I can't remember where, but I read there is a theory that English originated as a creole of Anglo-Saxon and Old Norse. The two languages had similar vocabulary (e.g. AS scirt -- modern English shirt -- vs ON skirt, both originally referring to a unisex knee-length tunic), but different inflections: the vocabulary was kept but much of the inflection was dropped, which is why English is morphologically so much simpler than, say, German.