When humans started counting we kept it simple. We used our digits to keep a note of how many things we had. To begin with it was unlikely that anyone had more than ten of anything.
As people gathered more things they needed to count them and used something like the rock method. This worked like counting with fingers except that when you ran out of fingers you grabbed a rock and threw it onto a pile. Each rock represented ten things so that when you finished counting you could tell that you had used 3 rocks and 6 fingers.
Some quick arithmetic tells us that this is (3 x 10 fingers) + (6) or 36.
Next: Number Bases