There’s a lot of science fiction books that ask the question “What is it like to be human in an alien world/society?” Recently, with the explosion in molecular paleontology that is taking place, I’ve found myself wondering what it means to be human in a world that was (once) full of other – well, were they humans?

I was going to give a definition here, but it’s rather difficult, as there’s a been a lot of chopping and changing over recent years, and it’s also horribly confusing, with lots of similar sounding names. Broadly speaking, hominids are the great apes like gorillas and chimpanzees, and including humans, and hominins are the human lineage after the evolutionary split from chimpanzees (see Wikipedia here for a rundown if you feel the need to punish yourself). The only surviving member of that is our own species, Homo sapiens, which is 150-200,000 years old. Until fairly recently, the picture of our recent evolutionary origins over the past half a million years or so was relatively clear: Homo heidelbergensis gave rise to Homo Neanderthalis (Neanderthals) and Homo sapiens. Neanderthals went extinct, leaving only us. You will note, however, in the below image (which is just an example – there are other, slightly varying trees) that further back in time it gets more muddy and complicated. This isn’t surprising: it’s very, very rare for remains of anything to fossilise, and many human species are just known from a few bone fragments. Oh, and also: nobody knows where the hobbits came from (Homo floresiensis).

Taken from the American Museum of Natural History website.

1. Dashed lines show how related species diverged from each other through a common ancestor.
2. Faded lines indicate very unclear origin or descent.
3. Branch points are called nodes. Nodes indicate a species that once lived, and was the common ancestor of two or more descendants.
4. Horizontal lines indicate time.
5. Orange vertical bars indicate how long a species is known, from fossils, to have existed.

When the ability was attained to extract and sequence ancient human DNA, however (largely down to the pioneering work of paleogeneticist Svante Pääbo) things started to get a whole lot more interesting, particularly down the more recent end of things (again, unsurprisingly, since the younger it is, the more likely you are to be able to get some readable DNA). First came the astonishing revelation, with the sequencing of the Neanderthal genome in 2010, that Neanderthals and humans of our own lineage must have interbred at least occasionally once we had migrated out of Africa. About 4% of the genome of non-African humans is shared with Neanderthals. The date for this occurring has, at least in one population, just been  pushed back considerably earlier than at first thought, from about 65,000 years ago to 100,000. Then more ancient humans were found, the Denisovans (so recent that they don’t have a formal taxonomic name yet), which again bred with certain subsets of ancient Homo sapiens, and also Neanderthals. Not to mention the hobbits, Homo floresiensis, which were thought to have existed until as recently as 12,000 years ago, but again, this picture has changed. It’s come out only this week that they probably went extinct closer to 50,000 years ago (I confess to some disappointment with this), suspiciously coinciding with the migration of humans down the coast of Asia towards Australia. It’s could be coincidence, but it isn’t unlikely that we out-competed them.

So you end up with fuzzy overlapping trees and diagrams like this:

Paleogenomics figure3
Taken from Lalueza-Fox & Gilbert, 2011, Current Biology

Red arrows mark genetic evidence of interbreeding among different hominin populations.
Black arrows mark suggested or possible additional gene flow. kya = 1000 years ago.

What does this even mean for the term “human?” Aren’t species supposed to not to be able to interbreed? Well, yes, that’s the definition we all learnt at school, but it’s over-simplistic. There are fuzzy boundaries – there have to be, because species are emerging by a gradual process of evolutionary divergence from each other. Take a simple example: there is a population of animals called Thingys; one group of Thingys migrates in search of new areas to live and wanders far north. Later, a barrier separates the two populations: perhaps the climate shifted and it’s too difficult for the populations to mix, perhaps they are literally separated by a new river course or sea level change. The Northern Thingys and Southern Thingys can no longer interbreed, and evolution carries on its merry way, so they start changing, in different ways (it’s important to note this won’t all be down to selection; there will be random genetic drift as well). At some point, after enough time, there are going to be two different species: they are now no longer just all Thingys; probably neither of the populations are Thingys anymore: the ancestral population were Thingys, and the current population in the south are now Thingybobs, whereas the northern population are now Thingywhatsits. There are points during this process where they will be similar enough that they can still interbreed and look virtually indistinguishable. However, if you wait long enough, and they probably won’t be able to. At what exact point do we consider that they are new species? There is an inherent degree of imprecision; all we can probably do is say when they were definitely the same species, and when they were definitely not, but if the two populations were able to mix again after not much time had passed and could still interbreed to a limited extent, are they really two species? Two sub-species?

