Two Alien Races, One Planet
Isaac Arthur's latest video poses the interesting question about whether or not two alien races can evolve on the same planet. Physical anthropology is my favorite aspect of biology so this video was interesting to me. Maybe the idea could be expanded to three or four alien species but there are already many factors limiting the possibility of two alien races evolving side-by-side.
if history had occured differently a separate hominid could have evolved on North America from the modern humans of Europe. It's interesting to think about that. That "other" hominid species would be genetically distinct enough for interbreeding with modern humans to be impossible but similarly intelligent.
If it could have happened Earth then maybe somewhere else in the galaxy or Universe. Who knows?