Pygmies, the most well-known group of diminutive humans, whose men on average grow to a maximum of five feet tall and their women about a half foot shorter, were thought to be endowed with their characteristic small body sizes due to poor nutrition and environmental conditions.

But the theories did not hold up, given that these populations—primarily hunter–gatherers—are found mostly in Africa but also in Southeast Asia and central South America, and thereby are exposed to varying climates and diets. Further, other populations who live under conditions of low sustenance, such as Kenya's Masai tribes, are among the world's tallest people.

So what could account for these pockets of people who grow so small?

According to University of Cambridge researchers, the key is the pygmies' life expectancy. "After going to the Philippines and interviewing the pygmies, I noticed this very distinctive feature of the population: very high mortality rates," says Andrea Migliano, a research fellow at Cambridge's Leverhulme Center for Human Evolutionary Studies and co-author of a new study published in Proceedings of the National Academy of Sciences USA. "Then, going back to life history theory, we noticed that their small body size was really linked to high mortality."

Migliano and her colleagues began their study by comparing the growth rates of two Filipino pygmy groups (the Aeta and the Batak) with data from African pygmies as well as from East African pastoralist (livestock-raising) tribes like the Masai and the lower echelon of the U.S. growth distribution (in essence, malnourished Americans). All these groups have low nutritional status but reach significantly different average height levels. The U.S. population showed the greatest growth rate, whereas both the pygmies and African pastoralists lagged behind. Although the pygmies plateaued around 13 years of age, the pastoralists kept growing, reaching their cessation point into their early twenties. Because the pygmy growth rate approximated the taller pastoralists, but had an earlier end point, the researchers concluded that their growth was not nutritionally stunted.

The group next examined the incredibly low life expectancy of different pygmy populations, ranging from roughly 16 to 24 years of age. (Pastoralists and other hunter–gatherer populations experience expectancies that are nearly one to two decades longer—a number that is still low, especially when compared with the 75- to 80-year life span expected of Americans.) Pygmies also reach their age of last reproduction a few years earlier than their taller counterparts, although there are many more pastoralist women than pygmies who reach this age at all.

Looking at fertility curves, the researchers noted Aeta appeared to reproduce on average when they were around 15 or 16 years old, which is about three years earlier than other hunter–gatherers. The tallest of these populations actually appeared to reproduce the latest. By having an early onset of reproductive abilities, the scientists say, the pygmies appear to trade off time spent growing, allowing them to continue on in the face of low life expectancy.

"Although the challenges posed by thermoregulation, locomotion in dense forests, exposure to tropical diseases, and poor nutrition do not account for the characteristics of all pygmy populations," the authors wrote, "they may jointly or partially contribute to the similarly high mortality rates in unrelated pygmy populations."

This research centered around women, but Migliano expects an analysis of males to mirror that of females, partly because the fertility of one would affect the other. Further, life history theory is anchored to the female because of the importance of reproduction as a variable. She adds that this paradigm could be used to help better understand the evolution of Homo floresiensis, the so-called "hobbit" found on the Indonesian island Flores in 2003.

"I think there is a great potential to use the theory to understand changes in body size during hominid evolution, such as the size of the hobbits and the relatively larger size of erectus," Migliano says. "But my main objective is to apply the theory to the understanding of the current human diversity."