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. And let's not even get started on Homo floresiensis, the so-called Hobbit people. 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.
The Most Annoying Star Trek Episode Ever Written
Even if you aren't a Star Trek fan, you are probably aware of the much-loathed Star Trek: TNG episode "The Chase," where we learn that humans, Romulans, Klingons and every other humanoid we've met are in fact from a common ancestor whom we'll call the Doughfaces (you can see why from this picture). Through a series of improbable events, the Enterprise eventually finds a secret holographic message from the now-long-gone Doughface representative, who says:
We knew that one day we would be gone, and nothing of us would survive - so we left you. Our scientists seeded the primordial oceans of many worlds, where life was in its infancy. The seed codes directed your evolution toward a physical form resembling ours: this body you see before you, which is of course shaped as yours is shaped, for you are the end result. The seed codes also contain this message, which is scattered in fragments on many different worlds.
OK, yes, this is absurd and you can see why people hate this episode. As Peggy Kolm has already pointed out brilliantly on Biology In Science Fiction, it makes no sense that every group would have evolved the same way on a variety of planets, and you can't "direct" evolution with "seed codes."
But if you ignore all that, you wind up with a pretty interesting portrait of evolution in Star Trek that happens in some ways to mirror our own on Earth. First, we already know that some groups of aliens can interbreed. There is the half-human, half-Vulcan Spock; there is the half-human, half-Klingon B'elanna Torres, the half-human, half-Betazoid Troi, and many other minor characters too. This only makes sense if they are all descended from the Doughfaces, though there are some episodes where it's suggested that the mixed-race people are the product of technological tinkering.
So what does this have to do with human evolution?
The Common Ancestor
Humans and our ancestors are called hominins (here's a good explanation of why), while the greater group of humans and apes are called hominids. All the groups I mentioned earlier are undeniably hominins, and all came from the same common ancestor as Homo sapiens did — the Doughfaces of humanity are called Homo ergaster or Homo erectus. Obviously Neanderthals didn't get their Romulan-esque brow ridges from some kind of panspermia event with "directed evolution". Instead, different human groups simply left Africa at different times, scattering across Eurasia. Because Homo erectus left Africa a million years before Homo sapiens did, the two groups evolved separately for quite a while. Same goes for the ancestor of the Neanderthals and Denisovans, who also left before H. sapiens did.
So humanity's million-year trek into Europe, Asia, and Australia was somewhat like what happened to the Doughface's progeny on many different planets. They started out as one species, but as they settled down in different regions of Earth, they began to look quite unlike each other. I've always found it amusing that what would have probably identified different human groups 200 thousand years ago would have been brow ridges and height — the same two features that are used in the Star Trek universe to make people look "alien." Of course, Denisovans wouldn't have had a silly, crinkle-cut french fry nose like a Bajoran. But Neanderthals would have had a thicker brow and receding chin than modern humans, while the Hobbits were significantly smaller than a typical Homo sapiens.
When Homo sapiens met the Neanderthals for the first time, would it have been like humans meeting the Klingons? Quite possibly, yes. Especially the part where they look slightly different and speak different languages, but are nevertheless able to have children together and engage in extensive trade back and forth (as well as having a war).
As I mentioned earlier, one of the big questions in Star Trek (and human evolutionary biology!) is whether Homo sapiens really could have children with other humanoid groups. On Star Trek, we hear different stories about how a half-human half-Vulcan child could be born — was it done in the wild, or in the lab? But we are absolutely certain that Vulcans and Romulans are related so closely that they are able to have children with no problems. This actually mirrors issues in evolutionary science, too.
When one species splits into two or more, that's called speciation. Usually it happens when two groups of the same species are separated for long enough that they evolve to the point where they are no longer able to produce offspring. The big question is, were groups like erectus and the Neanderthals another species, or were they humans who just had facial and body structures that were different from modern humans? There is now a lot of DNA evidence that Homo sapiens interbred with Neanderthals and Denisovans. So it's likely that all three groups were, in fact, the same species. But we still know almost nothing about Homo erectus, and are similarly in the dark when it comes to the Hobbits and other hominin groups that are still being discovered. It's possible that Homo sapiens interbred with Neanderthals, Vulcan/Romulan style, but couldn't interbreed with Homo erectus.
Speciation is a messy, uneven process that is rarely black or white. Sometimes two very different groups are basically like Vulcans and Romulans. They behave completely unlike each other but are genetically almost identical. Other groups may have a common ancestor, like humans and chimps, but they cannot mate. It is suggested in Star Trek: Enterprise that humans and Vulcans may be in this situation, if it's true that they require technological intervention to have viable offspring.
We just don't know all the answers. In the case of Star Trek, that's due to messy plotting and a lot of annoying retcons. In human evolution, it's because we're still trying to discover enough about our history to understand what happened while we evolved.
We're All Human
Many anthropologists use the word "human" or "people" to describe Neanderthals, Denisovans, and other hominins who were contemporary with modern humans. There's strong evidence that these groups used tools and fire, may have had language, and bore children with Homo sapiens. Similarly, in Star Trek, the humanoid aliens are always treated like people — even if they are Klingons who are killing everybody and eating piles of black gummi worms. They aren't called human, but they are clearly human-equivalent. Nobody uses the pronoun "it" to describe a Romulan. They may be enemies, but they aren't animals.
When we try to imagine what it would have been like for Homo sapiens to migrate out of Africa, only to discover Neanderthals and Denisovans and possibly lots of other hominins, it isn't entirely unreasonable to keep Star Trek in mind. What was it like to live in a world with lots of other intelligent hominins? Possibly something like being in a Federation with lots of other humanoids.