Pygmies
© Evan LeachMembers of a village in Cameroon who are Pygmies.

Pygmies have unique changes in their genomes that help their life in forested habitats and keep them short, a new analysis of their genes suggests.

Researchers analyzed the genomes, the "building code" that directs how an organism is put together, of Western African Pygmies in Cameroon, who average 4 feet, 11 inches tall, and compared them to their neighboring relatives, the Bantus, who average 5 feet, 6 inches, to see whether these differences were genetic or a factor of their environment.

"There's been a longstanding debate about why Pygmies are so short and whether it is an adaptation to living in a tropical environment," study researcher Sarah Tishkoff of the University of Pennsylvania said in a statement. "Our findings are telling us that the genetic basis of complex traits like height may be very different in globally diverse populations."

Short population

Pygmies_1
© Evan LeachStudy researcher Alain Froment, of the Museum of Man in France, in the striped shirt with a group of Pygmies.
The Pygmy and Bantu populations separated about 60,000 to 70,000 years ago, then came back into contact roughly 4,000 to 5,000 years ago, and started interbreeding.

Some Pygmy women have given birth to half-Bantu babies, which integrates Bantu genes into the Pygmy population, but offspring of male Pygmy and female Bantu are rare, so the Bantus don't have many Pygmy genes.

The researchers analyzed the genomes of 67 Pygmies and 58 Bantus for changes that would provide information about an individual's ancestry.

These changes are small, nonharmful misspellings in the code (the chemical bases A, C, T and G) that makes up the genome. For example, a Bantu might have an A where a Pygmy has a T.

By analyzing large numbers of these changes, researchers can tell how much of an individual's genome is Bantu and how much is Pygmy.

Selected for statue

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© Evan LeachStudy researcher Sarah Tishkoff (center) with Pygmy women from Cameroon.
The researchers also used this letter-change data to look for areas of the genome associated with height and those that were "naturally selected" for - parts of the genome that are passed down through the generations because they provide some sort of advantage.

The data said height had a genetic component related to Bantu ancestry: The more Bantu ancestry an individual from the Pygmy tribe had, the taller that individual tended to be. One part of the genome, on chromosome 3, was especially important in this trait, the researchers said.

"We kept seeing a lot of them [these single letter differences] highlight that region in chromosome 3," Tishkoff said. "It just seemed like a hot spot for selection and for very high differentiation and, as it turns out, very strong association with height as well."

Height genes

The researchers zoomed in on the genes in this area of the genome. One of the genes they found had already been associated with height changes in other populations, but some hadn't.

They found new changes in hormone pathways and immunity that seemed to correlate to the pygmy's short stature. These could have been selected for because of their influence on height or because changes in these genes play other roles in the body, Tishkoff said.

For example: An immunity component might be selected for because it helps the pygmies fight off infections, which are prevalent in their habitat. And the link to hormone pathways also makes sense, Tishkoff said, because changes to them could help the Pygmies reproduce earlier. Shorter height could just be a byproduct of these changes.

The study was published today (April 26) in the journal PLoS Genetics.