December 5, 2012
Perhaps no other human trait is as variable as human height. At 5’4″, I’d be dwarfed standing next to 6’3″ Kerri Walsh, the 2012 Olympic gold medalist in beach volleyball. But next to an African pygmy woman, I’d be a giant. The source of that variation is something that anthropologists have been trying to root out for decades. Diet, climate and environment are frequently linked to height differences across human populations.
More recently, researchers have implicated another factor: mortality rate. In a new study in the journal Current Anthropology, Andrea Bamberg Migliano and Myrtille Guillon, both of the University College London, make the case that people living in populations with low life expectancies don’t grow as tall as people living in groups with longer life spans. They also argue changes in mortality rates might account for the jump in body size from Australopithecus to Homo some 2 million years ago.
From an evolutionary standpoint, Migliano and Guillon note, it’s beneficial to start reproducing as soon as possible if you live in a society where individuals typically die young. That way you can have as many babies as possible in a short amount of time. Thus, you should stop growing relatively early in life and start devoting your energy to having children and taking care of them. Having a shorter developmental period means you can’t grow as tall, on average, as someone who has more time to mature. But getting big has reproductive benefits: Larger individuals tend to take in more energy and therefore can invest more energy in reproducing. So in societies with lower mortality rates, and longer adulthoods, it’s better to mature slowly and grow bigger and taller. Over time, populations experiencing different mortality rates will adapt to have shorter or longer developmental periods—and therefore be shorter or taller. (Of course, there is also variation within a population. But here, and throughout the post, I’m talking about population averages.)
To investigate this idea, Migliano and Guillon looked at previously collected height and mortality data from 89 small-scale populations from all over the world. These groups live in a variety of environments, including deserts, forests and savannas, and have different subsistence strategies, including hunter-gathering, pastoralism and agriculture. Using statistical analyses, the team wanted to see what kind of factors best explained the variation of heights in their data set.
In one analysis, three measures of survivorship—life expectancy at birth, life expectancy at age 15 and probability of survival to age 15—accounted for about 70 percent of height variance. The researchers also found evidence that people from societies with high mortality rates do indeed develop faster: Girls from groups that have low life expectancies start menstruating earlier than girls who are more likely to live longer. Environmental setting also influenced height, with people from savannas tending to be taller than people from forests. Diet, however, seemed to play a much smaller role, at least in the study samples.
Other variables not considered in the study may also contribute to height variation, the researchers point out. Temperature and humidity probably somehow factor in. For example, some work suggests shorter people generate less heat in hot, humid environments and therefore cool down more efficiently. That might explain why people living in tropical forests are shorter than those from savannas.
There are some situations, however, where the study’s findings don’t hold up. In modern Western societies, where mortality rates are low, growth is actually sped up because of an overabundance of food. Some studies now show that obesity may contribute to early puberty in girls. On the other hand, severe malnourishment can lead to delayed growth.
Based on the study’s findings, Migliano and Guillon suggest lower death rates probably contributed to changes in body size and height during the Australopithecus-Homo transition. In one study, anthropologists estimated early Homo species were about 30 percent bigger than australopithecines. Homo erectus grew even taller, within the range of variation of modern people. The larger brain of the genus Homo may have allowed the group to lower its mortality rate by outsmarting predators or foraging more efficiently than Australopithecus. Within H. erectus, differences in mortality rates between populations—which lived over a much larger geographic expanse than australopithecines—probably accounts for the variation of height seen in the fossil record of that species.
Much more investigation is needed to corroborate the link between death and height in the fossil record. But the work does highlight how even seemingly simple physical features have complex evolutionary histories.
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