show ten fingers, close both fists, show nine fingers. (eleven plus nine)
Anyway...there are other alternatives if you're going to attribute the origin of our numeric system to the number of fingers we have. A base 6 system would be really likely as well. One hand could represent the most significant digit, and the other could represent the least significant digit. Then you could easily count to 35 with your fingers before you finger overflow.
Or you could do a binary finger count where every finger represents a different significant digit. Then you're at 1023 before you finger overflow. This is more unlikely given the large number and the unintuitive idea that each finger represents exponentially more value than the previous one. Plus you end up with some really awkward finger positions to represent certain numbers.
But I'm just not buying the idea that base ten is most likely because we have ten fingers. I'd say the base 6 system is best for our fingers. Overflow in one hand just naturally increments the finger count in the other hand. And as with the base eleven or two systems, each finger always represents the same written glyph.