Different evolving language families were kept distinct by the same geographical barriers that impeded trade and communication.
The first type of barriers were water ones. Although some water features could aid transportation and communications, others such as the Atlantic and Pacific oceans, tended to serve as barriers. The disappearance of the Bering Land Bridge at the end of the Pleistocene ice age ended land travel between Siberia and North America, causing their languages to diverge. Within North America, east-west trade was limited by mountain ranges. North and South America are separated by the narrow strip of Mesoamerica.
Within Africa, the Sahara Desert served as a major barrier to trade, as did areas of dense jungle. Australia was geographically isolated by water. The Indian subcontinent is separated from the rest of Asia by high mountain ranges. Mountains also separate Inner Asia from Southwest Asia.