Seeing as how you brought up evolution; I doubt that many Neanderthals that chose a sedentary lifestyle made it out of their teens. Similarly; a debilitating injury as minor as a broken toe may have caused the same fate. At the time Homo Sapiens made the changed from nomadic hunter/gatherers to domesticating crops and animals, life expectancy dropped due to higher infection rates among settlements and lower dietary variety.
Thanks to advances in nutrition, public health, and lower infant mortality rates; the life expectancy in the last century has risen 64% from 47 years to 77 years. The bad news is that over the last 100 years the Industrial revolution, followed a larger technological revolution has changed us from a labour-intensive workforce to lazy chair-bound, car-bound slobs. The scariest numbers come from the U.S., where over the last 20 years adult obesity has doubled, and infant/adolescent obesity has tripled. Throw in smoking, AIDS/HIV, and cancer; I think we are on the cusp of a epidemic from our excesses.
Sorry I got off on a tangent there - where was I - Oh yeah - Get up and do something!

Legs,
OK - you asked - There are a dozen or so hypothesises as to why we evolved to a upright stance. All evidence points to the idea that we became bipedal before our brains expanded. According to Darwin, "Man could not have attained his present dominant position in the world without the use of his hands, which are so admirably adapted to the act of obedience of his will". It is commonly accepted that moving upright allows us to use our forearms to defend , attack, eat, wade, as well as situating our main sensory equipment (eyes, ears, nose) as high as possible. As we adapted to carry things, our spine became stronger but less flexible, to the point that only Humans and kangaroos are faster upright among mammals.
As for housework - I consider it exercise, because my significant other wont let me go out for my daily run without doing some, then makes me clean up my sweaty clothes when I get home.

My sister and I have always laughed at the various bizarre diet plans and exercise regimines that our contemporaries buy into.

We know the secret to long-lasting weight loss and it is simple: eat less, move around more.

Because there are doctors who are themselves healthy and actually care whether people get in shape?

I have been overweight, over-fat rather, at 43% bodyfat. I have looked for doctors' help and/or advise (not pills mind you) and always met people with dull skin, lifeless hair and un-clear eyes, some fat themselves, telling me to cut out gravies, cakes and alcohol. When I would tell them that I did not use any of the above other than once a week maximum, I was always met with a look of "right, another liar" and a shrug.

Result: I started reading nutrition books myself, and found a few internet sites which helped me help myself, because no one person would! To me, doctors ... barely good enough to write out certain prescriptions for ordinary things (birthcontrol pills, flu antibiotics, such things). Other than that, I stay clear of them all!

MPO