So were the Neanderthals human? The Denisovans? The hobbits? Doesn’t that depend on your definition of human? It’s nice to make up a label like “hominins” and apply it to the group, because then you still have a group, but you still have your nice different homo species. The distinctions are real, and they are important, but they are also, in a sense, not that important. A Neanderthal woman would probably look quite similar to a modern woman, but she would be significantly shorter than most and yet still probably about twice the weight: those people had serious muscles. And yet, if Neanderthals had survived, would they be the ones with a thriving technological civilisation? Quite possibly. It’s more likely, though, that if we were similar enough to mix completely, then our populations would have blended, re-merging the two lineages before they had time to split completely. After all, they are like the Thingys: the population that became Neanderthals migrated out of Africa early, whereas modern humans arose from those that migrated later.

Are hominins unusual in having this degree of species splitting and mixing? It’s not easy to say, because what do you compare it to? Other closely studied species tend to be model laboratory organisms. It is notable, though, that where an ecosystem results in lots of micro-ecosystems that are physically separated, species do tend to branch out quite prolifically. African cichlids in Lake Victoria are an example. A more pertinent question might be: why haven’t chimpanzees done this? Or gorillas? There are two species of chimpanzee, the common and the bonobo, and the common are divided into four sub-species. We don’t know how many ancestral ones there are, as such, but in our search for hominins/hominids, we haven’t found loads of different varieties of chimpanzee. Unfortunately, tropical forest habitats, where they live, don’t lend themselves to fossilisation.

I do think that there is something striking about hominins which may account for some of this diversity, and that’s the huge migrations even ancient humans species went on. We did not stay in the habitat in which we had originally evolved and were, presumably, adapted to: African savannah. We were able to use our technological skill (itself an adaptation, of course) to colonise widely different habitats very early in our evolutionary history. This of course would have resulted in many separated populations, particularly as the climate changed and sea levels rose and fell. It’s highly probable that several of these populations mixed and separated multiple times.

Suppose a spaceship landed in your back garden tomorrow and out walked an alien who tried to talk to you. A Star Trek style alien; something that looked at least vaguely like a human, walking on two legs. You would never say they were human. But you would probably say they were a person. How well you treated them is up to debate: but, unless you thought they were a threat, probably you would treat them as a person. This is an interesting point on two levels: you would be biologically more related to the earthworms in your lawn than your alien visitor, but you would regard the visitor as being more like you. Not just because they could walk and think and make spaceships, but because, well, they look a bit like you. And yet, humans conspicuously fail to treat other humans as people on the basis of utterly trivial biological distinctions such as skin colour and gender. Not only that, but on the basis of entirely superficial non-biological characteristics like religion and nationality – which people can change. We are a hypocritical species.

And if you planted an alien on a human world that still had Denisovans and Neanderthals? They probably wouldn’t be able to tell the difference.


  1. Lalueza-Fox & Gilbert, 2011: Paleogenomics of archaic humans. Current Biology 21, R1002–R1009. DOI 10.1016/j.cub.2011.11.021
  2. Kuhlwilm et al, 2016: Ancient gene flow from early modern humans into Eastern Neanderthals.  Nature 530, 429–433. doi:10.1038/nature16544
  3. Sutikna et al, 2016:  Revised stratigraphy and chronology for Homo floresiensis at Liang Bua in Indonesia. Nature. doi:10.1038/nature17179
  4. Cover photo: Neanderthal woman’s face reconstructed by Kennis & Kennis, photo by Joe McNally, National Geographic.


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