Life expectancy may have been that low, but overwhelmingly due to infant mortality. It's not like primitive humans were dying of old age in their 20s.

not in their 20s but definitely not in their 80s like today. if you reached 50s you were a boss. a lot of things that we consider routine to fix today would have ended your life (broken leg/hand, infections, severe diarrhea, etc). We really under-appreciate how many things we have as tools to expand our lifespan today.

From what I remember, it was more like if you made it past adolescence, making it to 50 was about normal. All kind of beside the point; you seemingly rejected the relevance of the primitive human diet based on their life expectancy. I don't think that follows.

Considering that soldiers in Ancient Greece could only retire after 30 years of service, I think it's fair to say there were plenty of 80 year old "primitive" humans.

I think we're talking prehistory, pre-agriculture here.

that what i also though. at least 10000 years ago if not more