I'm not an expert but knew this does occasionally happen. I think because the female "mom" mule is a "genetic defect"- a mule with an abnormal set of DNA for a mule. Just as genetic defects can make an animal of a species sterile, the opposite is also possible, but much less likely. I. e. a mule with a very unusual genetic defect can be fertile. When a human is genetically sterile, we don't says they are no longer part of the human species. Even though Down's syndrome children are genetic defects, they are still part of the human species. Likewise when a mule has a rare genetic defect and is able to give birth to a live mule-like animal, we don't say that mules are a new species. Rare genetic defects can exist without any effect on the "species or not" question. I do know of one troublesome case: There is an Arctic bird, (I forget the name) that lives all across Alaska and North Canada, that can mate with those in Iceland. The Iceland bird can mate with those of N. Norway, which can mate with those in N. Sweden. They can mate with those in NE Russia, and they with those in north/central Russia, and they with N. eastern Russia; But the birds of that species which live in eastern Russia differ genetically too much to mate with those that live in Western Alaska and produce fertile off springs! These birds tend to remain near where they were born, so as you go around the Arctic Crcle, small differences in the local gene pool have developed. There is very little mingling of the genes of the Eastern Asian gene pool with that of the Western Alaskan gene pool, so usually mating between them do not produce fertile birds.
In my mind as I probably mentioned . The Sahara was a populated region and there were many variety of animals to hunt for food. As the climate started to change, the inhabitants followed the animals or food. So probably some of the inhabitants moved north and some moved south . But in the north there were other inhabitants such as Neanderthal , and they probably intermixed, and so here we are you and me .
Although I think climate change could have been a factor in our early evolution, I don't think it had anything to do with leaving Africa. I don't think people followed animals out of Africa, animals were all over. It's more likely that when people inhabit any area they spread out. Our migration out of Africa probably happened so slowly that no one really noticed they were migrating.
Well, there has been plenty of population movement in that region, especially in the modern era. And we now have cosmetics and vitamin supplements that allow African people to live healthy lives in Greenland, and Norwegians to live healthy lives in Colombia. Nonetheless, the average Lithuanian has much lighter skin than the average Bengali. Indeed. The migration of the first humans into the Western Hemisphere happened so slowly that their range expanded only ten miles to the south per year. Nonetheless, at that speed humanity expanded from Alaska to Tierra del Fuego in one millennium. [My numbers are not terribly precise, but the premise still holds.]
Good you have your opinion and I have mine. Sahara have spotted aquifers, yet the population and animals is relative low comparing where surface water is in abundance. Animals and people move where water is in abundance
Why man migrated is no mystery. ALL life forms expand into viable environments. The mystery is why one small African tribe destroyed all others (and other humanoid forms) as it populated the world. One answer is here: http://www.sciforums.com/threads/modern-man-out-of-africa.151634/#post-3313586
Don't forget that the desertification of northern Africa was (paradoxically) a phenomenon of an ice age. As a greater portion of the Earth's water becomes trapped as ice in the glaciers and polar caps, there's less liquid water. This results in less rainfall (as well as lower sea levels) and less food. Our descendants will see the opposite phenomenon. We're living at the tail end of an ice age--human activities may accelerate this in a barely measurable way, but these cycles happen with or without us. The glaciers and ice caps are melting and sea level is rising. With more rainfall I suppose there will be more food. But since the end of the Stone Age, we've been creating our own problems. It's not clear that an abundance of food is going to make this a happier, more peaceful place. Not to mention, half of the earth's population lives near the oceans, seas, rivers and lakes, because when we started building cities, boats and ships were the only fast, economical transportation technology. Half of our population is going to have to move inland. Places like Florida and the Maldives will simply disappear.
You have a contradiction there - mules are more common partly because it's easier to breed a male donkey with a female horse, which is partly due to the relative size of the mare and foal. As far as people wandering: humans have been nomadic hunter/gatherers for all their history - why would their expansion of range and wandering puzzle anyone? All large land predators wander at some stage of life. Even very slow range expansion - a day's walk per generation, on average - would account for the known human migration.
