stuff on human ancestry 1: the australopithecines, mostly
All the evolution we know of proceeds from the vague to the definite
C S Pierce
I was in a bookshop yesterday, where I picked up a copy of Yuval Noah Harari’s book Sapiens and had a gander at the back cover. I read one sentence, which went something like ‘100,000 years ago there were at least six species of Homo sapiens, now there is only one.’ Or maybe it was just ‘six species of Homo‘. It resonated with me, because it’s been a while since I’ve researched and written about the ever-fascinating topic of human origins, a topic that resurfaced for me recently on reading an essay, ‘Lucy on the earth in stasis’ by Stephen Jay Gould in his 1996 collection Dinosaur in a haystack. The essay promoted his ‘punctuated equilibrium’ view of evolution, as it reported that Australopithecus afarensis appeared to be the only hominin type in existence for a period of almost a million years, from approximately 4.9 million years ago to 4 million years ago, after which there was a relatively rapid radiation of hominid species. I could only take the essay on trust, but I maintained the thought that I should investigate whether this claim still held, some twenty-three years later. And that, further, I should investigate whether we were any clearer about our descent, as the last surviving species of that apparent radiation.
And by the way, for my education’s sake I need to straighten out the difference between hominids and hominins. We humans are both, apparently. The hominids, or great apes, include four genera: Pongo, the orang-utangs, of which there are three extant species; Pan, of which there are two species, chimps and bonobos; Gorilla (two species), and Homo, of which there’s only one extant species, but many extinct ones including Neanderthals. The term ‘hominid’ has broadened over time. The term ‘hominin’ is more restrictive, referring only to those species ancestral or related to humans, since the split from the chimp and bonobo line. This explains, I hope, why we are both hominids and hominins. Clearly, though, I should stick to the term hominin for this post, or series of posts.
Anyway, I was surprised to read this claim about the state of human play 100,000 years ago. The old Bill Bryson question, How do they know that? came to mind, but I also felt skeptical, as I seemed to remember that the number was smaller – possibly dependent on whether you were a lumper or a splitter.
We know of course that our closest living relatives are (equally) chimps and bonobos, and the latest dating of our divergence from their line is 4 to 7 million years (according to Wikipedia, but Gould put it at 6 to 8 mya, and this video from the American Museum of Natural History gives it more ‘precisely’ at 7 mya, and another Wikipedia article gives the figure as 6.5 to 5.5 mya, so who knows?) There are a couple of possibilities for our last shared ancestor – Sahelanthropus tchadensis and Orrorin tugenensis – but their more or less competing claims are mired in uncertainty, due to the extreme sparsity of material. It may well be that neither of them fit the bill.
When they look at the evidence from early hominins, researchers are particularly interested in signs of bipedalism, which have been argued to exist in S tchadensis due to the placement of its foramen magnum (the hole in the skull through which the spinal cord passes) towards the back – though this placement has been disputed, quelle surprise. In any case, these earliest hominins evolved during the Pleiocene epoch into the definitely bipedal australopithecines. The bipedal adaptation is so important to the emergence of Homo sapiens that it has been the subject of a great deal of speculation, hypothesis and argumentation. It’s likely that there were a variety of converging factors that favoured this trait’s development. For example, it provided a wider visual field, especially on the ground; it left the hands free to grasp and carry food; it enabled long-distance running, and it reduced the expenditure of energy. However, bipedalism appears to have been a slow development, and early australopithecines such as A afarensis likely spent a lot of time in trees. This is supported by anatomical features such as longer arm-bones, curved fingers, a shallow rib-cage and strong clavicular anchors for brachiation (swinging from the arms among branches).
Over time there were anatomical changes favouring bipedality. These included greater robustness of the ankle and knee joints, and changed positioning of the foramen magnum, the femur and the spine, to support changes to the centre of gravity. But the changes which have had the most long-lasting, even at times dire effects, have been those to the pelvic region. The strengthening and widening of this region, including the ilium, ischium and pubis, to support an upright stance, has to a serious degree compromised the process of childbirth. It’s been observed that australopithecines share with modern humans a sexual dimorphism relating to the lumbar vertebrae, allowing the spinal curvature of females to become more pronounced during pregnancy, which helps to better distribute the weight of the unborn child and to reduce fatigue and maintain stability of posture for the mother. However, the changed shape of the pelvis and the consequent narrowing of the birth canal resulted in what has become known as ‘the obstetrical dilemma’. Unlike virtually every other mammalian species, humans face major difficulties and dangers in childbirth, which require others – midwives or other medical professionals – to assist in the process (for example, neonatal rotation is often necessary for safe delivery). A ‘solution’ to this dilemma, which appears to have evolved over time, is a comparatively short gestation period – the time spent in the womb – to give a greater opportunity for both mother and child to survive the birth. This of course leads to a longer period of childhood dependence as it develops outside the womb. Apparently, a modern human baby is born with approximately 25% of full brain development, compared to 45-50% in other primates. Brain size at birth is limited due to the obstetric dilemma, and greater neoteny is the result.
