Aeon has a long article on how the current human species emerged from something like 10 earlier pre-human species, including Neanderthals, Denisovans, and something called "archaics." See what the article concludes.
After thousands and millions of years, one lineage emerged to replace all the others. This probably explains something about our history, and our tendency towards war and conflict.
We may live in civilisation today, but the genes within us are those that made us the sole survivors of hundreds of thousands of years of intertribal conflicts and bloody, genocidal wars. We replaced all the other humans because we were more dangerous than all the others.
That's us: "more dangerous." We not only eradicated the competition, it's possible we hunted them as food. If we encounter other star-faring species, it is likely their backstory may be as violent as ours.
Heinlein's Starship Troopers could be in our future.
