Short lives, short size – why are pygmies small?

This article is reposted from the old WordPress incarnation of Not Exactly Rocket Science.

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For decades, anthropologists have debated over why pygmies have evolved to be short. Amid theories about their jungle homes and lack of food, new research suggests that we have been looking at the problem from the wrong angle. The diminutive stature of pygmies is not a direct adaptation to their environment, but the side-effect of an evolutionary push to start having children earlier.

Andrea Migliano at the University of Cambridge suggests that pygmies have opted for a ‘live fast, die short’ strategy. Their short lives gives them very limited time as potential parents, and they have adapted by becoming sexually mature at a young age. That puts a brake on their pubescent growth spurts, leaving them with shorter adult heights.

Pygmies are technically defined as groups of people whose men are, on average, shorter than 155cm (or 5 feet and an inch for the Imperial-minded). Strictly speaking, the word is restricted to several ethnic groups of African hunter-gatherers, like the Aka, Efe and Mbuti. But the world is surprisingly replete with shorter-than-average groups who also bear the colloquial moniker of pygmies, including some from Brazil, Bolivia, South-East Asia and Papua New Guinea.

The earlier explanations for a short stature worked for some of these groups, but they could never account for all of them. Some scientists suggested that smaller people move more easily through dense jungles, but some pygmies live outside forests. Other theorised that they could maintain their body temperature more easily, but many live in cool and dry climes.

One of the more popular theories put forward by Jared Diamond suggested that small people are more resilient to starvation and malnourishment when food becomes scarce. But this can’t be the whole story for Africa groups like the Turkana and Massai manage to be some of the tallest people on Earth despite facing similarly unstable food supplies!

Migliano found more evidence against this theory by comparing the growth patterns of three groups of genuine pygmies – the Filipino Aeta and Agta, and the central African Biaka – with the shortest Americans, whose malnourished childhoods landed them in the bottom 0.01% of the population in terms of adult height.

Together with Lucio Vinicius and Marta Lahr, she found that the true pygmies grew slightly more slowly than the undernourished Americans, their growth spurts ended much earlier, at age 12 rather than 15. Typically, groups who lack free-flowing calories grow slowly over a long time – the pygmies’ pattern matched the first part but not the second. The pygmies’ growth curves disproved the malnutrition idea, but their lifespan pointed Migliano towards a better explanation.

Pygmies around the world are short in life expectancy as well as height, with the average adult dying at 16-24 years of age. Only 30-50% of children survive to the age of 15 and less than a third of women live to see menopause at 37. Taller African groups like the Ache or Turkana have lower adult mortality and twice the average lifespan, and compared to them, the pygmies’ pattern is closer to that of chimps.

Migliano argues that their early deaths are the driving force behind both their small size and their shorter growth spurts. It pays pygmies to divert resources away from growth and towards having children as early as possible, to compensate for their limited years. Indeed, Migliano found that they reach a peak of fertility earlier than taller groups.

In general, people who grow taller and larger tend to be more fertile and have larger and more capable offspring. That’s obviously advantageous but not if adult mortality is so low that you may not get a chance to have children at all. In this perilous situation, natural selection favours those who mature and reproduce early, to the cost of their growth.

Migliano’s theory has one important missing piece that needs to be filled in – why do many pygmies die early? It is here that the other earlier explanations for their short size may come in, including tropical diseases, thick jungle environments, hot climates and poor nutrition. None of these factors alone can account for pygmy evolution around the world, but Migliano speculates that one or more of them could lower the life expectancies of different populations.

If she’s right, it means that small body size could be an example of convergent evolution, where different groups of people in disparate parts of the globe independently evolved similar solutions to the shared problem of short and hazardous lives.

Reference: Migliano, A.B., Vinicius, L., Lahr, M.M. (2007). Life history trade-offs explain the evolution of human pygmies. Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.0708024105

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