Intra-species competition would also have been my first guess. Homo erectus seems to have been quite a successful species, given the archeological evidence. And

Intra-species competition would also have been my first guess. Homo erectus seems to have been quite a successful species, given the archeological evidence. And given the time frame of the migration, I'm betting on a slow but steady expansion as the families and tribes grew larger and split to explore new territories.