That is exactly what I find so inconsistent about the Aryan Invasion Theory. If the Aryans did indeed immigrate from Europe to India, why did Aryans in India invent the Brahmi numerals (which eventually became the Hindu Numeral System, and when Arabs came in contact with Indian Mathematicians transported it to Europe) while Europe had dumbed down Roman numerals for centuries? Why was no Vedic Knowledge or Sanskrit left behind by these Steppe nomads? I don't recall even a single instance of any migratory group not leaving behind any trace of Knowledge of their culture/traditions in their native lands.