Our ancestor didn't know how to count numbers, probably the first step was one and two including more than two, reflecting singular and plural.Next step was one, two, and many, reflecting the first, second and the third person. The notion of three appears when first person noticed there are someone other than I and you.The concept of number has developed as times go by. We don't usually count 700 billion, 701 billion, but we will get accustomed to count numbers in unit billion if current economic crisis continues.