Pygmies show growth plasticity is key to human evolution

While the stature of pygmies is well-suited to tropical rainforests, the mechanisms underlying their growth remain poorly understood. In order to decipher these mechanisms, a team of scientists from the CNRS, IRD and UPMC1 studied a group of Baka pygmies in Cameroon. Their findings revealed that their growth rate differed completely from that of another pygmy cluster, despite a similar adult height, which implies that small stature appeared independently in the two clusters. This work is published on 28 July 2015 in Nature Communications.

Baka in southeast Cameroon [Credit: WWF]

The stature of pygmies has intrigued Westerners since their first encounter with them in 1865. This population is in fact made up of several ethnic groups, which belong to two main clusters. One is spread across Equatorial West Africa (Congo, Cameroon, Gabon, Central African Republic, Western DRC) while the other is found in East Africa, in Eastern DRC and Rwanda. They all live in forested regions, linked with Bantu farmers.

Although genetic factors are responsible for the small stature of pygmies, until now scientists were unable to produce reliable data on their age in order to analyze their growth patterns. Thanks to the registers of the Catholic mission in Moange-le-Bosquet, Cameroon, it was possible to study 500 members of the Baka ethnic group for eight years in order to establish the first growth curves for pygmies.

The scientists were thus able to show that although the body size at birth of the Baka was within normal limits, their growth rate then slowed significantly until the age of three years. Their growth curves subsequently paralleled global standards, with a growth spurt at adolescence and an adult size achieved on average at the same time as that seen throughout the world. However, they never made up for this initial retardation. On the other hand, pygmies in the eastern cluster were born with a smaller body size, so that their small stature resulted from growth processes different from those of the Baka.

Measuring a Baka child of known age. Establishing growth curves, 
which provide a means to evaluate and monitor individual health status, 
requires longitudinal studies over several years in people of known age 
[Credit: © Fernando Victor Ramirez Rozzi]

The pygmy morphology of these populations thus results from two different mechanisms, which may be linked to an imbalance between the growth hormone and the two IGF that has allowed them to adapt to rainforest conditions, under a mechanism of convergent evolution.

These pygmy clusters thus split apart between 8,000 and 13,000 years ago, which shows that human growth can evolve within a relatively short period of time. This growth plasticity may have played a determining role in the spread of Homo sapiens outside Africa, allowing this species to adapt rapidly to new environments.

These findings also highlight the fact that further longitudinal studies (i.e. the monitoring of individuals over time) are needed to enhance the research in genetics and endocrinology that is necessary to shed light on the growth mechanisms in play amongst pygmies, as well as in the rest of the world's population, in whom they are also poorly understood. The scientists now wish to determine the endocrine processes that cause the growth deceleration observed during infancy in the Baka, by identifying the hormones and cellular structures that are responsible for this particular growth pattern, targeting the underlying genes and comparing them with those found in East African pygmies.

Source: CNRS [July 28, 2015]