Indo-European refers to a broad smear of languages and peoples- Middle-eastern, Persian, other parts of central Asia as well as India and later, Europe. Because of proximity and relationships in trade as well as language similarities, developments in one part of the area could spread to another- hence something developed in Persia would show up in the Indian subcontinent and work its way west as well. Later, something from Moorish Spain could work its way north and west.
Because of the emphasis on Caesar, Marco Polo and others who publicized or politicized their trips, we have the idea of insular, settled peoples who normally did not have contact with those in other areas. Not true. Migration and trade affected all peoples.

Even in North America, trade was long ranging before the Europeans; turquoise from New Mexico went to the Valley of Mexico, feathers from Mexican birds were used in the American west, shells from the Gulf of California were peddled all over western North America, obsidian and other goods from Oregon traveled with trader Indians to Texas, etc.