> I haven't finished "Lights Out" yet, but the authors say that paleo humans
> only exercised strenuously when running for their lives, or when trying to
> kill something.  So they rarely experienced extreme exercise.

Hunting can be pretty energetic, even if you are not running after
animals. Try it some time.
>
> They feel that modern humans who exercise three or four times a week at the
> club doing strenuous activities are only harming themselves in the long term

Likely true. My college coach had us running up to 15 miles a day 7
days a week. We ranked in the top 20 nationally year after year, but
every year at the end of the season so many of the guys were broken
down with injuries and colds that we never even made it to nationals.
That guy was just stupid, IMHO. 3 days a week is my max now. Exercise
competes for energy with other systems of the body, such as healing/building.