One of the neat consequences of modern biological tools is the ability to dissect human ancestry via genetics. In particular, by tracing markers on mitochondrial DNA and Y chromosomes, we can gain a general understanding of how human populations have spread and intermixed, as well as getting a window into how that process has operated in a gender-specific manner.
In their recent paper The Peopling of Korea Revealed by Analyses of Mitochondrial DNA and Y-Chromosomal Markers, Jin, Tyler-Smith, and Kim look at mitochondrial DNA and a specific region on Y chromosomes in members of a number of Asian populations (Korean, Korean-Chinese, Mongolian, Manchurian, Han, Vietnamese, and Thai).
You can head over to the paper for details, but the upshot is that the the mitochondrial DNA evidence supports the idea of the Korean peninsula being populated largely by northeast Asians. This makes geographical sense:
Note that the portion of modern-day China between Mongolia and Korea was previously Korean territory, and still contains a high percentage of ethnic Koreans living in China (see Korean-Chinese in the population groups above).
Notably, the Y-chromosome region examined shows a considerable bias toward Southeast Asian populations. The authors suggest that this is coincident with the spread of rice agriculture into the area, as that proceeded from the southeast.
Overall, this doesn't shatter any ideas about Korean ancestry -- basically, they came from that swath of land that is now Mongolia and the Russian far east. However, it's interesting to see the influence of rice agriculture on the genetic composition of the population. One imagines young men of southeast Asia making their way up the modern Chinese coast, passing on their farming practices and their genes through generations until both arrived in the peninsula.