Our Species Has Spent Its History Mating with Anyone Close By

In graduate school for anthropology many decades ago, I learned that the phylogenic tree of human evolution was best represented by a sturdy central trunk. That trunk embodied the singular path of our species but there were also several weak branches sprouting off the truck that had withered and died. The roots of that central trunk were the first bipedal hominids, Australopithecines living about 4.5 million years ago, and the top canopy was Homo sapiens sapiens, modern humans. But that clear, straightforward trajectory has completely changed. These days the tree of human evolution appears to be more of a tangled bush of interconnected stems and leaves than a strong central trunk with independent dead branches.

This last year (2021) has been a time when many more of those branching twigs have been exposed and connected, especially during the past 400,000 years, ending forever the idea that there will be some sort of singular line from our African heritage to modern humans. Nor will we ever be able to explain the small bits of physical variation we might see in people across the world today, because our genes have been exchanged and shuffled about so much that we are all of the same human genetic blueprint. Our kind, it seems is way more connected than we realized.

What happened here was sex, plain old sex. It seems that humans are a species that just can stay away from others, even if they are outsiders, foreigners, or newcomers. We just don’t care. The urge to mate with others, even those who do not belong to one’s home group, appears to be the driving evolutionary force that has united our species even after the many migrations by human-adjacent groups into various geographic areas.

Our ancestors first left Africa about 2 million years ago when they had many features recognizable as human but were still different enough that paleontologists named them Homo erectus, meaning they were close enough to be in the same genus (Homo) but different enough to be called a different species (erectus and not sapiens). No one knows why this migration out of Africa happened. These creatures might have been following other creatures, the ones they hunted and ate, or perhaps there was some sort of climate change that made finding edible vegetation difficult and so they searched farther and farther for something to gather. It’s not as if these pre-people had a map of the world and pointed to China, the Middle East, Indonesia, and Europe and decided to go there just for fun. But off they went across the globe, settling down here and there. Examining their bones and skulls, it’s possible to identify some slight regional anatomical differences among these Homo erectus groups, but in the end, they are pretty homogeneous, even when separated by millions of miles.

Early humans continued to walk out of Africa even as they evolved into Homo sapiens sapiens, our genus, species, and subspecies today. Again, these humans spread out globally but they also laid down roots long enough in some places to evolve enough difference to warrant subspecies names. Homo sapiens neanderthalensis is a good example; Neanderthals, which appeared in the fossil record about 50,000 years ago, are like modern humans but not exactly like modern humans. They are of the same genus (Homo), same species (sapiens) but a different subspecies (neanderthalensis not sapiens) based on differences in anatomical features. Anthropologists have always wondered about the role of Neanderthals in human evolution and they have assumed that since Neanderthals were a small geographically isolated population roaming Europe, and they went extinct about 35,000 years ago, modern humans must have arrived and wiped them out. These days, an alternative view is that modern humans arrived into Neanderthal territory and mated with Neanderthals and that our current genome includes some Neanderthal DNA. In general, the idea now is that modem humans have had a long-standing pattern of arriving out of Africa and mating with other humanoids who had left Africa long before and settled in pockets around the globe. This is how we got to be one united species today.

There is always the caveat that not all these matings were consensual — as in modern times, forced sex might have been part of one group conquering another, but the mixing and matching of genes over the millennia are so widespread and so consistent, it seems more likely that voluntarily mating with outsiders is a universal pattern of our behavior. Since the purpose of mating is to reproduce and pass on genes, it makes sense that humans, even with their big thinking brains, would be pushed to mate at every opportunity, and to be a little adventurous in their choices.

The result of all this sex is that while the modern human species spread across the globe it did not divide into separate species. Instead, this mating free-for-all joined up what paleontologists classify as subspecies, dismissing their differences to the realm of superficial geographical variation. We are one and the same. We know this from our skeletons, blood, mentality, and similar thoughts and behaviors. And we have had confirmation of this every day for the past two years because Homo sapiens sapiens is one big, united host for a virus. But the good news is that as one species, we can all take the same vaccine, all 8 billion of us, and have a good chance of wiping that virus out; every single one of us is in one big worldwide pod.

Our kind evolved not as a branching tree but as a tangled interconnected bush. One look at modern airline routes, shipping charts, cell phone tower connections, and highways shows that humans simply cannot stop contacting each other and hanging out, socializing, and mating. And we’ve been doing this forever because it is our evolutionary strength, at least for now.

Also published on Medium.com