PloS Genetics has just published a new study identifying genes that could provide a partial explanation for why pygmies are so short. It’s a fascinating question: Haven’t we all been intrigued by pygmies since we first came across pictures of them in the National Geographic when we were kids?
The researchers used a gene chip to analyze the genes of three pygmy groups and three neighboring groups of Bantu-speaking people–who are taller, and with whom the pygmies have sometimes had children, leading to some mixing of genetic traits. They found regions in the genome that seemed related to the pygmies’ adaptation to their environments. Genes in these regions are related to insulin and insulin-like growth factor, and to immunity and reproduction and metabolism, according to the study’s abstract. The genetic analysis also revealed portions of the genome related to height and to “growth hormone-stimulated STAT5 signaling,” whatever that might be.
This is tough stuff. At one point in the introduction to the study, the authors say that their findings provide insights into the adaptations in the ancestors of West African pygmies and the genetic “architecture” of short stature. Further down in the study, the authors link the findings concerning height and adaptation. “Our results raise the possibility that the adaptive process that produced small body size in Pygmies may be the result of selection for traits other than stature, including early reproduction, metabolism, and immunity…”
The University of Pennsylvania press release is little help. The study, it says, identifies genes that might be associated with height. It goes on to say that the study “also provides evidence based on genetic signatures of natural selection to suggest why these groups evolved to be small, with signs pointing to hormonal pathways and immune system regulation as possible drivers.” What? A couple of quotes from the lead author of the study (Sarah Tishkoff, in the photo above) don’t help either.
But a few good reporters have helped to clear up the issues here.
At Scientific American, Gary Stix clearly draws a connection between adaptation and height. “It may be that genes that protect against microbes may also hinder growth. Diminished stature could be a byproduct of bolstering immune and metabolic defenses and not a direct adaptation to a hunter-gatherer lifestyle,” he writes. Interesting, isn’t it? The short height itself is not an adaptation to the harsh living conditions faced by the pygmies–but merely a side-effect, so to speak, of the adaptations that actually do help them survive.
At the Chronicle of Higher Education, Josh Fischman also neatly draws the link. He writes that the researchers have “identified a ‘master gene’ for immune response that is much more common in Pygmies than in their taller neighbors, the Bantu.” And he reports that “this same gene can disrupt growth-hormone pathways.”
At Nature, Erika Check Hayden takes a very different approach. Rather than focusing on the evolutionary tradeoffs, she takes great care to note the limitations of the study. That’s never a bad thing, but I’m not sure she gives the researchers their due. Her lede is that the pygmy study “underscores how tricky it can be to find the genetic underpinnings of human characteristics.” That caution arrives before we know what the researchers are claiming. She then reports the identification of genes possibly linked to height, without discussing the immune factors, or insulin or metabolism. She continues with a discussion of the shortcomings of the study and the difficulty of doing such research in remote populations.
Elizabeth Norton at Science Now raises the natural-selection question: Is the short height a selective advantage in dense jungles, as some have speculated? This study, she notes, argues against that. She gets down to a bit more detail than some of the others, discussing the role of the CISH gene in the study. (I’ll refer you to Norton for more on that.)
Dr. Christopher Token, in a blog post at ABC News, notes speculation that the pygmies’ short stature was due to a lack of Vitamin D, because the deep shadows of the rainforest keep them out of sunlight. No, he writes: It’s genes!
So far, there has been less coverage of the story than I would have expected. We’ll likely see more in the coming days. And we should. It’s newsworthy, and it’s a good opportunity to enlighten readers to some of the complexities of genetic research–and its value.
– Paul Raeburn