In article at
http://www.sciencedaily.com/releases/2003/02/030207072417.htm Jeffrey M.
Friedman says:

"In modern times, obesity and leptin resistance appear to be the residue
of genetic variants that were more adaptive in our previous
environments," he says. "It may be that the obese carry the
'hunter-gatherer' genes and the lean carry the 'Western' genes."