One of the big mysteries of human evolution is what happened to all the humans who shared the planet with Homo sapiens for hundreds of thousands of years. While Homo sapiens was evolving in Africa, there were also tool-making hominins in Europe and Asia, known as Homo erectus, the Denisovans, and the Neanderthals.
Did all those human groups meet at some point? Did they interbreed or kill each other? Is it even appropriate to call them all human, or were some human and some animals?
Star Trek has the answers to these questions. And they are just as tangled and frustrating in science fiction as they are in real evolutionary science.