The earth was “of one language” (Gen 11:1) because relatively few generations had passed since the sons of Noah disembarked. If the scattering of the tribes and languages of earth happened with the generation of Peleg, that was merely four generations after Shem. Moreover, the text makes clear that all humanity lived within close proximity of each other. In Peleg’s day “was the earth divided” (Gen 10:25); the people fear being “scattered abroad” (Gen 11:4); but God does so scatter them (Gen 11:8). Besides, the text makes it abundantly clear that all his names ancestors going back to Noah are still alive in the time of Peleg. So this is, essentially, a large tribe.