First, the people portrayed in the number of notches on wood or stone. Time passed, and our ancestors learned to build houses, ships and many other things, why the need for complex calculations. But without numbers it is impossible to do.
The first number appeared simultaneously with the letters. The ancients did not know of zeros. For example, the Greeks wrote down the digits with letters: a - 1, B - 2, G - 3...
In Ancient Rome digits knew, but they were not very comfortable. One, two and three depicted sticks: III, five - this icon: V. And if you had to write four, first wrote the unit, followed by the five. See Roman this icon will produce a simple arithmetical operation (5 subtract 1) and smcnet that this Quartet - IV. Want to write a 6 - first writes five, and then one. This icon is obtained: VI. Seven - five (V) and two ones (II); (VII). So the eight, only instead of 2 of ones writing 3.
But in order to write nine, it was not necessary to add and subtract. 10 (and the Romans a dozen portrayed like that - X) took a one that is initially portrayed unit, and then a dozen IX.
Whence came the word "digit"?