When did user accounts using UIDs above 1000 become normal? And why?

We have a really old NIS domain which is mapping some user accounts to UIDs less than 1000. I'm wondering * when did using UIDs greater than or equal to 1000 for human users become 'standard', * why did it become a convention? Are there any practical implications or reasons for this? (i.e. anything similar like the 'convention' of trusting ports below 1000)