The multiregional evolution hypothesis asserts that modern humans are the present manifestation of older species of hominids including Homo neanderthalensis and H. erectus. The replacement hypothesis, however, states that modern humans are a new species and that the older species mentioned above were replaced.
In the latter hypothesis, transition of archaic H. sapiens to modern doesn't occur anywhere in the world except Africa at around 200,000 years ago. Anatomically modern humans then dispersed outward to other regions, replacing other hominid species by out-competing them for resources or by displacing them from environments optimal for their continued survival.
There is, however, a very persistent group of paleoanthropologists who adhere to the multiregional evolution argument, which doesn't, by the way, imply that there was parallel evolution or multiple origins of modern humans. This theory suggest that genetic exchange explains how differentiation, geographic variation, and evolutionary change within humans occurred.
The arguments have gone back and forth between the two camps for many years, but new research is supporting the multiregional evolution hypothesis. In a recent article by National Geographic , "Neandertals, Modern Humans Interbred, Bone Study Suggests", Erik Trinkaus, an anthropologist at Washington University in St. Louis, Missouri. comments on a fossil remains found at Petera Muierii ("Cave of the Old Woman") in Romania, which date to about 30,000 years ago.
Unfortunately, the National Geographic article doesn't go into a great amount of detail regarding the cranial and post-cranial morphology of the remains found in Romania, but classic Neanderthal features include a distinct brow ridge, lack of a chin, and occipital bun and their cranial capacities were significantly larger than that of modern humans. Trinkaus does say in the article:
While the remains are largely typical of modern humans, they also show some distinctly Neandertal traits, says Trinkaus. [...] These telltale skeletal features include the shape of the lower jaw and the back of the skull.
"The only way I can explain the anatomy of these fossils and the fossils from a number of other sites across Europe is that there was a fair amount of interbreeding."
According to National Geographic, the research is reported in the Proceedings of the National Academy of Sciences , though, I've not been able to locate a citation yet to the research article itself. I'm sure this won't settle the Multiregional vs. Replacement debate, but it certainly is thought provoking.
Hat tips go to Abnormal Interests who set me to looking for the PNAS article and clicking on the National Geographic article with a post of his own on the subject and another to a friend that emailed the PNAS article to me this morning: thanks!