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Did Dodging Disease Keep Pygmies Small?
26 April 2012 5:00 pm
Pygmies are found throughout the world, but they all have certain things in common. They're small (the word pygmy refers to any group in whom adult males are under 1.52 meters), they live and hunt in tropical forests, and they die young. The average life span for a pygmy is only about 17 years; a leading cause of death is infectious disease carried by the miasma of microbes that flourish in the hot, moist jungle.
Researchers believe pygmies' short stature evolved to equip them for their brief lives in the jungle; smaller bodies may move more nimbly through dense forest or be better able to dissipate heat. Another possibility is that pygmies are evolutionarily programmed to stop growing right after puberty so that they'll have enough time to bear children, the idea being that their body's resources can be channeled into bearing offspring. Genetic evidence to support any one scenario is lacking. But a new study points to a hotspot of DNA in which growth and immunity converge. The finding presents unexpected evidence that short stature is a byproduct of other traits, not the direct consequence of natural selection.
In the study, geneticist Sarah Tishkoff of the University of Pennsylvania and colleagues scanned the entire genomes of about 70 individuals from three pygmy groups in Cameroon, and another 60 from three neighboring Bantu groups (tall, thin, farming tribes with whom pygmies have intermarried for generations). The researchers were looking for variants called single-nucleotide polymorphisms (SNPs, pronounced "snips") that might account for traits unique to pygmies. A SNP is an alternative spelling in the genetic code; similar SNPs turning up among many individuals in a family or a group are likely contribute to that group's characteristics.
The genetic dragnet, reported online today in PLoS Genetics, yielded telltale variants near genes involved in several of the brain's chemical messenger systems, including the receptor for thyrotropin-releasing hormone, which is significant because pygmies do not seem to develop thyroid problems, such as goiter, that typically occur in the iodine-deficient areas where they live. Some of the SNPs cluster in an area of chromosome 3 around genes that could be involved in height. One, called CISH, shuts down the receptor to a key growth hormone; mice that overproduce the receptor are smaller and grow more slowly than normal. Investigating this gene further, study co-author Joseph Jarvis, who is now at the Coriell Institute for Medical Research in Camden, New Jersey, found that it also confers immunity to diseases such as malaria and tuberculosis.
The fact that pygmies seem to show genetic differences in CISH suggests that growth and immunity are physiologically linked, Tishkoff says. "Our finding raises the intriguing possibility that short stature could be the result of better disease resistance—which is likely given the many microbes that pygmies are exposed to," says Tishkoff. "Even though they do die young, pygmies might have evolved with a slight edge in the constant arms race between the human immune system and the pathogens."
She says the study doesn't refute other hypotheses; it shows that the picture is more complex. "The pygmies' size can't be explained by one or two height genes" she says. "Clearly, these groups evolved with growth, metabolism, and immunity all interconnected."
Evolutionary biologist Michael Hammer of the University of Arizona in Tucson calls the study exciting. "The numbers of individuals are too small to turn up a smoking gun, but no one else has gone into Cameroon to get samples from two neighboring groups that differ in height," he says. "The team is using a very creative approach, and they've provided an interesting candidate region of DNA to home in on."