This is second in my current series about human origins, which began with Who you aren’t. Now I’m talking mainly about our own genus, Homo, regarding a kind of diversity which outweighs the ordinary use of the word by a solid quantum.
I realized I’ll have to save some historical taxonomic hysteria about humans/apes for some later posting; please picture me twitching with suppressed fury.
Let’s start this one with the big difference between alpha taxonomy vs. systematic revision. The first is what you call creatures or their remains when you first collect them from somewhere, which is also what drawer you first stick them into in a museum collection. It’s explicitly provisional. The second is what you call them given a lot of work on organizing the phylogeny they’re part of, and after reviewing any and all names they could be called and have been called, and it leads to reorganizing and re-labeling a lot of museum drawers. A revision is serious science and usually entails intense debate. It’s certainly harder regarding paleontological remains, which is understandable, and particularly fraught regarding humans, because the naming is shot through and through with constructing heartfelt narratives. The latter situation led to a lot of resistance and tradition in preserving the alpha taxonomy for fossil humans and their relatives, and things are still a bit rattly. (To esteemed colleagues: so you’re just gonna have to put up with some choices I made about whom to call what.)
Technical term #1: mosaic evolution, which is to say that across a bunch of closely related species, you can see different parts undergoing independent evolutionary change. In a complex group like this one, a later species can end up with lots of changes that did not evolve at the same time or in tandem in any other way. Crudely, we stood up first, our heads got big later; also crudely, Paranthropus went “jaw” and Homo (eventually) went “brain.” See also my replies in the comments for My favorite human.But rather than dive into that, try this basic point: don’t use Homo sapiens as the “search image” for who was or wasn’t “truly” human, or “fully” bipedal, or “truly/fully” any other thing. This group of creatures messed with lots of variables and we happen to have one particular profile of changed and unchanged versions.
Now I’m refining the focus to the early known forms we put into Homo, however dubiously, and with however much uncertainty about how many species are represented. The most familiar name is Homo habilis, with not a whole lot of visible difference from Australopithecus except for – as I see it too – an interesting bulbous aspect to the cranium, although not much size change. But the coolest one, available in tons of finds all over Africa, is H. ergaster, to the right, or if you insist, African H. erectus, whose postcranial skeleton is strikingly similar to ours – nice long gams and all the details of bipedalism to use’em with. Some researchers doubt that Australopithecus was a “true” biped (I think they split some unnecessary hairs there), but H. ergaster nails it that our genus stood up long, long before we got our freakishly huge cranium.
For a long time, during the celebrity personality-dominated era of physical anthropology, there were considered to be three non-sapiens species of Homo: H. habilis, H. erectus, and H. neanderthalensis, making a too-neat and usually not well-depicted “climb up from bestiality” which has characterized narratives of human evolution throughout its history. Partly through cladistic thinking, it’s been completely dismantled. Some of you may recall the first phase in which much hair was torn regarding “single-origin” vs. “multiregional” stories (well, people said “hypotheses,” but …), itself riddled with complications regarding new technologies like DNA sequencing and new systematic techniques. That’s been dismantled too. It turns out we didn’t have too little fossil evidence for nifty species of Homo to work with; we had too much, and when younger biologists got into the collections, they realized that no real revision had ever taken place.
At present there are at least six well-supported named species of fossil species of Homo, and their history is at least now divided into solid-evidence units vs. open questions.
I am endlessly fascinated by the nuances of reconstruction: cover hair distribution, vibrissae hair prominence (e.g. eyebrows), breasts, penis size, small iris (of the eye) … no one knows to what extent these appeared among the relevant species or in which order or combinations. Another post someday for sure!
Here’s technical term #2, and this is a big one: cladogenesis.
Every non-biologist I know thinks speciation works like anagenesis, on the left: a species is replaced by its own descendants, going extinct in the process. However, if this ever happens in the real world, it’s extremely rare. Cladogenesis is the observed default: a species splits with its older form persisting and some subset, usually a small group in an isolated situation, is the new species. The appearance of the new form doesn’t include the disappearance of the prior one. Therefore we talk about ancestral vs. derived traits, but since the 1980s, we are, or should be, no longer so hung up about who was whose ancestral species.
Just as the three genera I discussed first do not present a “Y replaces X, then Z replaces Y” history, neither do the various species within Z, relative to one another.
Crucially and related, we regard extinction as a completely different phenomenon which has nothing to do with some new “superior” form coming along. When you apply cladistic thinking to human species history, it instantly junks all talk of who replaced or outdid whom, or of each new species being “better” at being human. Forget all that crap about the “march to humanity.”
Now for the more recent species in Homo.
There’s the technical term #3 for the post: relict. By definition, all relictual species are funny-looking compared to their living relatives. In many mammal groups today, you can see the effects of both background and mass extinction, the latter sustained by all large land mammals during the Pleistocene, not just us. Furthermore, our persistence is most likely due to our broad, populous distribution rather than any particular toughness of form or character or intellect. The result: we’re a very typical relict, a single species which is rather funny-looking compared to its living relatives, due to its closest relatives being absent and therefore invisible.
Whereas the observing eye and mind, of course, interprets this as saltation, a rapid “jump” in evolutionary change. Saltation does occur and is a cool thing to discuss, but here it’s an illusion. Unfortunately, it’s an illusion which feeds our need to see something special and even heroic. Nearly all accounts of human evolution include this heroic element, even using terms like “achievement” and “unique” … despite relictual mammals being all over the place and in many cases much funnier-looking, relative to their living relatives, than we are. You don’t see people blithering similarly about elephants, (i) whose difference from their living relatives is profound and (ii) whose social and cognitive lives are clearly sophisticated. No one lauds their distinctiveness as an unusual evolutionary event; it’s immediately explainable upon examining the wide array of diverse related fossil species, some of which show different “variables profiles” and many of which are more ordinary-mammal looking. Sound familiar?
In particular, nothing suggests that speech, cognition, and culture had anything to do with our persistence, and although the debate rages, just how much of those things were the norm for humans until surprisingly recently, is still under debate. Is there more to say about this? You bet! The next post in this series concerns the history, dispersal, and diversity of our own species.
Next: A world of debate in a strand of hair