So, I have been kind of wondering, every now and then, about the current state of our understanding of hominid evolution, because that is the sort of thing I sometimes wonder about.
You may have heard of the discovery and description of Homo naledi in South Africa a few years ago. I did, and read this excellent National Geographic article with fascination (I think you have to now register (free) to read this article). (There’s also a video, but I don’t know if you have to register to view it.) The head paleontologist for all this was Lee Berger, and he did lots of cool things, including allowing free access to the real fossils and casts of them, and encouraging dozens of publications by heaps of paleontologists. So now there’s this book of his, Almost Human, published in 2017, which I happened across just a few weeks ago.
I picked up a copy at once and read it immediately and I have to say, if you’re the least bit interested in hominid evolution, and you like narrative nonfiction, you should absolutely read this book. It’s not technical at all. It’s very much a story about these discoveries.
I really want to look up some of the more technical papers cited in the bibliography, plus I need to do a literature survey and see what Berger’s been up to in the last couple years, but this was a great reorientation to the field plus it’s just a fun read. Let me quote a little bit that includes a wonderful analogy:
For their work, the preparators use an “air scribe,” a metal device about the size of a large fountain pen, attached to a compressed air hose. The business end of the scribe is a tungsten-tipped metal implement with a sharp, pencil-like point. As compressed air is injected into the scribe, the whole tip vibrates rapidly up and down, a few microns at each pulse. It is as if an ant had invented a jackhammer …
I laughed when I got to that phrase. That may be the best single line in the book. I mean, it’s hard to top! I also bookmarked the section about the safety procedures used when excavating the Homo naledi fossils from the very difficult cave where they were found and made my mother read that section. (She went on to read the whole book, which should tell you something, because this is not her normal kind of nonfiction reading.) I don’t dislike caves, quite the contrary, I’ve been caving and I enjoy reasonably safe caves and I’m not particularly claustrophobic, but this one, well, let’s just say it would hardly be possible to go overboard about safety in this particular cave. Not sure I would be willing to go all the way into the part where the bones were found.
Anyway, Berger was first responsible for finding some great fossils of a new species, now called Australopithecus sediba, in 2008. This is a mosaic species, with lots of primitive features and lots of derived features, so it was basically a matter of lining up lots of important anatomical features and asking, Okay, Australopithecus or Homo? Small brain, primitive. Small teeth, derived. Relative proportions of molars, primitive. Anatomy of the foot, primitive. Anatomy of the pelvis, more derived. And so on and so forth. As you see, eventually Berger decided to assign this new species to Australopithecus, for lots of reasons but partly on the basis of primitive characteristics of the legs and feet.
Could this species be in the direct line of descent to modern humans? Sure, could be! Or maybe not! That’s the thing, there are SO MANY DIFFERENT hominid species, and they have such different combinations of traits. This one has the sort of mosaic of traits that would be expected for SOME transition between Australopithecus and Homo, but there is absolutely no reason to expect there weren’t lots of other hominid species with completely different mosaics of traits.
In 2013, Berger found another such species. This was Homo naledi. Berger was responsible for finding tons and tons of fossil material in the Rising Star cave system, in the same basic area as the A. sediba – a very thoroughly explored area, and yet here’s another completely new hominid species. Getting to those bones required getting through a nine-inch gap (!) and creeping through this extremely narrow forty-foot-long chute. Lots of people just could not fit, including Berger himself, so he wound up recruiting a team of six young female paleontologists plus support staff. Here is an interview with one of the women recruited for this excavation. They took out about 700 bones in three weeks and – I did not know this – there are thousands and thousands of bones probably still in that cave, if someone had the funding and inclination to go get them. Then they located another spot in the same cave system and recovered hundreds more bones from that location, same species. They got complete hands, complete feet, they got almost every possible bone of the body, all ages, it was just an incredible find. Berger then organized a workshop, pulled in thirty young paleontologists from all over, and studied the heck out of these Rising Star fossils.
