It is her best-known work, pioneering the use of linguistic typology as a tool for understanding human migrations in prehistory.
For example, head marking is more common in the residual zones, which Nichols suggests is a result of long-term language contact.
At the broadest level, Nichols divides the world of languages into three large regions: The Old World is geographically largest, but has the least typological diversity and lowest density of language families, suggesting that repeated spreads from its center have eliminated much diversity which previously existed, especially at the edges of the Afro-Eurasia supercontinent.
One interpretation is that these patterns resulted from chance; another is that the New World was colonized from a Pacific region which was formerly larger and included unknown archaic languages of coastal East Asia.
Based on the latter interpretation, Nichols suggests a relatively early date (pre-Clovis) for the initial peopling of the Americas.