It's not another tall tale: Evolutionary biologists have developed a new understanding of the genetic basis of short stature in humans. Also known as the pygmy phenotype, a study published Monday in the Proceedings of the National Academy of Sciences shows that this trait has evolved several times over the course of human history.
In the Central African rain forest, several groups of hunter-gatherers are significantly shorter than their agricultural neighbors are. Both the Batwa people in the east and the Baka in the west are commonly referred to as pygmies.
But exactly what factors were contributing to their reduced height wasn't clear. By analyzing the genomes of the Batwa and the Baka, and comparing them with the genomes of their average-height neighbors, the researchers were able to show that these two groups of humans showed variations in the region of the genome that codes for human growth hormone receptors and bone formation.
"We have found the strongest evidence yet that the pygmy phenotype is controlled by genetics," said Luis Barreiro of the University of Montreal and the senior author of this recent study.
Although height is a tremendously variable trait among humans, several rain forest-dwelling populations in Asia and Africa have been noted for their unusually short stature. The average height among Batwa men (60.1 inches, 152.9 centimeters) and women (57.4 inches, 145.7 centimeters) is significantly lower than in neighboring Bakiga men (65.1 inches, 165.4 centimeters) and women (61.0 inches, 155.1 centimeters).
Biologists soon began to wonder why height was so different in these populations.
Some scientists hypothesized that environmental factors were the main cause. Rain forests might be biodiverse, but it's extraordinarily hard for humans to find food there. People like the Batwa may be short simply because they are not receiving proper nourishment to grow to their full height.
Other scientists, including Barreiro, thought that gene variations might be contributing to the pygmy phenotype. For one, it could be easier for shorter individuals to keep their bodies from overheating in the tropical humidity. Some of Barreiro's own work revealed that traveling through the rain forest involves a lot of ducking under things, which requires a lot of energy. Shorter people don't have to duck as much and consequently burn fewer calories as they walk through the rain forest.
Significant Genetic Differences
Barreiro and colleagues gathered genetic data from the Batwa and Baka peoples, as well as from three neighboring agricultural groups of average height. When they scanned different regions of the genome, they found significant genetic differences among the Batwa and Baka in an area of the genome that is known to code for the receptors for human growth hormones.
When the researchers looked more closely, they found that these genetic differences weren't just random chance and that the first Batwa and Baka people just happened to be short. Instead, these genetic differences were somehow benefiting the individuals living in these rain forest environments. It's an example of convergent evolution, Barreiro says, in that the same trait (short stature) evolved independently in several different populations.
When they looked at when these mutations might have happened, Barreiro and colleagues found that they were relatively recent events, having occurred separately in both the Batwa and the Baka. This showed that whatever factors were selecting for short stature were fairly strong and could exert their effects relatively quickly.
"This study is one of the most significant advances made on the genetic determination of the pygmy phenotype so far," said Paul Verdu, an anthropologist and ethnobiologist at France's National Museum of Natural History.
The results will help provide an understanding not just of the pygmy phenotype, but also of the evolution of the tremendous amount of diversity in our species.