Biologists agree on the big picture when it comes to Darwinian evolution, but they're still working out many details. One longstanding puzzle is why there's so much variation from person to person, why some people like broccoli, for example, and others hate it.
There's evidence that people who hate broccoli are not being childish or stubborn. Some broccoli haters are reacting to compounds called glucosinolates, which are also present in brussels sprouts and other so-called cruciferous vegetables.
Some people taste these as intolerably bitter, and others can't taste them at all. The difference is genetic. The gene responsible, called hTAS2R38, comes in several forms, some of which enable people to detect this stuff.
The standard theory explaining this gene is that it helps us avoid harmful plant toxins.
But University of Pennsylvania geneticist Sarah Tishkoff didn't buy this. If it's so important, she asked, why would half of us be missing the ability to taste a whole class of dangerous plant-toxins?
To investigate, she teamed Penn geneticist Michael Campbell as well as researchers at Monell Chemical Senses Center, Rutgers, and National Institutes of Health, and went off to Africa, since it's the source of humanity and much of our variation.
Also on the team were researchers from French and African universities.
Luckily for the scientists, they didn't have to feed hundreds of subjects vegetables. People's reaction to natural glucosinolates can be tested by their reaction to a synthetic relative, called PTC. When they spike water with PTC, people taste either nothing or something nastily bitter.
Tishkoff said she thought they would find a simple evolutionary explanation - some people probably needed this particular toxin-detection system more than others depending on local food sources. "I thought this would be a way to see how people adapt to diverse diets."
What they found was a surprise, published in a recent paper in Molecular Biology and Evolution. The sensitivity to bitter taste varied far more than expected.
From Kenya to Tanzania to Cameroon, Tishkoff and Campbell studied how people differed in their taste perception and their DNA. They included 57 groups, including hunter-gatherers such as Pygmies, as well as a variety of farmers and herders.
"This was not trivial," she said. They had to reach some of the remotest parts of Africa lugging around 40 bottles of the PTC at different concentrations. The subjects volunteered to taste increasing concentrations of the stuff until, as Tishkoff described it, "they made a face and went 'eeww.' "
Some people were repelled by even the most dilute solutions, she said, while others tasted nothing. The researchers also saw gradients in between - more so in Africa than among U.S. subjects, who tend to cluster around the extremes. That's not surprising, she said, because Africans are more diverse than other populations.
She also found a number of versions of the bitter-perception gene among Africans that hadn't shown up in other populations.
Tishkoff and her collaborators say they assumed they would find big differences from one group to another. But instead, they found about the same spectrum of PTC tasters and nontasters.
Sometimes variability increases when a trait loses its importance for survival. That has happened to some extent with our receptors for smell. We have about 1,000 genes that encode our smelling ability but only about half of them work - a different half in each person.
But that's not what the researchers think happened with bitter taste. If it were just a case of degeneration, they wouldn't see such a strong pattern, Tishkoff said, with all the African groups showing the same genetic variants in about the same proportions.
That, they say, suggests Darwinian evolution is driving the maintenance of diversity. They call this pro-diversity force "balancing selection." It can happen when multiple types of a gene offer different advantages. They don't know what the advantage is of the nontaster version, but there are several possibilities. Taste genes are active in our guts and our lungs, Tishkoff said, so maybe the gene they studied has some other job in those regions.
In other words, we may vary in our taste for vegetables thanks to some reasons that have nothing to do with tasting.
Stephen Wooding of the University of Texas Southwestern Medical Center finds this particular taste-making gene so fascinating that he studies it in other primates.
Chimps also vary in their ability to taste PTC - which he has determined by watching the animals' reaction to apples soaked in the stuff compared to apples that aren't.
In 2006, he published a paper showing that there's a completely different mutation that disables the PTC tasting in chimps. Nontasting must have evolved independently in two species.
"The interesting thing about bitter taste is that it plays a basic role in survival of animals," he said. It's part of a plant-animal arms race in which toxic and bitter compounds help the plants avoid being eaten by the animals. The bitter receptors help the animals avoid being poisoned by the plants.
We humans have about 25 genes controlling our ability to taste bitter compounds, Wooding said, and the one Tishkoff's group studied helps us detect toxins that attack the thyroid and cause goiter.
He agrees with the study's authors that the variation they saw among Africans suggests an upside to the nontasting version.
Wherever these bitter receptors show up in the body, he said, they work to detect toxins, and so the versions that don't detect PTC probably pick up something else. We all have two copies of this gene, he said, so if evolution favors those with one of each, you'll see a uniform mix of the two types. Those people would be gaining both advantages.
That still doesn't explain why vegetables such as broccoli have compounds that raise the toxin alarm in some people. Could those of us who can't taste PTC be in danger of broccoli poisoning?
Probably not, the scientists say. The nasty stuff shows up in genuinely toxic plants. The bad taste associated with cruciferous vegetables is a curious side-effect.
The consensus on broccoli and its relatives is that they're still good for you, whether you like them or not.