The glow of the firefly is as mysterious as it is alluring. Scientists have long argued over why the trait evolved in these adult beetles: Was it to deter predators—or identify mates? A new study argues for the latter, finding the ability arose more than 130 million years ago, tens of millions of years before firefly hunters like bats and birds existed.
“It is nice to see convincing evidence pointing in one direction,” says Sarah Lower, a biologist at Bucknell University. Lower was not involved with the study, but she separately analyzed the same data and got nearly identical results.
Like all beetles, the firefly cycles through a complete metamorphosis—hatching from its egg as a crawling larva before pupating into a mature adult. Scientists believe the larval stage first evolved the ability to light up as a way to signal their fowl taste to predators. But researchers have disagreed about when and why adults began flashing, though some believe this behavior also arose to ward off predators.
To test this idea, Gareth Powell, then a doctoral student at Brigham Young University, and his colleagues set out to re-create the timeline of firefly evolution. In 2019, the lab analyzed DNA from 88 species of glowing beetles to construct a giant family tree. But the tree lacked dates, which require fossils for calibration. Then, earlier this year, another team provided the missing link: a 99-million-year-old bioluminescent beetle fossil, the oldest ever described.
The scientists placed the new fossil and three others from fireflies at branches on the family tree where they shared the most physical characteristics. They then used algorithms to come up with the most likely date for the origin of beetle bioluminescence. The researchers ran multiple models on two data sets of beetle DNA, which converged on the same conclusion: Fireflies started to glow at least 141 million years ago on the ground (as larvae) and 133 million years ago in the air, the team reports in a study posted to the preprint server, bioRxiv.
Bats, by contrast, didn’t show up until about 65 million years ago, and modern birds are only 10 million years older than that. The study doesn’t rule out whether extinct flying predators such as pterosaurs played a role, but there’s no evidence that such creatures fed on these insects, Powell says.
Jesse Barber, a biologist at Boise State University who first formally posited that adult fireflies evolved their glow to evade predators, is convinced. “This new preprint nicely shows that this is not the case.”
Rather, Powell says, the findings support the idea that nature’s flying night light emerged for sex. Fireflies today use species-specific flash patterns to find mates in the dark. If a female likes the direction, duration, and color of a male’s signal, she’ll send a particular response to guide the male to her.
By contextualizing these “evolutionary stories” with dates, Powell says, the new study suggests the firefly’s courtship ritual has a long and important history.