If Pygmies are known for one trait, it is their short stature: Pygmy men stand just 4’11" on average. But the reason why these groups are so short and neighboring groups are not remains unclear. Scientists have proposed various theories based on natural selection, including that Pygmies’ reduced size lowered nutritional requirements, helped them better handle hot climates, or allowed them to reach sexual maturity at an earlier age.
Now a new study of the Western African Pygmies in Cameroon, led by geneticists from the University of Pennsylvania, identifies genes that may be responsible for the Pygmies’ relatively small size.
The work 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.
"There’s been a longstanding debate about why Pygmies are so short and whether it is an adaptation to living in a tropical environment," said Sarah Tishkoff , senior author on the study and a Penn Integrates Knowledge professor with appointments in the genetics department of the Perelman School of Medicine and in the biology department of the School of Arts and Sciences. "I think our findings are telling us that the genetic basis of complex traits like height may be very different in globally diverse populations."
While hundreds of studies have sought and identified genes that play a role in height variations in European populations -- nearly 180 such genes have been pinpointed -- this is the first genome-wide study of genes that contribute to stature in African Pygmy populations.
"By performing a detailed genetic analysis, Tishkoff and her colleagues have identified many candidate genes that have played an adaptive role in Pygmy populations, including several related to stature," said Irene Eckstrand, who oversees evolutionary-biology grants at the National Institutes of Health’s National Institute of General Medical Sciences , which partially funded the work. "This research illustrates the value of studying human traits in their evolutionary and ecological contexts for understanding how humans adapted to their local environments."
Tishkoff led the study with Joseph Jarvis, a Penn postdoctoral researcher at the time the study was conducted and now a senior research scientist at the Coriell Institute for Medical Research. Other Penn contributors included Laura Scheinfeldt, Sameer Soi, Charla Lambert, Bart Ferwerda and William Beggs of the Department of Genetics.
The Penn researchers collaborated with Larsson Omberg, Gabriel Hoffman and Jason Mezey of Cornell University; Alain Froment of the Musée de l’Homme in France; and Jean-Marie Bodo of the Ministère de la Recherche Scientifique et de l’Innovation in Cameroon.
Their paper was published today in the journal PLoS Genetics.
Africa is the birthplace of humankind and remains a continent rife with examples of the variation within our species, including variation in body size, ranging from short-statured Pygmy hunter-gatherers to tall-statured East African pastoralists. Individuals in one of these unique groups, Western African Pygmies, are 17 centimeters shorter on average than their Bantu-speaking neighbors.
These two groups diverged genetically 60-70,000 years ago. But roughly 4-5,000 years ago, Bantu-speaking groups expanded into the tropical forests where Pygmies’ ancestors lived. The resulting led to population mixing, typically Bantu men reproducing with Pygmy women. Their children would return to Pygmy villages; therefore Pygmies’ genomes reflect the contributions of Bantus, while Bantu genomes contain little to no input from Pygmies.
Using a variety of analyses, Tishkoff’s team took advantage of the stature differences and history of genetic exchange between the Bantus and Pygmies to seek candidate genes responsible for the Pygmies’ small size.
The geneticists scanned the genomes of 67 Pygmies and 58 Bantus for particular mutations -- known as single nucleotide polymorphisms, or SNPs -- that provide information about individuals’ ancestry. They confirmed that height differences had a genetic component, as the more Bantu ancestry a Pygmy had, the taller that individual tended to be.