Our planet is populated by a dazzling variety of creatures that bump into each other to reproduce. Cats do it. Dogs do it. Birds and bees certainly do it. But what were the first animals to have sex?
Animals have been reproducing sexually since their evolution, 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 probably appeared over the last 800 million yearslived in the ocean and looked like sponges.
Sponges in our oceans today reproduce sexually by ejecting sperm and eggs into the water, which combine to form new sponge larvae, according to the Discovering 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 way. Indeed, life forms were having sex before animals appeared.
“The first animals to have sex had sex before they were animals.” John Logsdonassociate professor of biology at the University of Iowa, told Live Science.
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Logsdon traces 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 had the ability to do meiosis or have the ability to do it,” Logsdon said. “The logical conclusion is that a common ancestor of all of them 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 a kind of genetic exchange.
But sex between sponges and bacteria is very different from the sexual intercourse, or copulation, that humans and many other animals have, which relies on more intimate internal fertilization. For the first evidence, scientists looked to ancient fish fossils.
“The earliest evidence of intimate sexual reproduction using copulation comes from placoderm fishes of the Devonian period. [419.2 million to 358.9 million years ago]as Microbrachius dicki” John LongProfessor of Paleontology at Flinders University in Australia and author of “The Dawn of the Act: The Prehistoric Origins of Sex” (The University of Chicago Press, 2012), he told Live Science in an email.
Fossils reveal that Mr. Dicki males had paired podoptera to inseminate females internally, while females had reciprocal genital plates. Long and his team found that male and female fish hovered side by side during copulation with their arm-like limbs linked, so the first sexual act would have occurred It looked like a square dance.
“We have placoderms to thank for both the joy of intercourse and the labors of childbirth,” Long writes in his book, “The Secret History of Sharks” (Ballantine Books, 2024).
Sexual reproduction has many advantages. For one thing, the offspring receives genes from both parents, unlike asexual reproduction, in which the offspring only receives genes from one parent. This mixing of genes allows animals to better adapt to changes in their environment.
“Sexual reproduction means that the genetic makeup of the offspring is more diverse than asexual creatures that simply clone themselves (like jellyfish), so it is much less likely that the entire population of the species will be wiped out by disease,” Long said. “This greater variability in the gene pool not only improves survival [against] “Pathogens, but also environmental changes, for example climate change, or even better tolerance to chemical toxicity if, for example, volcanic eruptions change the chemistry of seawater.”
