The issue with Native Americans is an interesting question and probably not something I am fully equipped to answer. However, to hazard a guess (help me out here, cormac) I'd have to say it's due more to their own genetic mixture. Native Americans journeying across the Bering Straight thousands of years ago were a different race from Europeans, having come from ancient Asian populations. And to this day most Asians have brownish-colored skin. The first Europeans, on the other hand, came from a more "Middle Eastern" ethnicity, and to this day many Middle Easterners can be rather light skinned.
But the idea of living in caves is no longer accepted, docy. Extensive acrhcaeological excavations in Europe have found sites of Paleolithic inhabitations largely in river valleys and other sheltered locations (including alongside large rock overhangs), but to this day there is no demonstrable evidence that Paleolithic peoples of Europe lived in caves.
Ever see the movie Nell how to two different languages can make up a whole new language.