It is interesting that people that lived 25,000 years ago in a rock shelter near my home in Thailand had somewhat different tools than the first inhabitants of Australia. The Negritos, Andaman Islanders and Australians are all quite different than the people that have lived for considerable time right next door both in South Asia (Dravidian) and Southeast Asia. It is too bad that abuses of science of the beginning of the last century and before have poisoned the well of speculation.
Indeed. But with DNA analysis we have a non-subjective tool for determining descent (as opposed to the influence of climate and environment).
River valley civilizations emerged in different parts of the world at about the same times but without any real evidence of commmerce.
Given the way that humans like to marry those like them, any inbreeding would be limited to, say. captives. So did the human race begin with slavery?