And the average human lifespan was like 50 years, so maybe they were dying of contaminated waters.
Where we have good data, people lived to about the same age as today - or even longer.[1]
See, for example: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2672390/
[1] Obvious caveat that particularly awful periods of history in some locations had bad life expectancies - wars that killed a third of the population, the Black Plague, genocide, etc. The idea that human life expectancy has improved over time is a myth - unless you start counting in 1900. It's gone up and down throughout history, but ours is nothing special.