> I would say molesting children for fun, being absolutely morally wrong, is something we can't deny. But why should we care what each other thinks if morality is arbitrary?

You were just told why. Morality is not arbitrary, but only because social interaction gives it meaning.

As for your requests for hard evidence on the math matter, the earliest that exists is probably http://aleph0.clarku.edu/~djoyce/mathhist/plimpnote.html - the problem is that the development of arithmatic precedes that of writing, thus there can be no record of it. (Actually, the development of writing was very much driven by the need to do complex calculations.)