Unlike A. sediba, H. naledi has a very, very human-like foot and leg, except the neck and head of the femur is very Australopithecus-like and so is the pelvis. The wrist is very human-like too, but the thumb has a unique metacarpal and there is a distinct curvature of the finger bones. The shoulder is much more primitive. The general summation I first read about this species is that the more medial parts of the skeleton are more primitive and the distal parts more derived, and you see that’s about right. Then the spine is very similar to the Neanderthal spine. The teeth were very small, the skull shaped much like some of the skulls assigned to Homo erectus, but the brain was about a third the size of the modern human brain.
Despite the small size of the brain, it seems almost certain that those many fossils got into those highly inaccessible cave locations because living H. naledi carried their dead into those locations. Water definitely did not wash those bones into those locations. Animals most certainly did not carry bones into those places. And because there are so many bones in two different locations, I’m happy to say my alternative hypothesis, which I never liked much, does not seem to be likely – I was afraid that predators or enemies might have forced those H naledi individuals to creep into very inaccessible locations and hide there until they died. I do think now that a burial type of behavior seems more likely, which is much nicer to think about.
At the time I first read about H naledi, the age of the fossils had not been established. The rock formations surrounding the bones couldn’t be dated, there weren’t any contemporary animal bones to date, and so that was a tough problem. But finally Berger and his colleagues decided they had so many bones and especially teeth that they could sacrifice a few to destructive testing. So now we know those bones are less than 450,000 years old, maybe as young as 250,000 years. This is remarkable, as just guessing from anatomy, most people were leaning toward those bones being more like two million years old. If those bones are as young as 250,000, they more than likely overlapped in time with early modern humans, during the period when modern humans were anatomically identical to contemporary modern populations but way before there is any evidence of tool use. At this time, there were definitely a lot of hominid species overlapping in time and space, and evidently Homo naledi was one more.
So … what can we conclude about the place of H naledi in our own evolutionary history? We can’t conclude much! I’m so pleased Lee Berger emphasizes that. Maybe H naledi evolved early from H erectus or some similar species and lasted a long time. Maybe some earlier ancestor gave rise to both H naledi and H erectus. Maybe something else.
At this point, as Berger points out, we have four known fossil specimens from Africa which share a basic skull shape with living people. Each of those skull types looks much more similar to modern humans than they do to H naledi or other, definitely older, species. But at the same time, each of those skulls is more different from the others than any modern human is different today. Do they all represent different species, distinct from each other and from modern humans? Probably! We already know Neanderthals and Denisovans were contemporaries of modern humans and interbred with them, and we see genetic traces of at least one other species in our own DNA too. Now we have H. naledi, which may have, almost certain did, live surrounded by some or all these other species with their much bigger brains, quite possibly competing successfully for a long time. H. naledi was gracile, but overlaps in size with modern humans; the feet and legs were adapted for walking long distances and the hands seem about as well designed for grasping, which means this is a species that occupied the same broad niche as modern humans.
Hominids were SO much more diverse than the impression you’d get from pop culture. For nearly the whole span of hominid evolution, it was just normal to have way more than one species around in any given ecosystem at any given time. I mean, think of sub-Saharan Africa. We currently have African painted dogs, black-backed jackals, side-striped jackals, golden jackals, Ethiopian wolves, Cape foxes, and bat-eared foxes living in the same basic ecosystems right now. That is seven species of canids in four different genera. That is what it was like to be a hominid for most of prehistory! That is just so amazing to think about!
Personally, I would suggest reading this book in combination with Henrich’s The Secret of Our Success, which is about unique aspects of human behavior, not intelligence but social behavior, that in his opinion are more likely to have made modern humans more successful than other hominids or apes. Henrich argues that it is not clear that individual intelligence was the specific advantage that led to the success of modern humans. The plausible long-term competitive success of small-brained species like H naledi lends support, in my opinion, for Henrich’s argument that our eventual success may have depended primarily on social learning and especially on the transmission of adaptive learned behavior from individual to individual across generations. Alternatively, it seems possible that social learning might have provided a small-brained species like H naledi a competitive advantage over other hominid species that may have been more intelligent individually, allowing them to live for a long time in competition with many other hominids. We don’t know! This is just so neat!
It’s things like this, rather than ideas about what might happen if someone went back and shot Hitler, that REALLY make me want a time machine — or at least a way to look back through time.