Humans are prone to focussing on inconsequential differences within their own species and thus tend to believe that humans are quite diverse. An intelligent Brassica would point out that, despite a light spicing of genes from Neanderthal, Denisovan, and other now-extinct humans, we’re all members of a single species, anatomically modern Homo sapiens, and that the range of traits displayed is extremely narrow… at least compared to, say, the Brassica.

In fact, you would not have to go too far back in time to find an era when multiple humans species shared the Earth: Neanderthal, Denisovan, anatomically modern Homo sapiens, Homo floresiensis, and Homo erectus, perhaps others. The current state of affairs, when there is just one human species, is at least a little peculiar.

This does raise the question of whether modern humans are incredibly successful or simply a currently numerous but clearly doomed last holdout of a generally unsuccessful genus1. More importantly, it presents SF authors2 with a challenge: if they want to expand the possibilities and craft a setting with greater biodiversity in the human population, what can they imagine and how can they justify it? As I see it, there are at least five plausible approaches…

The Past

As previously mentioned, not too long ago there were numerous human species. Simply set your story back then and bob’s your uncle. Conveniently for authors, our knowledge of archaic humans is limited to those aspects that leave durable evidence3. This means authors have considerable leeway for speculation about archaic human culture without earning more than a grumpy frown from John Hawks4.

The obvious example here is Jean M. Auel’s The Clan of the Cave Bear*, *which among other things speculates about how Neanderthal genes found their way into modern humans… something that was not established to be the case until long after Cave Bear was published.

Cryptic Populations

Perhaps archaic Homo survived into the modern day and we’ve simply, somehow, overlooked them. It does seem a bit of a stretch but after all, bonobos were only recognized as their own species of Pan in the 1920s. It would help plausibility considerably if the archaic Homo population was located somewhere out of the way, such as an isolated island5.

David Kerr’s “Epiphany for Aliens,” first published in Again, Dangerous Visions, provides an example. A small community of modern-day Neanderthals is discovered in a particular desolate region of Corsica. Obscurity and isolation spared these Neanderthals the fate of their mainland kin. How will they fare discovery by their Homo sapiens kin?

Extremely poorly, as it turns out. Well, the story was in ADV so a happy ending was likely ruled out from the start.

Sideways in Time

Anatomically modern humans displaced or assimilated every other flavour of Homo in our timeline. Not so in other timelines! Modern descendants of lineages long vanished on our Earth might be found in trouser-legs of time that diverged from ours ages ago. Some may be happily puttering along with the reliable Stone Age technology that served our ancestors so well for so long. Others may possess technology well in advance of ours.

Take, for example, Keith Laumer’s The Other Side of Time, in which Imperial Intelligence Agent Brion Bayard encounters two different species that might be kin to humans: the Xonijeel, whose last common ancestor with Homo sapiens was long enough ago that while the Xonijeel are definitely primates, they may not be Homo, and the Hagroon, very definitely Homo and far more unpleasant than the Xonijeel.

The Future

Diversifying the genus Homo could be as easy as arranging for a lengthy period of isolation. A calamity sufficient to knock us down to small pockets of neo-paleolithic survivors might do the trick (or perhaps not: Stone Age Homo sapiens got around). Alternatively, one could imagine spreading humans across the galaxy on habitable worlds dissimilar to Earth, while denying them the means to rapidly share genes.

Unfortunately, the best example of this ploy is also a story for which listing it here would be a huge spoiler. Consider instead H.G. Wells’ classic The Time Machine, in which two very distinct, yet symbiotic, species of humans are created through simple socio-economic isolation and time.

Science!

Absent the ability to time travel backwards, sideways, or forwards, one can always turn to present day or near future science chappies for some means to rapidly diversify Homo. Genetic engineering is popular, although it lacks the simple elegance of irradiating the planet.

In the backstory of M.A. Foster’s The Gameplayers of Zan, scientists set out to create the Superman. They created instead the Other, the ler: behaviourally distinct and reproductively incompatible with Homo sapiens, but neither superior nor inferior. The Earth being crowded, and humans not known for their toleration of other human species, ler survival demands that bold steps be taken.

Have I overlooked something obvious? Feel free to point out the omission in comments below.

1. See, for example, Lystrosaurus, which in the early Triassic accounted for 95% of the fossils in some fossil beds. It’s easy for lucky survivors to thrive, at least for a time, if they somehow make it through a mass extinction. Humans are, as you know, survivors of the Late Pleistocene extinctions. Possibly the cause as well. In any case, our success may prove short-lived. ↩︎
2. Fantasy authors have it easy. Gods are notoriously lazy, and frequently plagiarize each other’s homework. Thus, a myriad of human-like species just barely different enough from each other to avoid a repeat of Marduk versus Jehovah. ↩︎
3. Archaeological evidence ranging from obvious things like stone tools and cave art, to indirect evidence, such as the implications of lice evolution as a marker for the time when humans adopted clothing. ↩︎
4. That said, I’d probably draw the line short of having one species of archaic Homo able to generate electric shocks like an electric eel, a detail in a 1970s-era story whose title and author I now forget. ↩︎
5. I only omit Marc Miller’s Traveller, in which human populations were taken to other worlds 300,000 years ago, because I mention it so often. ↩︎