The small body size associated with the pygmy phenotype could be selective adaptation for hunting and gathering in the rainforest.

Since all African pygmy phenotypes do not have these genetic characteristics researchers suggest they are newer than we thought, Penn State reported.

"I'm interested in how rainforest hunter-gatherers have adapted to their very challenging environments," said George H. Perry, assistant professor of anthropology and biology, Penn State. "Tropical rainforests are difficult for humans to live in. It is extremely hot and humid with limited food, especially when fruit is not in season."

A phenotype is "the outward expression of genetic makeup," Penn State reported. The pygmy phenotype is usually associated with rainforest hunter-gathers who do not farm. The researchers looked at the genetics of the Batwa rainforest hunter-gatherers of Uganda and compared them with farmers who lived nearby. These two groups often made change and have been known to intermarry. They also looked at Baka rainforest hunter-gatherers and their farming neighbors the Nzebi/Nzime.

The average height of Batwa men is men is five foot and women are around foot eight. This short stature is caused by a number of genetic changes throughout the genome. The team investigated 16 different genetic locations that were associated with height; several of the regions contained genes involved in human growth.

Genetic mutations are constantly occurring in populations, but if they have a negative effect on the individual they tend to quickly disappear; mutations that have positive impacts tend to spread throughout the population.

The results of the genetic comparison suggest a statistical difference between the two groups, suggesting multi-gene adaptation. The distribution of the genes suggests Pygmy phenotypes arose independently and may have showed up later than previously believed.

"What we think we see is that regions of the genome that are involved in the Batwa's Pygmy phenotype do not look the same in West Africa," said Perry. "If the Pygmy phenotype were really old, then we would have expected the locations to be similar."

Short stature may be adaptive for rainforest dwelling individuals for a number of reasons; smaller bodies require less food, generate less heat, and are also more agile.

The findings wer published Aug 18 in the Proceedings of the National Academies of Science.