I look at the Book of Ether as reconciling the DNA issues of our day - once we move past the former assumptions of the early saints (including the leaders). The record states quite clearly that the Nephites were religious separatists by nature, so it's perfectly reasonable to assume that the Lamanites outnumbered them so heavily (despite disproportionate death in warfare) because the Lamanites assimilated some of the remnants of the Jaredite nation that didn't die in the great battle that destroyed the nation itself. That assimilation could even have been of a more numerous people (like the Nephites and the Mulekites), if the Jaredites lacked social cohesion and wanted social stability they had lost. In fact, I think that is the most logical conclusion, since even AFTER the Nephites assimilated the Mulekites the Lamanites STILL outnumbered them substantially.
There's no reason to demand that they all were wiped out in that battle (Coriantumr's claim notwithstanding) - that the Jaredites were any different than the Nephites (or any other hyperbolic historical description of total annihilation) in that regard. Many could have fled and refused to gather, joining a separate civilization instead. The Nephites certainly wouldn't have distinguished them as anything other than "Lamanites" in their records - since "Lamanites" meant "anybody not with us" right from the start.
Therefore, the "Lamanites" (as defined expansively by the Nephites themselves) not only would have outnumbered the Nephites, but they easily could have been primarily of Jaredite descent - and the description of the Jaredites leads me to believe they originated in the Upper East Asian steppes. (Hugh Nibley also reached this conclusion in "The World of the Jaredites".) If that is true, the "Lamanites" would have been primarily of Asian descent quite quickly in their existence, which the current DNA research indicates is the best possibility.