The difficulty I have with this explanation is that the Earth is a big niche that we've come to dominate. At every point the sapiens sapiens migratated to a ne

The difficulty I have with this explanation is that the Earth is a big niche that we've come to dominate. At every point the sapiens sapiens migratated to a new area already populated by a previous human group (a previous wave of migration from Africa) the sapiens sapiens would have to be dominant in that niche - even though we know the Earth is extremely varied. A different explanation is that we were the only hominid group that survived a dramatic climate change (change out of Ice Age, or volcanic change etc) and our 'niche' was the ability to adapt to a radically changed environment in all parts of the planet that we inhabited. I'm not an ecologist, though, so I might be missing something. Also, "relatively small" may have been up to 10,000 years, for Neanderthal and sapiens in Europe. I know this is relatively small on evolutionary time, but its less than an order of magnitude from the amount of time that we've inhabited many places on earth, suggesting that it might have been possible for other human species to survive to modern times (ie if the land bridge between America and Asia hadn't formed until just a little bit later).