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By Rachel A. Becker, National Geographic

PUBLISHED July 28, 2015

People with small body sizes, known as Pygmies, begin life at a typical size but grow slowly in early childhood, a new study shows. The results may cast doubt on long-held beliefs about how and why these groups developed shorter statures.

New evidence published Tuesday in the journal Nature Communications suggests that East and West African Pygmy children have different patterns of growth, a finding that may also shed light on how these groups evolved. Fernando Ramirez Rozzi, a National Geographic grantee at the French National Center for Scientific Research and lead author of this study, spent eight years tracking age, growth, and fertility in a Pygmy population in Cameroon called the Baka.

The name Pygmy describes rainforest hunter-gatherer populations around the globe that share heights of less than around five feet tall. This short stature is genetic, research has shown, not merely the result of malnutrition. Less clear, though, is the question of how diminutive body size evolved, and whether it did so independently in different African Pygmy groups.

Growth curves

“There are just so few studies of this kind on Pygmies,” says University of Pennsylvania professor Sarah Tishkoff. Tishkoff studies the Baka but was not involved in this work.

Nomadic hunter-gatherer groups such as the Baka are notoriously difficult to study. Rozzi described the village as completely in the rainforest, with a shifting population. “You go one time in the year and you find some people. You go again six months later and the people have moved—you have a new family there,” he said.

Another challenge, added George Perry, an anthropologist and geneticist at Pennsylvania State University, is that hunter-gatherers don’t always know how old they are. “If you don’t have that information, or if that information is prone to error, it’s very hard to actually get accurate growth curves,” said Luis Barreiro, an evolutionary biologist from the University of Montreal.

Rozzi’s group got accurate ages from a group of nuns at a nearby Catholic mission who recorded birth dates and weights since the late 1980s. Rozzi’s team combined the information with their own measurements of children and adults whose ages they knew to create a growth curve of Baka people from birth to age 25.

The growth curves revealed that Baka infants are born the same weight as French infants, but after three months Baka weights drop, and never catch up. This contradicts the reigning theory—at least for this population—that Pygmy people are short-statured because they lack a growth spurt during puberty. (“They demonstrate a growth spurt pretty convincingly,” Perry says.)

East African Pygmy populations have a different growth pattern: Infants are born small and stay small, according to published studies that Rozzi’s team used for comparison. The difference between the two groups could mean that Pygmies’ shorter statures did not start with their common ancestor, but instead evolved independently in response to similar environments.

Evolved twice?

Pygmy populations in Africa have a common ancestor thought to have split from typically-sized populations around 60,000 years ago, before splitting again into East African and West African groups around 20,000 years ago.

Several theories suggest that short stature is an adaptation to life in the tropical rainforest: small bodies regulate heat better, are more agile when moving through dense vegetation, need less food, and, according to one theory, can reproduce at younger ages.