"From a genetic standpoint, humans living today are Stone Age hunter-gatherers displaced through time to a world that differs from that for which our genetic constitution was selected." Source: "Stone Agers In The Fast Lane: Chronic Degenerative Diseases In Evolutionary Perspective [PDF]"
This interesting paper argues that the rise of civilization, especially modern industrial civilization, has had a devastating impact on our health and general fitness. Physiologically, we are Stone Age hunter-gatherers, but we're locked into a high-stress alien environment which is literally killing us:
"Although our genes have hardly changed, our culture has been transformed almost beyond recognition during the past 10,000 years, especially since the Industrial Revolution. There is increasing evidence that the resulting mismatch fosters 'diseases of civilization' that together cause 75 percent of all deaths in Western nations, but that are rare among persons whose lifeways reflect those of our preagricultural ancestors."
This isn't a new argument, but in some ways it makes a lot of sense. After all, the Stone Age lasted for something like three million years, so humans have spent most of their time as hunter-gatherers living in an underpopulated, wilderness environment. We're not adapted to living in cities, for one thing. The difference between the Paleolithic world and the modern world is so dramatic it's almost like we've been transported to a completely different planet.
"It is generally thought that life expectancy in the past was less that it is today for our species as a whole and in the case of industrialized countries in particular. However, this belief counts as a falsehood not because it is untrue (it is, in fact, true) but because many people get this idea wrong in a few different ways." Source: "Falsehood: 'If This Was The Stone Age, I'd Be Dead By Now'", Science Blogs.
The common assumption that people routinely died at young ages back in the Stone Age, not to mention the classical and medieval periods, isn't really accurate. High rates of infant mortality and deaths due to disease, wars, hunting accidents, etc., skew the statistics and make it seem like people in the ancient world had biologically shorter lifespans. In reality they weren't that much different from us physiologically, though they were probably stronger with more endurance and didn't have the high levels of mental disorders common to the modern world.
As usual, your lifespan depends on your particular circumstances regardless of what period you're living in. Life expectancy has increased over the last few centuries, but your quality of life declines so much in your eighties and nineties I'm not sure living longer is that much of an advantage. One way or another, though, it's difficult to see our decaying western "civilization" as natural and healthy, either physically or psychologically.
Perhaps the fact that we're vulnerable to "diseases of civilization" means that civilization itself, in many ways, is actually a kind of disease.