We probably use base ten because we have ten fingers and started counting with our fingers. Got to doubt that aliens are going to have ten fingers or even tententacles.
Yup. It fits.
Let's restrict ourselves to words for positive integers, since other kinds of numbers are formed from them, and since zero is the nothing number.
Many low-tech people only have words for low numbers:
- 1, many
- 1, 2, many
- 1, 2, 3, many
- 1, 2, 3, 4, many
This is from not having much that they want to count.
Going further means having to have a lot of individual words or else constructing numbers from other ones, and when constructing them, one uses some number base or sequence of number bases.
The simplest one is unary, base 1. Repeating 1:
1, 1+1, 1+1+1, 1+1+1+1, ...
That quickly becomes impractical, so everybody uses higher bases to continue. However, it is used in Giuseppe Peano's axiomatic formulation of the nonnegative integers.
The next one is binary, base 2. I don't know of any natural language that has binary number words, but there are some in Australia, New Guinea, and South America that have a sort of unary-binary system:
1, 2, 2+1, 2+2, 2+2+1, 2+2+2, ...
Next, quaternary, base 4. Speakers of languages with this base in New Guinea and nearby islands often use "dog" for "4" because a dog has four legs.
Next, quinary, base 5. That's obviously derived from the fingers of one hand.
Next, senary, base 6. That may be derived from the fingers and palm of one hand.
Next, octal, base 8. That may be derived from the spaces between the fingers of both hands.
Next, decimal, base 10. That's obviously derived from the fingers of both hands. That's *very* widespread. It is sometimes implemented with a biquinary mixed-base system: 2, 5 -- (0,1) then (0,1,2,3,4).
Next, duodecimal, base 12. The fingers and palms of both hands? That is common in West Africa.
Next, vigesimal, base 20. The fingers of both hands and the toes of both feet? It is sometimes implemented with a quadquinary mixed-base system: 4, 5 -- (0,1,2,3) then (0,1,2,3,4).
Next, quadrovigesimal, base 24. One language has mixed-base system 4, 6 -- (0,1,2,3) then (0,1,2,3,4,5)
Next, duotrigesimal, base 32. Very rare.
Next, sexagesimal, base 60. Sumerian had mixed-base system 6, 10 -- that's the ultimate origin of base-60 time units.
Next, octogesimal, base 80. One language has mixed-base system 4, 2, 2, 5
The larger bases are often implemented in mixed-base form.