Why do we count in tens?

Chris Smith asked mathematician Bobby Seagull, from Cambridge University, to drum up an answer to Tim's question on Facebook.  

Bobby - Good question. Demonstration: put your hands up, count how many fingers you have - digits. There we go. I’m hoping all of us have ten.

Chris - What about toes?

Bobby - Yeah. You could do. The reason we count in tens historically, is because we have ten fingers. But other cultures, other civilisations use different things: Aztecs use 20, the Babylonians have used 60. There’s some indigenous groups of people in South America who use 3 or 4. And again, if we happened to be based in the Simpson’s land, Homer has eight fingers so he could equally count in base 8. So really it’s just a quirk of our ten fingers being there.