I'm no biologist, but it's pretty easy to imagine a plausible mechanism.

Stress doesn't cause your body to "immediately grenade itself", it causes gradual problems over an extended period. But you can imagine that evolutionarily speaking, a stressful situation would frequently be a situation involving imminent danger. A response that prioritizes short-term survival (e.g. releasing adrenaline and cortisol) regardless of the long-term consequences could easily improve evolutionary fitness.

I suppose the stress was running away from a lion, not having the all-eating thought of meeting deadlines, a bad boss, backstabbing coworkers and so on, i.e. intense, but short, not persistent and chronic.

There always has been a lot of stress, but there used to be relaxed moments as well. The issue is that we don't calm down anymore.

Relevant video by Kurzgesagt – In a Nutshell: https://www.youtube.com/watch?v=Mo1A45ShcMo

Considering that aging only happens later in life, it wouldn't have much of an effect on how efficient humans produce offspring, so it wouldn't be selected against. It is the same explanation for why Huntington's and Alzheimer's are still around.

The idea that pre-agricultural societies are subjected to constant stress reflects modern biases somewhat. I remember years ago I saw a talk from Spencer Wells where he described how pre-agricultural life might have been like in Anatolia. Fields and fields of wild wheat stretching across the horizon. Long periods of seasonal abundance. Occasional periods of hunger and privation where families would often have to rely on the help of friends and acquantainces, with the understanding that one day the debt would be repaid if similar ill circumstances befell your neighbors.

You can see some of how life might have been like in modern Papua New Guinea. War is nothing like modern warfare with artillery and guns. Its two groups of people firing arrows at each other from stand off distances while shouting at each other with minimal casualties. If you look at casualty rates from some of the early Greek warfare during the hoplite era, it's also surprisingly low.

Also note that in this post, and from research, caloric restriction counteracts some of these inflammatory effects. In many ways modern life can be stressful in its own ways, especially as we remove ourselves more and more from the active lifestyles that necessitated survival in previous eras.

You only need to get to reproduction once, regularly, for a gene to keep getting passed on.

Other mitigating factors: not eating much of the ultra-processed garbage we have today, regular high-intensity exercise, and a vastly-more interconnected community. Multigenerational homes make it easier to dump the kids off with grandma and go for a long hike with the wife or a party with the village or whatever. Etc. etc. etc.

Were safer and healthier, but I don’t think we’re necessarily happier.

> Wouldn't we have evolved in an environment with constant stress?

Well, humans did not reach an old age if you look back to history. Only in recent times, say, 2000 years or so, or, if you want to be strict, the last 100 years, did humans reach very old ages. Back in older days, 30 years was considered old. Then 40 or so.