People didn’t live as they do today for 99% of the time since civilization started.
That should have been “live as long as they do today”.
It is entirely possible that civilization in its earliest incarnations may have on occasion actually supressed average lifespans. The rise of agriculture allowed for massive population increases in certain favorable areas, but also probably led to more unbalanced diets. A monotonous peasant diet of boiled or baked grains, with minimal supplemental vegetables or protein certainly seems to have been rough on the ancient Egyptians. It scarcely affected the civilization as a whole as everyone reproduced in their teens and twenties. But life was neither particularly happy nor long for the poor masses.
This is a good question in general and one that know one has all the answers to. I am typing this looking at my very cute and sweet chinchilla. He is a rodent fairly closely related to squirrels, rats and all the rest but chinchillas are another extreme exception to the general lifespan rule. Chinchillas live longer than most dogs and cats do with many making it to 20+ years with sustained care. I have never heard a good explanation why they can live literally many times longer than their close relatives. It is also very hard to tell how old a chinchilla is until they are completely geriatric. It is possible for some species to slow down the aging process for whatever reason and they have done it.
No, it isn’t. Life span was mostly short due to simple misfortune. Infectious diseases, injuries, bad years for hunting or gathering. Someone most likely lucked out there and lived to be a hundred even back then.
Life span 300 years ago was short for the same reasons, with malnutrition and constant hard work adding to the burden for a poor peasant, and you still have the occasional peasant living past 100.
Just of note is that chinchillas, compared to their close relatives, havemuch lower heart rates, 100 to 150 bbm compared to say 300 to 500 for rats and so on. That correlates with a much lower metabolic rate than would be expected for their size.
Heart rate is no absolute law but heart rate does correlate with accumulated shear stress on the vessels and valves and is a good marker of metabolic rate. Routinely run the machine faster and yes it wears faster. In geneal.
Again, that’s life expectancy, not life span.