By DR. CHRISTOPHER TOKIN

Why do we walk on two legs instead of crawling around on all fours?  Why are sons so often taller than their fathers?  And why are pygmies so short?

It is human to ask these questions, and the answers can sometimes be found in the smallest places.

Researchers from the University of Pennsylvania announced on Thursday that they may have discovered why pygmies are short - and the answer lies in the footprints of natural selection in the human genome.

Pygmy tribes are found all over the world and represent the largest group of mobile hunter-gatherers.  Pygmies are unusual in that their average height is a meager 4 feet, 11 inches.  They grow up just like other humans until they become teenagers, at which point they typically fail to undergo a normal growth spurt. Their short stature mirrors their short lifespan, with average life expectancy a mere 17 years.

These tribes have captured the interest of social scientists and biological researchers who, for years, have tried to understand why pygmies diverged from the norm. Theories on their short stature have ranged from suggestions that it was a natural adaption to their difficult lifestyle, to the notion that the thick forest kept them away from sunlight, decreasing vitamin D and leading to low calcium levels and slow bone growth.

More recently, it has been suggested that, due to their short life-span, the bodies of pygmies have evolved to shunt energy originally devoted to growth, in favor of efforts towards early reproduction.

But in the new study, published in the journal PLoS Genetics, researchers for the first time were able to apply new genetic tools to address this question - and it looks like genes could hold the answer.

The researchers not only found genes linked to pygmy height, but they also found that those same genes are implicated in reproductive hormone activation and immune system function - providing some explanation to how the trait has survived for 2,800 years.

"The truth is we don't know where pygmies came from," said lead researcher Dr. Sarah Tishkoff, associate professor of genetics and biology and the University of Pennsylvania.  "By looking at a million genetic variants across the genome, we finally have a good understanding of their ancestry."

Tishkoff notes that pygmies' genomes are a veritable toolbox that allows them to take on their challenging existence. They found that a gene associated with height was also linked to oxytocin, the hormone responsible for nipple stimulation and breast feeding in women, linking it to the theory of early reproduction and species preservation in such short-lived people.

Another gene, linked to bacterial resistance and immune function, also happened to shut down the actions of human growth hormone in the pygmies' bodies.

"Everything is intricately linked," Tishkoff said.  "As evolution is tinkering with one of these systems, others are affected as well."

Tishkoff says she studies pygmy genes because she is interested in the science behind human adaption - the what, when, where and why of human origins.

And while these findings certainly help us understand why a pygmy is shorter than the average Joe, they also show us how humans have adapted to their environments over time - information that may help the rest of us adjust appropriately to our own futures.