Humanity has had a few long time companions over the millennia, including dogs, lice and the plague. Among the most annoying, however, is the common fruit fly, Drosophila melanogaster, the tiny little red-eyed insect that tends to spoil fresh fruit. Though the little buggers seem to have followed humans all across the world and into the laboratory, their exact origin story was unknown.
According to Nell Greenfieldboyce at NPR, a new study presents an answer. Researchers understood that the flies likely started out somewhere in Africa, but they have never been found living in the wild. During a recent survey of fruit fly genetics with sub-Saharan ancestry, it was found that the most diverse set of fruit fly genes come from Zambia and Zimbabwe, suggesting that the wild ancestors of the flies might originate in the forests of south-central Africa.
But Marcus Stensmyer from the University of Lund in Sweden and co-author of the study Current Biology tells Greenfieldboyce that expeditions to find the flies in the area struck out. Then he and his team began to think perhaps unlike in our kitchens, where the flies lay their eggs on all types of overripe or rotting fruit and vegetables, the flies were picky eaters in the wild, attracted to one type of fruit. The team took a look at the wild fruits available in the region and decided that the marula, a sweet plum-sized fruit, most closely resembled the fruits that flies tend to prefer in the kitchen.
The team set out fruit fly traps near marula trees in Matobo National Park in Zimbabwe and, lo and behold, they caught loads of wild fruit flies going after the rotting fruit. They also found the flies were particularly attracted to ethyl isovalerate, a compound found in the fruit. When researchers set out rotting oranges near the marula fruit, the flies still chose the marula, though they chose oranges spiked with ethyl isovalerate equally.
“They are drawn to particular aromatic substances from marula that activate receptors on the antennae. When these are activated, it’s a sign that it’s a good place to lay eggs,” Stensmyr says in a press release.
The association with marula fruit also helps researchers understand how fruit flies ended up in our kitchens. According to the study, archeologists have found that ancient San tribes native to the area have relied on marula fruit for thousands of years. In one cave, they found 24 million walnut-sized 8,000- to 12,000-year-old marula pits discarded by generations of humans snacking on the fruit. The scent of all that luscious overripe fruit likely attracted lots of flies. The team even tested whether the flies would enter the dark caves, finding that, indeed they would take the risk for a little taste of the marula sweetness.
Over time the people and flies forged their lasting bond in these caves. “The fly has developed into a generalist that eats and breeds in all sorts of fruit,” Stensmyr says in the release. “But originally it was a real specialist that only lived where there was marula fruit.”
While some might wish the San had kept the flies out of their caves, so that they would have never ended up in our households, that’s not the case with scientists. Common fruit flies are an animal model in genetics research and they have contributed to five Nobel prize winning studies. Fruit flies have led to the understanding of thousands of genes that are also found in humans. Which, if you think about it, is worth a little spoiled fruit.