these kinds of mating parts, which then got turned off and then on again far later in evolution: "Once that gene was set in the vertebrate body plan, it could come back later."