Humans have always lived past their 40s. You've misunderstand life expectancy statistics, because human age-at-death forms a bimodal distribution: A lot of kids and infants die (especially historically), and then your body starts falling apart in your 50s. So while the average life expectancy at birth for a lot of history might have been in the 30-40 year span, it has never been common for people to die in their 30s. Because once you make it to ~15, your life expectancy is another 30-40 years. And if you make it to 30, your life expectancy was still another 25-30 years.