The only way that human migration can be traced before the advent of written language, is by the evidence of their existence which is dug out of the ground, ie bones, tools, artifacts etc
To my knowledge, this is incorrect. Genetics can do this to. How else does the Genographic Project from National Geographic work? All you do is send in a DNA sample and you receive a nice DVD with the full migration path of your ancestors ever since they left africa 10s of thousands of years ago.