"The short stature of Pygmy groups around the world has long intrigued anthropologists. It is generally accepted that their small body size is a result of genetic adaptation; however, which genes were selected, and the nature of the underlying selective force(s), remain unknown. The various hypotheses proposed include adaptations to food limitation, thermoregulation, mobility in the forest, and/or short lifespan. A recent study of the HGDP-CEPH populations identified a signal of selection in the insulin growth factor signalling pathway in Biaka Pygmies, which might be associated with short stature, but this signal was not shared with Mbuti Pygmies. By contrast, we found strong signals for selection in both African Pygmy groups at two genes involved in the iodide-dependent thyroid hormone pathway: TRIP4 in Mbuti Pygmies; and IYD in Biaka Pygmies (Fig. 7). Intriguingly, a previous study found a significantly lower frequency of goiter in Efe Pygmies (9.4%) than in Lese Bantu farmers (42.9%). The Efe and Lese live in close proximity to one another in the iodine-deficient Ituri Forest and share similar diets. Moreover, the frequency of goiter in Efe women living in Bantu villages was similar to that of Efe women living in the forest, and the frequency of goiter in offspring with an Efe mother and a Lese father was intermediate between that of Efe and Lese. These observations suggest that the Efe have adapted genetically to an iodine-deficient diet; we suggest that the signals of recent positive selection that we observe at TRIP4 in Mbuti Pygmies and IYD in Biaka Pygmies may reflect such genetic adaptations to an iodine-deficient diet. Furthermore, alterations in the thyroid hormone pathway can cause short stature. We therefore suggest that short stature in these Pygmy groups may have arisen as a consequence of genetic alterations in the thyroid hormone pathway. If this scenario is true, then there are two important implications. First, this would suggest that short stature was not selected for directly in the ancestors of Pygmy groups, but rather arose as an indirect consequence of selection in response to an iodine-deficient diet. Second, since different genes in the thyroid hormone pathway show signals of selection in Mbuti vs. Biaka Pygmies, this would suggest that short stature arose independently in the ancestors of Mbuti and Biaka Pygmies, and not in a common ancestral population. Moreover, most Pygmy-like groups around the world dwell in tropical forests, and hence are likely to have iodine-deficient diets. The possibility that independent adaptations to an iodine-deficient diet might therefore have contributed to the convergent evolution of the short stature phenotype in Pygmy-like groups around the world deserves further investigation."