agreed, and even more sensible would be to just think in regions. Bosporus, Black Sea and the Mediterranean are natural boundaries for the European region, the Himalayas are the natural boundaries between East/Central and South Asia, the Saharan / Arabian / Afghan and Kazakh deserts and steppes are natural boundaries of the Middle East / North African region. Whenever a civilization has tried to cross over from one of those regions into the other, big world events ensued.