Average life expectancy is a commonly misunderstood (and widely misrepresented) concept. We hear things like &quot;Humans really only lived into their 40s in the Stone Age,&quot; or what have you. This is untrue.<p><i>Average</i> (arithmetic mean) life expectancy was a lot lower because it was heavily skewed by infant mortality. Up until very recently, many more babies died within their first few years of life. We&#x27;ve dramatically improved our ability to produce and distribute food and medicine, which has had a profound effect on infant survivability. But we haven&#x27;t actually lengthened the average life expectancy of an adult who survives the first few years of childhood by all that much.<p>Point is, plenty of people have been living into their 70s, 80s, and 90s for as long as we&#x27;ve existed as a species. That said, the prevalance of diseases like Alzheimer&#x27;s might be a fairly modern aberration, perhaps owing to lifestyle, environmental, and dietary factors.