The primitive notion of numbers, requiring thousands of years to be extracted from repeated concrete situations, appears to have evolved from the many physical contrasts prevelent in nature - the difference between one tree and a forest, one sheep and a herd. Likewise, it was noticed that objects within a group could be placed in one-to-one correspondence with objects from other groups; the hands can be matched (placed in one-to-one correspondence) with the feet. Consequently, there was a recognition of an abstract property shared by some groups: that ability to be placed in a one-to-one correspondence with each other. This property is what we now call number. Those groups which could not be placed in one-to-one correspondence did not share this property and are thus said to be different in number. Out of this realisation of "sameness" was born the concept of number in mathematics.
It is of great importance to recognize that the concept of number was born directly from observations of real physical phenomena exhibited in nature. The development of the concept of number was a consequence of observing both the likeness and contrasts found within nature: the likeness of groups that could be placed in one-to-one correspondence, the contrast of a single object against many objects of the same form. Indeed, the very notion of form and size was developed from observing the likeness and contrasts of objects. Our ability to percieve and interpret such likeness and contrast is what ultimitely led to the concept of number.
The development of language was essential to the rise of mathematical thinking, for language is the means of communicating thinking. Yet, words expressing numerical ideas were slow in arising. Number signs preceeded number words; it is easier to cut notches in a stick than it is to establish a universally acknowledged phrase to identify a number. Had language been more advanced then number systems other than the decimal (base 10) system may have made a greater headway.