How a fruit-loving San artist is linked to the pesky fly in your bowl
The San’s taste for marulas was key to setting fruit flies on course to global domination, according to scientists
As diminutive San artists worked on their cave paintings 10,000 years ago, swarms of fruit flies buzzed round their heads.
We know this from the fact that millions of marula stones have been found in the caves, and because fruit flies love marula.
Now scientists have concluded that the San’s taste for the marula, which they famously share with the elephant, was key to setting fruit flies on course to global domination.
The finding emerges in a paper published in the journal Current Biology, which concludes that pesky flies in fruit bowls all over the world are the direct descendants of the creatures that buzzed past San artists’ heads.
Their evidence comes from the 424km² Matobo National Park, just south of Bulawayo in southwestern Zimbabwe – a wilderness area that also hosts the grave of Cecil John Rhodes.
There, Lund University biologists Suzan Mansourian, Marcus Stensmyr and colleagues went in search of fruit flies (Drosophila melanogaster). They baited fly traps with marula as well as orange, the fruit favoured by the insects when they want to lay eggs in the 21st century.
Their work took place near the Pomongwe cave, home to spectacular San paintings and the remains of 24 million carbonised marula stones, which they said were only a fraction of the total brought into the cave.
They found fruit flies showed a strong preference for marula, and that in areas without marula trees there were hardly any flies. Then they wanted to see if the fruit flies would enter a cave in pursuit of marula, so they placed traps along the far wall of the Ntswatugi cave and caught dozens of the insects over three days.
Stensmyr said they had uncovered enough evidence to conclude that “the flies in your kitchen fruit bowl are the direct ancestors of a group of flies that lived on marula in a distant forest”.
He added: “Some 10,000 years ago, these flies moved in with their human neighbours, and their offspring then colonised the world. That’s pretty cool.”
Before the Swedish research – and in spite of the fact that the fruit fly is thought to be the most intensively studied organism on Earth – the insects had never been scientifically observed in undisturbed wilderness.
The researchers also found that fruit flies from other parts of the world also prefer marula over citrus, even though they have never seen it before. “The ancestral marula preference is accordingly conserved in non-African flies,” they said.
The flies are attracted to the main chemicals released by marula, which activate odour receptors known to influence their selection of good egg-laying spots.
But the Swedish scientists found the link with Africa’s indigenous people particularly intriguing. “The San evidently spent considerable time collecting and processing marula, which would have been the staple food item during many months of the year,” they said. “Thus, just like [the fruit fly], these San tribes appear to have been seasonal specialists on marula as well.”
They suggest this may explain how fruit flies first came to live among humans, attracted to them by the scent of marula. Once inside the caves, they would have benefited from protection from predators and bad weather.
Over time, the researchers suggested, the cave flies adapted, becoming more willing to enter dark enclosures and increasingly tolerant of the ethanol produced by fermenting marula.
“Flies would have found a steady supply of marula and fermenting leftovers inside the caves, long after the fruit’s presence in the surrounding woodlands had diminished,” said the scientists.
Stensmyr added: “The fly has always been considered an opportunist and generalist, feeding and breeding in all manners of fruit.
“In its native environment, however, the flies show a quite specialised lifestyle, being only found with marula fruit.”
Archaeological evidence shows that intensive marula use began about 12,000 years ago and reached “massive proportions” 9,500 years ago, before ebbing away 8,000 years ago.
The scientists wanted to return to the Matopos hills on their next expedition, he said, “to really figure out what the flies are doing in their native habitat”.