For 65 years, scientists believed that because humans and chimpanzees have similar variations in the ability to taste a bitter flavor, this trait must have come from a common ancestor.
But in the cover story of the latest edition of the journal Nature, University of Utah scientist Stephen Wooding and colleagues have disproved the idea. Both humans and chimps have similar responses to the synthetic compound PTC, but they came upon them independently.
PTC, shorthand for phenylthiocarbamide, is a man-made compound, but it's much like the flavor of some toxic substances in plants. Because of genetic variations, not everybody can taste it, and not every chimp can, either.
Worldwide, about 75 percent of people can detect PTC. As Deseret Morning News reporter Lois Collins described it in 2004 when Wooding identified the genes involved, "It's bitter. It's nasty bitter." Others tested, however, could not sense the flavor.
Sixty-five years ago, a famous statistician, Sir Ronald Fisher, noted that "chimpanzees and humans share this trait," with some able to sense the taste and others not, Wooding said.
At the time, Fisher assumed the reason was that the two species share some mutations that must have emerged before humans and chimps became separate.
"We wanted to find out if this was true," Wooding said.
Wooding and Dr. Michael Bamshad of the University of Washington and formerly of the U. are the senior authors of the letter published by Nature. Other authors are from the U.; the German Institute of Human Nutrition; the Southwest Foundation for Biomedical Research in San Antonio, Texas; and Arizona State University, Tempe.
U. Department of Human Genetics authors besides Wooding and Bamshad are Michael T. Howard, Diane M. Dunn and Robert B. Weiss.
A human can taste for ability to sense PTC by putting on the tongue a paper strip soaked in a non-dangerous level of PTC. But chimpanzees are not likely to go along with that sort of test.
In what may have seemed like a dirty trick to most of their hairy subjects, investigators "soaked slices of apples in a very mild solution of PTC" and gave them to chimpanzees, he said. Then they "watched their reaction."
The chimpanzees, at the Southwest Foundation for Biomedical Research, had obvious reactions of distaste if they could sense PTC. Those that could not would happily snack on the apples.
Then researchers could examine the chimpanzees' DNA to check for the mutations.
"In fact, what we see now is that chimps, like humans, have two different forms of the PTC tasting gene," Wooding said. One of these genes is functional. But somewhere along the line, a mutation introduced a "broken form" of the gene into the population.
Like humans, each chimp receives two copies of the gene. They could both be functional, one functional and one not, or both broken. "As long as you have at least one functional copy, you can taste this stuff," he said.
Humans also have two forms of the gene. But they aren't similar in function to the genes of the chimps. In humans, the genes involve binding to chemicals other than PTC.
"Although humans and chimpanzees outwardly show a very similar pattern," Wooding said, the gene's mutations are completely different on a molecular level.
Humans and chimps independently developed their PTC responses.
Wooding said the fact chimps have a broken form of the gene "suggests the possibility that at some point in the ancient past, chimps were released" from the need to taste this particular type of toxic material. Maybe they wandered into areas where such plants didn't grow.
In humans, the new form of the gene seems to bind with a different chemical.
"Perhaps humans were under pressure to taste something new," he said. "But it's very hard to be certain."
Wooding thinks the discoveries "give us a little bit more insight into the ways that natural selection has affected patterns of variation in human taste sensitivity."
The research may lead to a better understanding of the relationship between genetics and behavior.
For example, in humans, the inability to taste PTC is considerably higher among smokers than that of the population as a whole, according to an earlier study by the National Institute on Alcohol Abuse and Alcoholism. But cruciferous vegetables like cabbage and broccoli, which are delicious to some, may taste unpleasant to some people who can taste PTC.
What's next?
"Well, this gene we've been working on is just one of approximately 25 such genes," Wooding said, "and the surface has just barely been scratched with respect to the others."
E-mail: bau@desnews.com