a bonobo humanity?

yeah – a few ‘awkward’ species missing from this group

Canto: So we’d all like to solve the riddle – or many riddles, of human ancestry, but the problems are manifold, it seems.

Jacinta: Yes, we’re not just talking about Homo sapiens, or H sapiens sapiens as some would put it, but the whole Homo genus, including neanderthalenis, denisova, floresiensis, naledi, heidelbergensis, rudolfensis, erectus and habilis, and I’m not sure if I’ve got them all.

Canto: Yes and there are lumpers and splitters, but I’m talking even further back, to Paranthropus and the Australopithecines. I watched a DW doco recently that piqued my interest, making me wonder at what date, round-about, did the Homo genus emerge, and what genus did it emerge from?

Jacinta: Well this is a problem for all species and genera really. Think of our favourite apes, the bonobos. In the book Who we are and how we got here, which is all about the new science of genomics and how it’s transforming our understanding of human populations , David Reich wrote of –

a new method to estimate the suddenness of separation of the ancestors of two present-day species from genetic data… When they applied the method to study the separation time of common chimpanzees and their cousins, bonobos, they found evidence that the separation was very sudden, consistent with the hypothesis that the species were separated by a huge river (the Congo) that formed rather suddenly one to two million years ago

D Reich, Who we are and how we got here, p46

Which is all very fascinating, but one to two million years is rather a long time frame.

Canto: Yes, in the DW doco the time frames were also rather flexible – which I suppose needs must. Australopithecines were described as emerging perhaps 3 million years ago and disappearing 2 million years ago, with the Paranthropus genus preceding them by about a million years – or was it the other way around?

Jacinta: And other types are mentioned – often from the most meagre remains. Sahelanthropus, Orrorin, Ardipithecus, and Danuvius guggenmosi, beloved of Madelaine Böhme among others.

Canto: Well D guggenmosi was an interesting but isolated find, dating to around 11.6 million years ago, and of course the remains are fragmentary so there are arguments about its bipedalism and other features. It was a tiny ape, quite a bit smaller than bonobos, the smallest of the extant great apes. Böhme is arguing, I believe, that these discoveries (three specimens were discovered) could push the chimp-human last common ancestor (CHLCA) back a few million years. The CHLCA date is usually given as between 6 and 7 million years ago, but Wikipedia is, currently at least, being more open to a wider range:

The chimpanzee–human last common ancestor (CHLCA) is the last common ancestor shared by the extant Homo (human) and Pan (chimpanzee and bonobo) genera of Hominini. Due to complex hybrid speciation, it is not currently possible to give a precise estimate on the age of this ancestral population. While “original divergence” between populations may have occurred as early as 13 million years ago (Miocene), hybridization may have been ongoing until as recently as 4 million years ago (Pliocene).

Chimpanzee–human last common ancestor, Wikipedia Jan 21 2023

Jacinta: Interesting, and it suggests a lot of work still to be done, and that’s just in relation to the Homo genus. I’d certainly be interested in pursuing the evidence and the debate in future posts, but for now I’m wondering about the immediate ancestors of our species.

Canto: Well, the book Who we are and how we got here tries to sort all that out through the study of population genetics and genomics, though much of it, so far, deals with migratory populations over the last tens of thousands of years…

Jacinta: Homo heidelbergensis has struck many palaeoanthropologists as the likely common ancestor of both H sapiens and H neanderthalensis. The Smithsonian dates the species to about 700,000 to 200,000 years ago, but there’s also this from their website:

This species may reach back to 1.3 million years ago, and include early humans from Spain (‘Homo antecessor’ fossils and archeological evidence from 800,000 to 1.3 million years old), England (archeological remains back to about 1 million years old), and Italy (from the site of Ceprano, possibly as old as 1 million years)

Canto: So yes, again, lumpers and splitters, and we’re no experts. From the term Homo antecessor I’d conjecture that they’ve been hailed as direct antecedents…. but other specimens, named H cepranensis, and H rhodesiensis, as well as H heidelbergensis, are in the mix, and the remains are often hard to identify and date, with DNA and the proteins made from them being tricky to isolate from warmer climes…

Jacinta: The Australian Museum gives us this interesting info about H heidelbergensis versus H antecessor, in describing the largest find of specimens:

• The remains of at least 6 individuals found at the site of Gran Dolina, Atapuerca, in Spain. They lived about 800,000 to 1 million years ago in Europe and are the oldest human remains found in that continent. Although many experts consider these remains to be part of an early and variable Homo heidelbergensis population, the discoverers believe the fossils are different enough to be given a new species name Homo antecessor.

