Human populations living in the rainforests of Bolivia and Malaysia appear to have evolved to be short in order to navigate the dense vegetation, scientists have discovered.
People with the human pygmy phenotype are found across the globe, normally in tropical regions. Adult males are normally less than 5 feet 2 inches in height. Recent research has shown their short stature is the result of convergent evolution—where different populations evolve the same trait because of the similar environment in which they live. However, it is not entirely clear what the adaptive benefit of being so short is.
In one of the first studies to look the human pygmy phenotype for its benefits, researchers examined two short-statured populations who forage in the rainforests. These were the Batek of Peninsular Malaysia and the Tsimane of the Bolivian Amazon. Their findings are published in the Proceedings of the Royal Society B: Biological Sciences.
Vivek Venkataraman, from Harvard’s Department of Human Evolutionary Biology, and colleagues tested out the hypothesis that specific locomotor constraints—walking ability dependent on step length—would have an impact on evolutionary success in a rainforest terrain. They used theoretical models and field experiments to work out what would be better when foraging in a rainforest.
“The rainforest is one of the most challenging environments on Earth for humans,” Venkataraman told Newsweek. “Rainforests are hot, humid, and structurally dense environments with little food (for humans at least), and lots of pathogens that cause disease. Something about these factors evidently makes it advantageous to evolve short stature.”
She said that it is difficult to study the adaptive benefits because the rainforest is so difficult to work in and because humans live for so long—gathering the data needed to show evolutionary changes would probably involve a long wait.
Instead, the team focused in on the intuitive hypothesis that being short must help people navigate the terrain. “It is highly intuitive—anyone who has trekked off-trail in a rainforest knows how difficult it is,” Venkataraman said. “Especially if you are tall, you feel extremely clumsy. But for people who live in these environments, it seems easy for them to move around. This advantage seems pretty clearly related to how short they are.”
Research showed being shorter—therfore having a shorter stride—helped people forage in the rainforest in both Malaysia and Bolivia, indicating the human pygmy phenotype has an adaptive purpose.
Venkataraman says this is just the first step in understanding the benefits of being shorter in a rainforest environment: “The idea for this study came from many hours of walking in the rainforest with these populations and feeling rather incompetent. There are many more avenues to investigate regarding human physiology in rainforests. For example, does small stature enable one to dissipate heat more effectively in a hot, humid, and windless rainforest? It should, on a theoretical basis, but the idea hasn’t been explored yet.”
Christina Bergey, from the Department of Anthropology at Penn State University, who was not involved in the research, said the findings turn what was a speculative idea into a testable hypothesis. “They zero in on step length as being key,” she told Newsweek. “It's well known that taller humans have a higher preferred walking speed, which they achieve by taking longer steps. In dense rainforests with tons of obstacles to avoid, the longer stride may be impossible, giving people with shorter legs an advantage.”
She said the paper provides a mechanism linking body size and fitness, while also combining two hypothesis about the evolution of a small body size in rainforests—limited food and locomotion efficiency.
“I'll be excited to see the work they do in the future on other aspects of gait. I think the evolution of small body size in humans living in rainforests is one of the most striking adaptations in all of human evolution, and it will be interesting to see if this pattern holds true in other species or other human populations that live in habitats with rugged terrain.”