when , in the 1920s and 30s , the cities of muhenjo-daro and harappa were unearthed and it soon became clear that these sophisticated urban centers dated from well before ( at least a thousand years before ) the supposed 1500 bc date imagined for the aryan invasion , a revised theory proposed that the aryans had overrun a river valley civilization which had flourished for at least a thousand years before their arrival ; a civilization more advanced but weaker in technology and military prowess .