Historians of science find the obvious explanation for the base-10 system at the fingertips of human beings, all ten of them. Not so coincidentally, the word digit comes from digital, the Latin word for finger or toe.
Counting on the fingers (base 10) or the fingers and toes (base 20) is the apparent root of many ancient written number systems.
The French language retains traces of a base-20 system, based on multiples of 20, like quatrevingt, or four 20s, for 80. The Maya also developed a base-20 system 1,500 years ago.
The Babylonian-Sumerian base-60 system survives in the measurement of time, angles, and geographical position; some scientists suspect this was also derived from finger counting.
But others think it might have arisen because of trade with some other people with a system of weights based on a different, larger unit, resulting in the need to find a common way to express fractional weights.