There is a huge difference between life expectancy and life span and this article [1] covers it pretty well.

Life expectancy is skewed by infant mortality rates:

> Our conclusion is that there is a characteristic life span for our species, in which mortality decreases sharply from infancy through childhood, followed by a period in which mortality rates remain essentially constant to about age 40 years, after which mortality rises steadily in Gompertz fashion. The modal age of adult death is about seven decades, before which time humans remain vigorous producers, and after which senescence rapidly occurs and people die.

[1] https://condensedscience.wordpress.com/2011/06/28/life-expec...

PDF: http://www.anth.ucsb.edu/faculty/gurven/papers/GurvenKaplan2...