In the case of an "acute trauma" type death, we modern people rely very heavily on modern medicine. This is where the bulk of our medical efforts should go, because you really can't just let that bone popping out of your arm sit there and heal itself. Same for infant illnesses.
In the case of a "chronic illness" type death, most modern medicines just try to coat the whole situation with prescription-strength pill bandages. We spend billions of dollars to cure symptoms, yet people live in a doped-up misery with these illnesses for decades sometimes. Sure, they haven't died from the specific disease, but they have all those pesky side effects, for which they take yet more pills. Just look at the name: Chronic (persisting for a long time or constantly recurring). The whole reason these things are called chronic diseases is because they manifest and then they linger. They build and build until you kick the bucket. If you're doing things wrong. We would have to check for the precursors (or just cursors) for these diseases in the remains of the people we know to be well past their 40s. (If we assume an average lifespan of 30 and that infant mortality is very high, then that means there's a good portion of people living past 40, too)
Mark Sisson recently wrote a fairly good summary of a study(pdf) that looked at several different cultures. The take-away, as he puts it:
"This data shows that human longevity is not a product of modern living. It shows that we have inherent proclivities toward long life, as long as we satisfy certain criteria – namely, the steady acquisition of food and shelter and the avoidance of infection, trauma, illness, and violent injury. The evolutionary lifestyle that eschews modern industrial processed food and promotes healthy levels of activity is the same one that supported our evolution into long-living Homo sapiens. Modern technology, sanitation, and medical advances are merely the cherries on top of an already solid framework."
There is also some research (I can't find it right this second) that looked at mummified Egyptians (neolithic diet, mind you) which showed signs of our classic auto-immune diseases, yet never showed signs of cancers. Perhaps we have several tiers of chronic illness resulting from many different layers of our modern lifestyle.
Also, why would we have an increase in auto-immune disorders and a drop in skeletal stature with the advent of agriculture? If we were just plugging along as a race, making steady medical advances throughout our transition from paleolithic to modern, shouldn't we see health increasing proportionally? We do to an extent, but I believe this is just our ability to solve acute problems -- overshadowing the presence of chronic problems. Largely, we have only gained the ability to solve the acute manifestations of chronic problems, so we survive just a little bit longer. That's backwards, but it makes people money, so we're stuck with it on an institutional level.
Comparing modern <30 year olds to our ancestors is also interesting. I've seen children with cancer. I've also seen TONS of <30y/os with auto-immune disorders. I had a friend with completely unexplainable gut problems. After a year, doctors still couldn't pin it down. They didn't even want to say it was IBS, their usual catch-all. Diabetes is creeping younger and younger and is certainly one of those illnesses that can "select" you out of the gene pool if you don't keep on top of the "acute" problems.
For me, there's so much going on with "lifespan" that I would much rather just look at the science. We know the biochemistry mechanisms of gluten and lectins. We can see how they interact with our gut lining. These things have been studied, and the conclusions don't look so tasty.
Ok. Off to buy some grassfed beef liver/meat and raw milk. Cheers!