It is believed by some scholars that early in the first millennium BC, several waves of Celts invaded Portugal from Central Europe and inter-married with the local populations, forming different tribes. Another theory suggests that Celts inhabited western Iberia / Portugal well before any large Celtic migrations from Central Europe. In addition, a number of linguists expert in ancient Celtic have presented compelling evidence that the Tartessian language, once spoken in parts of SW Spain and SW Portugal, is at least proto-Celtic in structure.