Dr. Hawks, I greatly enjoyed your course on the rise of humans I purchased through the Teaching Company. I could not find the answer to this question: if humans migrated out of Africa and met Neandertals and interbred, where did the Neandertals originally come from? I am sure you are a busy man but I find this puzzling me. Thank you in advance for answering this question.
Thank you so much for your kind words!
We don't strictly know where Neandertals originated. We do know that their population and the African population began to differentiate sometime before 250,000 years ago. I think it is likely that the ancestors of Neandertals migrated out of Africa at that time and began to evolve within western Eurasia, later to come into contact with Africans again. But there are fossil humans who seem to have some Neandertal-like features in Europe far earlier, as early as 600,000 years ago. One possibility is that the ancestors of Africans and Neandertals actually lived outside of Africa, and Neandertals stayed there as other people moved into Africa. Another is that a population representing most of the ancestry of Neandertals left Africa more recently, maybe within the last 150,000 years, and mixed with an earlier European population. It is even possible that the Neandertal and African ancestors lived long-term in Europe and Africa, respectively, with a high rate of gene flow between them for their entire history.
At this level things seem uncertain and will remain that way until we have a better fossil record in Africa. It's an exciting time for those of us who study that time period!