Encephalisation, which refers to a growth in brain size or mass relative to body size, is now seen as a later development in the human story than bipedalism. Brain size in general has become very questionable as a measure of complex evolutionary development – witness those smart corvids – and it’s worth noting that the Neanderthal brain is on average larger than ours. What’s important, though, is brain structure – something we can’t really look at vis-a-vis our ancestors. However it is reasonable to assume that our much larger brain size compared to australopithecines is largely due to growth in the temporal lobes and the prefrontal cortex. In fact all regions have grown, including the cerebellum, traditionally associated with fine motor control and balance, but more recently connected with cognitive function and language.
But let me return to the hunt for the hominin links from the other great apes to Homo sapiens. In the mid-nineties, two new examples of early hominins were discovered, Australopithicus anamensis and Ardipithecus ramidus. I’m guessing that Gould didn’t know about these discoveries when he wrote his essay, as they seem to have punctured his punctuated equilibrium thesis, at least as regards hominins. Anyway the A anamensis species is believed to have lived from about 4.2 to 3.8 million years ago, and the A ramidus specimens have been dated to around 4.4 million years ago, but interestingly, A afarensis, the principal subject of Gould’s essay, is now believed to have lived from 3.9 million years ago to 2.9 million years ago – that’s a million years after Gould’s stated range. The australopithecines first came to our attention in 1925 when Raymond Dart described Australopithecus africanus from specimens found in South Africa. A africanus is a more gracile type, and may well be in the direct line to humans, though there’s been a lot of dispute about the dating and classifying of different specimens. A africanus is generally thought to be a more recent species than A afarensis, another gracile type. So maybe we can link A africanus back to A afarensis, which in turn can be linked back to S tchadensis, with some intermediate missing links. But then there’s another recently discovered species, Australopithecus sediba, which has been dated to around 2 million years ago and is thought to be a transitional species between A africanus and either Homo habilis (which some prefer to describe as Australopithecus habilis) or Homo erectus. Another gracile species discovered in the nineties, A garhi, dating to about 2.5 million years ago, also seems to fit as a species connecting Australopithecus and Homo. From what I’m reading, the fragmentary nature of these finds, together with obvious questions as to whether particular specimens are typical of whole species (type specimens are often juveniles, which might not be such a good idea), are the main barriers to pinning down the precise lines of succession. That’s why every new discovery is such a treasure.
I haven’t mentioned Ardipithecus or Paranthropus as yet. In the nineties specimens were found in the Afar triangle in East Africa, and classified as Ardipithecus ramidus (around 4.4 million years ago, with uncertain evidence of bipedality, and some evidence of reduced sexual dimorphism) and Ardipithecus kadabba (about 5.6 mya, possibly an ancestor of A ramidus, but known from only a few teeth and bones – the type specimen being a bit of mandible with an attached molar). It’s possible, according to some researchers, that Ardipithecus, Orrorin, and Sahelanthropus all belong to the same genus.
I’ll have a look at Paranthropus, apparently a more robust distant cousin of ours, then move forward to the Homo genus, next time.
References
Australopithecus Evolution (video), by Henry the PaleoGuy, 2019
Seven million years of human evolution (video), American Museum of Natural History, 2018
https://en.wikipedia.org/wiki/Human_evolution
https://en.wikipedia.org/wiki/Australopithecus
https://en.wikipedia.org/wiki/Australopithecus_afarensis
https://en.wikipedia.org/wiki/Australopithecus_africanus
https://en.wikipedia.org/wiki/Australopithecus_sediba
https://en.wikipedia.org/wiki/Australopithecus_anamensis
https://en.wikipedia.org/wiki/Ardipithecus
http://humanorigins.si.edu/evidence/human-fossils/species/ardipithecus-ramidus
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