Picture this scenario: all of humanity in its beginning is on a single island roughly the right size to support 1/2/3 million people. For some reason (sea monsters, too much ocean, whatever reason you want) it is impossible to get to another land and colonize that. For another reason of your choice, once the population reaches about 1,500,000 infertility problems are widespread enough that the population doesn't grow, remains stable.
Would linguistic variation/evolution happen? IF so, how much?