Is life expectancy that low in such communities? I know it was around 30 in medieval Europe, but it had worse climate, epidemics carried over across the continent, overpopulation, constant wars and kings/lords/church robbing people of a big part of their food.
That said, diet probably isn't as crucial to longevity as we'd like to think. Hunter/gatherer populations got (and get) a lot of exercise and lead relatively low-stress lives. For example, the Tsimane of Bolivia eat animals and starch and have practically no heart disease. They walk around a lot and are generally a happy bunch. Probably the only thing that would increase life expectancies more would be actual medical care, particularly regarding parasites.
Drop of infant mortality (particularly, through having enough food) is what the advent of agriculture achieved, so this is like saying "people died younger, except those who didn't".
What does modern agriculture have to do with infant mortality? It makes sense to exclude infants, because the reduction of those deaths are likely due to improvements in modern medicine and personal hygiene.
The thread above was not about modern agriculture; it was about the pre-historic time when agriculture was adopted and the availability of more food enabled more children to survive.
Infants eat breast milk, and if the mother does not have enough to eat, there is not enough milk. Sufficient and stable nutrition for mother is important to avoid child mortality.
In context of ancient world, nutrition of the family also had other mechanisms that impact the survival of children (strength to build housing, or strength to defend against hostile people, etc).
I still did not write about modern medicine; I don't know why you bring that up. This was about the transition from hunter-gatherers to farming, first in Mesopotamia and a few other places, about 10 000 years ago or so.
That was the "advent of agriculture", also known as "Neolithic revolution".
However, overall I'd say that starvation vs. having sufficient food impacts mortality rates more than modern medicine. You can't eat the best of hygiene and medication doesn't nourish you.
And that is also incorrect in assuming a strict hunter-gatherer mode of production. Husbandry and some agriculture were part of the late hunter-gatherer people's life, as seen in Scandinavia for example, and this does not necessarily mean that this was a mere transition to full-fledged farming either. The debate is still wide open.
Keep in mind that the average lifespans were so low only because of high childhood mortality. If someone made it past the age of six or so, they had a good chance of making it in to their 60's, even millennia ago.
