What do you mean, our ancestors weren’t fat?

I’ve read a few times, around the internet, people who have asserted that “our ancestors weren’t fat”, and therefore, obesity cannot be hereditary.  I’ve been wanting to examine the evidence of this for a while.
The movie Fiddler on the Roof,released in 1971, was set in Czarist Russia 1905. One of the lines in the song [...]