Mid-term exam: Essay question

 Here is a historical graph of life expectancy at birth, for Homo sapiens on planet Earth.  The picture for the U.S. specifically is very similar, although the upturn started a bit earlier. (That mysterious dip around 1959 is the Chinese famine resulting from the so-called Great Leap Forward. In a graph of just the U.S., you would see a similar dip around 1918, from the influenza pandemic.) You can extend that horizontal tail back 6,000 years or more. In any given local area it might have gone up a bit in good times and down in times of plague or famine, but it basically stayed at around 30 years, never above 40, everywhere and always.So here ' s your question. What happened around 1900 to make that line start going up?
Source: Stayin' Alive - Category: American Health Source Type: blogs