Comment 16 by rod-the-farmer :

I would be interested to know if Richard has "done the math" on that chain of humanity back to a common ancestor. How many generations would that be ? And if translated into individuals holding hands, how many kilometres would it extend ? (Just something to give a visual image to the fence-sitters.)

Check out the article I linked to above: Gaps in the Mind.

Last year I wrote an article using the same idea for the local student magazine I edit, in which I traced the line of ancestors, at one metre per generation, through the streets of our own town. (Vividly conveys the idea if you happen to know the town, useless if you don't; hence, something you can only really do in a local publication.) Though I cheated on the calculations as I extended it, simply assuming four metres per century throughout time, rather than recalculating body size and generation time as if the ancestors really were holding hands.