Canto: I’m wondering about that Morocco specimen that has recently, no doubt controversially, been reclassified as H sapiens, though it dates from 320,000 to 300,000 years ago, pushing the age of our species back by a hundred thousand years or so.

Jacinta: Yes you’re talking about the finds at the Jebel Irhoud site, and it’s complicated, because most researchers don’t identify that region as the birthplace of H sapiens. They mostly agree that the species was ‘born’ in southern and Eastern Africa. The Smithsonian seems to me a bit confusing and unconvincing on this point:

The remains of five individuals at Jebel Irhoud exhibit traits of a face that looks compellingly modern, mixed with other traits like an elongated brain case reminiscent of more archaic humans. The remains’ presence in the northwestern corner of Africa isn’t evidence of our origin point, but rather of how widely spread humans were across Africa even at this early date.

They’re saying that the oldest human remains found are in north-west Africa, but humans probably originated in south-east Africa, though we haven’t got specimens from there that are older than 260,000 years, at most. Hmmm.

Canto: The paucity of the fossil record is probably to blame. But then – Homo naledi. I’ve just watched John Hawks giving a talk on naledi in 2017 – Hawks is a hero of mine, I used to follow his website regularly – and he talked of more and more discoveries in that diabolical underground cave system called – maybe ironically? – Rising Star, in South Africa. They now have more fossil remains of naledi than of any other ancient Homo apart from neanderthalensis. And they’ve managed to narrow the dating from about 320,000 to 240,000 years ago, from memory. So they may well have lived alongside the earliest H sapiens.

Jacinta: Complexifying the picture in ways some find fascinating, others frustrating. And they were much smaller and smaller-brained, right? Like floresiensis, another mystery. So there will be questions about how ‘advanced’ they were. Is there evidence of tool use? Of fire? And remember, it’s not brain size that matters so much but brain organisation. Think of corvids – tool users, problem solvers, complex family systems, brains the size of a walnut but packed with as many neurons as some monkey species.

Canto: Yes, I agree, we can’t make too many assumptions based on size. Bonobos females are smaller than the males but much smarter, right? But one major difficulty about the naledi lifestyle is that we know nothing about it. It seems these remains were placed, or dropped, in the cave after death. And as far as I know, we have no trace of naledi above-ground, which is kind of bizarre.

Jacinta: Okay, so I’m watching the ever-reliable North 02 vid on naledi. They first thought these remains were probably well over a million years old, due to various features, especially skull size, though there were plenty of anomalies, but they were eventually able to date some of the teeth, using electron spin resonance and uranium-thorium dating, and yes, your dating is about right. As to skull or brain size, smaller than habilis and quite a bit smaller than erectus, but actually larger than floresiensis, which clearly tells us, doesn’t it, that there hasn’t necessarily been this enlargement of brain size over time for all members of the Homo genus.

Canto: Yes, interesting – floresiensis, do we know anything of their lifestyle, tools, decorations…?

Jacinta: The most recent dating of floresiensis has them living until about 50,000 years ago at the latest. So much more recent than naledi. The cave where they were found yielded over 10,000 stone artefacts similar to those associated with the much larger-brained H erectus, from whom they may have learned a few things. With a brain quite a bit smaller than that of H naledi. Surely a cautionary tale.

Canto: Right – and they’re not even looking at brains, they’re looking at skulls, and making possibly unwarranted assumptions.

Jacinta: Okay so there’s a lot more to say on this topic – about Homo naledi alone, never mind the many other species or pseudo-species, so we’ll have to turn this into an ongoing series. I’ve been reading and listening to a lot of very smart people, which has made me feel quite dumb and shallow on the topic – no Dunning-Kruger effect for me at least.

Canto: Well, even as dilettantes we’ve come up with some reasonable skeptical queries – about brain size, and more on that next time, about tool use or the lack of evidence for it, and the lack of evidence of anything re H naledi outside of Rising Star. So, next time…

References

How did humans come to be? DW documentary

Homo Naledi – Ancient Human (North 02 – video)

D Reich, Who we are and how we got here, 2018

M Böhme et al, Ancient bones, 2020

https://en.wikipedia.org/wiki/Chimpanzee–human_last_common_ancestor

https://humanorigins.si.edu/evidence/human-fossils/species/homo-heidelbergensis

https://australian.museum/learn/science/human-evolution/homo-antecessor/

How Homo Naledi is Changing Human Origins | University Place (video)