Researchers have traced the act of copulation back to its most primitive form. The antiarch fish—an ancient lake-dwelling placoderm—is now the oldest species known to have sex, according to a new study published in Nature.
As paleontologist John Long told Nature News, they did it “sideways, square-dance style.”
But these early vertebrates didn’t keep it up for long. The bony fishes that follow placoderms in the evolutionary tree show no evidence of internal fertilization—meaning that at some point, antiarchs gave up on their new method and went back to spawning in water. Later on, their descendants evolved sex organs that reintroduced the antiarch’s pioneering strategy.
The finding, which rewrites the entire history of sex, was an accident—and a moment of scientific inspiration. Here’s Ian Sample, writing for Guardian Science:
Like so many in science, the latest discovery came about by chance. Last year, Long was working in a palaeontologist’s laboratory in Tallinn, Estonia, when he was handed a box of placoderm bones. Among them he found a plate with a strange, grooved bone attached. He had studied placoderms all his life but was at a loss to explain what it was. Later that day, the penny dropped: “It was a clasper, a sex organ, and it was the oldest and the most primitive one yet found on the planet,” he said.
The chance finding prompted a search for other samples that led them to private collections of fossils in the UK and the Netherlands. From those they gathered more evidence of male antiarchs with their claspers still attached. The grooves in the organs are used to transfer sperm from male to female.
Further examination of the antiarchs revealed the first evidence of discrete female sexual organs, in the form of small genital plates in exactly the right position to facilitate sex.
Until now, biologists had no idea that the switch from internal to external fertilization was evolutionarily possible.
Placoderms were previously considered by some to be an evolutionary dead end, but this study complicates matters. Internally-fertilizing antiarch fish could mean that placoderms are more unified than expected, suggesting that these specimens may be among the earliest members of a branch on the evolutionary tree that eventually leads to Homo sapiens.