Indica comes from Asia and Sativa comes from South America but everything started out on the single continent of Pangea.
While Indica has been found in certain parts of Asia, I believe I read somewhere that Sativa grew in northern parts of Asia, but am unsure. In addition, the word Indica literally means "of Indian origin" which was first classified as a seperate sub-species of the cannabis plant sometime in the late 19th century. Before that, it was either referred to as hemp, cannabis or specifically cannabis sativa in the english language.
Sativa plants are believed to be native in Africa, and were probably introduced to South America by the ancestors of the Native Americans who probably brought cannabis with them when they crossed the Bering strait when there was still a land mass there.
Some believe cannabis sativa evolved from cannabis indica. Over the centuries we grew it in equatorial regions which would explain for its increased vigor and height as well as the stretchy quality of its leaves. More light, co2, humidity, and general soil quality in the equatorial and southern hemisphere might have slowly, through generations of farming, caused evolutionarily different traits to appear in cannabis until it developed into a whole new sub-species. This, though, is a belief and barely constitutes enough research to be risen to the level of even theory, though it is an entertaining thought.