  One explanation is certainly the Chomskyan one that they are  consequences of speakers' innate knowledge of UG. Another is that  they derive from other, non-linguistically-specific features of  cognition, such as memory or processing constraints (e.g., Berwick  and Weinberg 1983 trace certain constraints on movement to  limitations on parsing imposed by the structure of human memory). Yet another is that they derive from universal demands of the  communication situation (e.g., Sapir 1921, argued that the  distinction between nouns and verbs arises from the fact that  language is used to communicate propositions, hence needs a way to  bring an object subject to mind and a way to say something about it). Finally, as Putnam 1971 speculated, universals might be relics of an ancestral Ur-language from which all other languages evolved. This  last hypothesis has generally been rejected as lacking in empirical  support. However, recent findings in genetics and historical  linguistics are converging to suggest that all human populations  evolved from a small group migrating from Africa in the fairly recent past, and that all human languages have probably evolved from the  language spoken by that group. (Cavalli-Sforza 1997.)