What were the first animals to have sex?

Our planet is filled with a dizzying array of creatures that bump into each other to reproduce. Cats do it. Dogs do it. Birds and bees definitely do it. But what were the first animals to have sex?

Animals have been reproducing sexually since they evolved, so the first animals to have sex were the first animals to exist. Researchers are still looking for direct evidence of the first animals, but they likely arose in the last 800 million yearslived in the ocean and looked like sponges.

Sponges in our oceans currently reproduce sexually, ejecting sperm and eggs into the water, which combine to form new sponge larvae, according to the Exploring our fluid Earth website hosted by the University of Hawaii.

But while ancient sponges may have been among the first animals to reproduce sexually, the act itself predates them by a long time. That’s because life forms were having sex before animals came on the scene.

“The first animals to have sex were already having sex before they were animals”, John Logsdonassociate professor of biology at the University of Iowa, told Live Science.

Related: Do animals have friends?

Logsdon tracks sexual reproduction by looking for the presence of meiosisa form of cell division that creates reproductive cells in eukaryotes — organisms with a nucleus in their cells, such as animals, plants, and fungi.

“It’s pretty clear that all eukaryotes either had the ability to do meiosis or have the ability to do meiosis,” Logsdon said. “The logical inference there is that a common ancestor of all of us did it.”

So when did the first eukaryotes evolve? According to Logsdon, the answer is about 2 billion years ago, when simple bacteria would have participated in some type of genetic exchange.

But sex between sea sponges and bacteria is quite different from the intercourse, or copulation, that humans and many other animals engage in, which relies on more intimate internal fertilization. For the first evidence of this, scientists look to fossils of ancient fish.

“The oldest evidence of intimate sexual reproduction using copulation comes from placoderm fishes from the Devonian period (419.2 million to 358.9 million years ago), such as Microbrachius dicki,” John Longprofessor of paleontology at Flinders University in Australia and author of “The Dawn of Action: The Prehistoric Origins of Sex” (The University of Chicago Press, 2012), he told Live Science in an email.

An illustration of a Devonian placoderm. (Image credit: Nobumichi Tamura/Stocktrek Images via Getty Images)

Fossils reveal that M. Dicki males had paired claspers to inseminate the females internally, while the females had reciprocal genital plates. Long and his team found that the male and female fish would have hovered side by side during copulation with their arm-like limbs linked, so the first sexual act would have it looked like square dancing.

“We have placoderms to thank for both the joy of sex and the labor of childbirth,” Long wrote in his book, “The Secret History of Sharks” (Ballantine Books, 2024).

Sexual reproduction has many benefits. For one, offspring receive genes from both parents, unlike asexual reproduction, in which offspring receive genes from only one parent. This mixing of genes allows animals to better adapt to changes in your environment.

“Sexual reproduction means that the genetic makeup of the offspring is more diverse than that of asexual creatures that simply clone themselves (like jellyfish), so it is much less likely that the entire population of the species will be susceptible to being wiped out by disease,” Long said. “This greater variability in the gene pool increases survival not only against pathogens, but also to environmental changes, for example climate change, or even better tolerance to chemical toxicity if, say, volcanic eruptions alter the chemistry of seawater.”

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