PNAS, Proceedings of the National Academy of Science, has published a paper concerning the teeth of a fossil known as Lagar Velho I or the LV 1 in scientific parlance. It seems, from a superficial glance at the paper, which was written by several experts who did the study, and who have long had ties with the ongoing research on the Lagar Velho fossil, that there were things about the child's teeth that seem more like that of some Neandertals, than of "modern" humans.
To give anyone reading this a background, the Lagar Vellho fossil was discovered in, IIRC, 1995. It was thought at first that the child, who was apparently about four or five years old when it died, was an example of early "modern" burial culture, and that was about it. Then Erik Trinkaus, a Neandertal expert and one of the authors of the present paper, took a closer look. He said he found that the child's skeletal proportions suggested a mixture of early "modern" and Neandertal traits. João Zilhão, one of the original discoverers, and also an author of the paper, declared at the time, and has been declaring ever since, that Neandertals "took their last stand" so to speak, on the Iberian Peninsula, and the early "modern" population was a "mixed" one. Needless to say, these assertions have been disputed, just as almost everything concerning Neandertals has been.
Whether or not one believes these claims, it is certainly true that early "modern" humans, e.g. Homo(sapiens)sapiens were more "robust", and in some ways, more like Neandertals, than later ones. This, as Razib of Gene Expression points out, is probably due in part, at least, to the introduction of agriculture in the so-called Neolithic period. You just don't need as much muscle and bone bulk to grow crops, as you do to go out and hunt or gather your own meals, so it's no wonder people switched to agriculture! By that time, of course, there weren't any Neandertals, and, when people started switching to agriculture, there probably weren't that many "modern" foragers(e.g., what have previously been called "hunter-gatherers"). Agriculturalist populations were larger, and probably "swamped" the hunter-gatherers.
Which is food for thought. At the time they existed, Neandertal populations were even smaller than later "forager" populations, and little groups of them apparently hung on is relative semi-isolation on the Iberian Peninsula rather late. "Modern" populations eventually filtered their way into the peninsula, but there may not have been very many of them, either, at the time. Which may account for the "mixed" population, if there was a "mixed" population(people have disagreed about this, too). In any case, assuming "moderns" and Neandertals "mixed" in the Iberian Peninsula or anywhere else(and they might have, in some places, at various times), there weren't very many of the "mixed" populations, either. Which would make them easy to "swamp" genetically, even if more "gracile" people didn't originally have some genetic advantage. In any case, as the PNAS article seems to be suggesting, the "modern European origins" story is rather more complex than some people like to believe.