When And Where Fruit Flies First Bugged Humans

Enlarge this image toggle caption Courtesy of Marcus Stensmyr Courtesy of Marcus Stensmyr

The next time you swat a fruit fly in your kitchen, take heart from the fact that people have apparently been struggling with these fly infestations for around 10,000 years.

A study published Thursday suggests Drosophila melanogaster first shacked up with humans when the insects flew into the elaborately painted caves of ancient people living in southern Africa.

That's according to a report published Thursday in the journal Current Biology.

Scientists say the flies would have been following the alluring smell of stored marula fruit, which were collected and stored by cave-dwelling people in Africa. This tasty yellow fruit was a staple in the region in those days — and was also the fruit that wild flies apparently evolved to depend on in nearby forests.

The humble fruit fly now lives with humans all over the planet and is one of the world's most studied creatures. For more than a century, biology and medical laboratories have depended on this fly — one scientist notes that at least nine times, the Nobel Prize in Physiology or Medicine has been awarded for research on Drosophila. One of those prizes was won by Thomas Hunt Morgan of Columbia University, whose fly research in the early 1900s plucked this species from obscurity and transformed it into a mainstay of genetics.

"It's small; it's cheap to raise; it has interesting genetics," explains Thomas Kaufman, a biologist at Indiana University in Bloomington. "We think that flies are quite charismatic. They're wonderful. They're beautiful little animals, and we love them. Seriously."

But despite all that love and study, the origins of this fly, and how it first moved in with people, have been a mystery.

"I've been wondering about this for the past 20 years," says Marcus Stensmyr, a biologist at Lund University in Sweden who uses these flies to study the olfactory system. "It's really been kind of a life-long ambition, if you wish, to find where they come from."

Scientists have known for decades that, like people, the flies seem to have started out in Africa — somewhere.

"You find them in your kitchen. You find them in my kitchen — you find them in everyone's kitchen," says Stensmyr. "But if you go out into the forest, you simply don't find them."

Recently, researchers collected flies from around Africa and looked at their genes. They found that the greatest genetic diversity was found in flies from Zambia and Zimbabwe, suggesting that this species got its start in the southern-central region of the continent.

But trips to that region failed to turn up much of anything.

"After a number of failed excursions down to Africa," says Stensmyr, "we thought, 'OK, so maybe they are associated with some specific fruit in their original home.' "

Stensmyr and his colleagues studied a long list of possible fruits, looking for all the features that D. melanogaster is known to prefer. The flies favor citrus fruits — like oranges, for example.

Enlarge this image toggle caption Courtesy of Marcus Stensmyr Courtesy of Marcus Stensmyr

"We came to a candidate fruit — that was marula fruit," says Stensmyr. The yellow fruit is about the size of a large plum, with a hard stone in the middle. "It has a sweet and nice taste."

The researchers traveled to the woodlands of the Matobo National Park in Zimbabwe. They found fruiting marula trees and put out traps. Bingo — they caught D. melanogaster.

"We found tons of flies," Stensmyr recalls.

Further study showed that wild D. melanogaster strongly prefer marula fruit over oranges.

What's more, so does a breed of fly that's commonly used in labs. This strain was established in 1916 from a fly population in Canton, Ohio.

"They actually have retained the preference for marula," says Stensmyr. "They would go for the marula as well."

The researchers isolated one particular chemical in this fruit — ethyl isovalerate — that seemed particularly important. Flies that were given a choice between marula and oranges spiked with this chemical failed to pick one over the other, suggesting that the two choices seemed the same to the flies.

All of this provides an intriguing clue for how these insects may have started to make their home with people. Near where the researchers found the wild flies, there are caves where the San tribes once lived. These people left behind beautiful cave paintings — as well as the pits of marula fruit that they had eaten. From one cave alone, excavators turned up 24 million marula stones.

"They really, really loved marula," says Stensmyr, who points out that the stones found date from about 12,000 years ago to about 8,000 years ago. "During the times when these caves were inhabited, the San people must have brought enormous quantities of marula into the caves."

That means marula was likely stored there, and available there long after the marula in the forest had been eaten up by wildlife. The strong smell of all this marula would have attracted the flies.

To test whether or not wild flies would actually enter a cave, the research team put traps baited with fermenting marula along the far wall of the Nswatugi cave. Sure enough, over a period of a few days, these traps caught a number of D. melanogaster flies.

The study, and the story it tells, has completely delighted other scientists who study fruit flies.

"I particularly liked going and catching the flies in the painted caves," says Kaufman. "That was inspired. It's really a neat paper."

"I thought it was fantastic," agrees Celeste Berg, a developmental geneticist at the University of Washington, Seattle who has used flies in her research for 30 years. "I thought their data was really quite striking."

Berg says she wonders exactly how the flies would have spread from these caves to the rest of the world.

"I think it's exciting to learn the origins of fruit flies and, even if you're not an ecologist or a population geneticist, I think it's just natural to be interested in the history of the organism you study," says Berg. "I had assumed that fruit flies liked all kinds of fruit — especially bananas. I didn't even realize that they preferred citrus. And it's not even really citrus that they prefer; they prefer this particular marula plant, which I also had never heard of."

Debbie Andrew, a developmental biologist at the Johns Hopkins School of Medicine who has worked with fruit flies for four decades, also says she loved the paper.

"They built a good story," says Andrew. "It's very hard to prove something that happened 10,000 years ago or more. I like the story."

As to whether all the details are right, she says, "I don't know; it does seem plausible, based on the amount of marula fruit stones they found in the cave."

Based on this paper, says Andrew, the old saying, "time flies like an arrow, and fruit flies like a banana," should really be changed.

"Time flies like an arrow," she says, "and fruit flies like an orange, or a marula fruit, or perhaps an orange spiked with ethyl isovalerate."