Pygmies are a branch of Homo Sapiens.
Theories to their small stature are that:
Some studies suggest that it could be related to adaptation to low ultraviolet light levels in rainforests.
This might mean that relatively little vitamin D can be made in human skin, thereby limiting calcium uptake from the diet for bone growth and maintenance, and leading to the evolution of the small skeletal size.
Other evidence points towards unusually low levels of expression of the genes encoding the growth hormone receptor and growth hormone compared to the related tribal groups, associated with low serum levels of
insulin-like growth factor-1 and short stature.
New evidence suggests East and West African Pygmy children have different growth patterns. The difference between the two groups may indicate the Pygmies’ short stature did not start with their common ancestor, but instead evolved independently in adapting to similar environments, which adds support that some sets of genes related to height were advantageous in Eastern Pygmy populations, but not in Western Pygmy populations.