In 1900, the average life expectancy in the United States was forty-eight years. By 2000, just one hundred years later, it was seventy-seven. For every three days we lived in the twentieth century we gained a day of life expectancy. Consider that in light of the fact that early humans in the form of Homo erectus emerged 2.4 million years ago, and it took us until 1900 to achieve our forty-eight-year life expectancy. That means it took 80,000 generations to reach the 1900-era life expectancy, and only about 4 to reach our current level. With clean water, sewer systems, safer food, pasteurized
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