The Straight Dope: Why do we nod our heads for "yes" and shake them for "no"?

"Dear Cecil:

Why do we nod our heads for 'yes' and shake them for 'no,' instead of the other way around? Are there any peoples who reverse the gestures? --Have to Know, Chicago

Cecil replies:

Believe it or not, H., some people think this is a silly question. Little do they know. No less a personage than Charles Darwin looked into it and wrote up his findings in a book called The Expression of the Emotions in Man and Animals (1872). Darwin was interested in finding out whether there were universal gestures and expressions, so he sent out a questionnaire to missionaries and whatnot that, among other things, asked what gesticulations the locals used to convey 'yes' and 'no.' Nodding and head-shaking turned out to be pretty common, but there were some striking exceptions. For example, certain Australian natives, when uttering a negative, 'don't shake the head, but holding up the right hand, shake it by turning it half round and back again two or three times.' One Captain Speedy--I can't say the name inspires much confidence--told Darwin that the Abyssinians said 'no' by jerking the head to the right shoulder and making a slight cluck, while 'yes' was expressed by the head being thrown backwards and the eyebrows raised for an instant. The Dyaks of Borneo supposedly raised their eyebrows for 'yes' and slightly contracted them, 'together with a peculiar look of the eyes,' for 'no.' Eskimoes nodded for 'yes' and winked for 'no.'" (more) The Straight Dope: Why do we nod our heads for "yes" and shake them for "no"?