My theory is that modern humans wiped out all other humanoid beings. The fossil record clearly shows there were other hominid evolutionary paths, inlcuding the Neanderthals, who went extinct around the same time that modern humans arrived in Europe. We killed off our competition either directly or by simply out-competing them in their niche. To me, this is also a convenient explaination for some of our odd behaviours such as racism/xenophobia; warring cultures; and nearly universal ancient legends of tiny humanoids (elves etc.), giant humanoids, bigfoot/yeti, etc.