Theorizing on evolutionary effects and diversity in human life span via PhysOrg.com: "researchers point out that one characteristic unique to but common among many pygmy populations is their short lifespan compared to other humans. With this in mind, the researchers suggest that pygmies represent the 'fast' extreme of life history strategies, with short stature being a side effect. ... when I went to the field, and started to interview them, I noticed the very high mortality rates – really high compared to any other population. So when we checked that different pygmy groups followed the same pattern, we thought that these facts should be linked. Also, life history theory has been used for a long time to understand body size diversity among mammals, and we thought it should also apply to the understanding of human diversity ... Because of their short life expectancies, the researchers speculate that pygmies have had to shift their reproductive years forward. The average life expectancy at birth for different pygmy populations ranges from just 16 years to 24 years." We humans are not immune from the historical environments that have shaped animal species - and we will extend our own healthy life spans, just as we are extending the healthy life spans of laboratory animals today.