It's not just early death. The CDC released a report a few years ago that found that the mildly obese suffer fewer health problems than thin people. This is because people who are obese are generally so not because of poor habits, but because of genetic and epigenetic predisposition. They can see that they are fat, they get chided about being fat, and suffer other social consequences for being fat, and therefore trying to be healthy, whether it's eating nutritiously or exercising, is at the top of their priorities. And they pursue it vigorously, even if it's ineffective toward changing their appearance. The thin people, however, have no such motivation -- they can eat all the chips and ice cream that they want, and veg out on the couch all day, and not suffer any of the obvious physical signals of poor health. It's like not feeling any pain -- you could break your leg and walk around on it, not realizing that you're making it much, much worse. Therefore, it's the thin people who suffer strokes and heart attacks because they didn't realize their cardiovascular health was so poor until it's too late. Mildly obese people are often much healthier than their thinner counterparts throughout most of their lives, but catch up and surpass them however, because being active is harder when you're fat, especially as you age. Eventually, it's too hard to keep trying, and so they give in to just relaxing, enjoying life, living with the consequences, and awaiting death. In the end, anyone who abused their health -- whether fat or thin -- dies first, and it's the thin people who bothered to eat healthy and exercise, or have the genetic predisposition for longevity, that live the longest -- which is a very small group.

As far as evolution is concerned, there's little to select for longevity. Few people will die before about 50 from natural causes, statistically speaking, no matter their health habits. Once you have your children and raise them, however, you are expendable. After that, the body doesn't put up with abuse. The next generation already exists and is well on their way to creating yet another generation, and so you are obsolete. If you want to live long after that, you have to treat your body like a machine, like a classic car, keeping all of the parts meticulously clean and in good working order, keeping all of the fluids clean and topped off, driving it often enough to keep the engine from gunking up, but not driving it too hard either. Ignore basic maintenance and you'll drive it straight into the ground.