At the dawning of Homo sapiens, our ancestors were born into a world we would find utterly surreal. It's not so much that the climate and sea levels or the plants and the animals were different, although of course they were — it's that there were other kinds of humans alive at the same time. For most of H. sapiens' existence, in fact, multiple human species walked the earth. In Africa, where our species got its start, large-brained Homo heidelbergensis and small-brained Homo naledi also roamed. In Asia, there was Homo erectus, a mysterious group dubbed the Denisovans and, later, Homo floresiensis — a hobbitlike creature, tiny but for its large feet. The stocky, heavy-browed Neandertals, for their part, ruled Europe and western Asia. And there were probably even more forms, as yet undiscovered.How was that any different from today?
All humans today are considered the same species, but it is not known that the above groups were really distinct species either. In fact, those that have been DNA sequenced are known to have interbred. And the hobbit was not really later, as it was dated incorrectly.
The difference between Europeans and Africans is about the same today as it was 100k years ago.
If humans were aware of the rival human sub-species back then, would they have been concerned that alien invaders would wipe them out? The answer is likely to be yes. Basic evolutionary theory teaches that the sub-species will fight to the death, leaving one surviving group. Someday Europe may have another fight to the death with African invaders.