[singular]

It was indeed an (uncited) quote from a wikipedia article which is hardly the same as concrete evidence :-)

Regardless, it is certainly the case that we have evolved to be able to sit comfortably and take weight off our legs - that, I think, is indisputable.

I would argue that the buttocks provide a structural base for the weight of the body which would otherwise be transmitted through the upper parts of the legs more so than they are in reality. That is something of a guess, however.