Human Evolution: Australopithecus

Before getting started, a word on terminology: I will try to use genus names (Homo, Australopithecus, etc) or their abbreviations as much as possible, instead of saying ‘hominid’ or ‘hominin’, the reason being that those two words have traditionally been thrown around erroneously and without definition (is it only Homo? Is it all the Old World apes?). If I do happen to say hominin, I will mean those apes that diverged from the chimps already, not necessarily the genus Homo. Hominid I reserve to members of Homo.

The australopithecines are of particular interest to palaeoanthropology because in our current view of human evolution, the genus Homo is descended from a species of Australopithecus, with certainty being clouded by necessary ambiguities in the interpretation of the earliest representatives of Homo and thus the link from Australopithecus to Homo.
That missing link has putatively been described just this year (2010, for the time travellers), highlighted in the cladogram above in red, and a CT reconstruction of the skull shown below.
This is Australopithecus sediba, a 1.95-1.78 Ma descendant of Australopithecus africanus and the australopithecine most closely resembling Homo, a realisation that is both intuitive and supported by the cladogram, found in the Cradle of Humankind, South Africa, a Plio-Pleistocene complex of cave deposits renowned for its palaeontological (especially palaeoanthropological) finds. It shares a number of traits with Homo erectus (but not H. rudolfensisH.habilis, who form a separate clade that does not lead to H. sapiens), including the structure of the nasal area and the area around the eyes (I am trying to avoid those idiotically draconian names the vertebrate people give to bone structures). However, it is not quite within the Homos, as we can tell from its postcranial skeleton (i.e. anything not belonging to the head) and some dental features – it supports the leading hypothesis of human evolution by mosaic evolution: the appearance of traits in different areas at different times, not ‘all at once’.

First it was the body that changed, to go from an ape adapted to living in trees (Australopithecus) to an ape adapted to a bipedal lifestyle (Homo). This involved changes especially in the pelvic region, the hips and the legs. But these changes had largely occurred in Australopithecus; the cranial size that is a hallmark of Homo had not been developed yet, even though changes in dentition that resemble that seen in Homo started appearing – this is a pattern typical of mosaic evolution.

But the topic here is Australopithecus, not Homo or the evolution of humans. That intro was just to point out why they are particularly important to study. The australopithecines, like other early hominids, had relatively small brains (compared to humans) and exhibited other similarities to the great apes, including a similar life history (the concept of a childhood and adolescence is non-existent in the great apes and is a characteristic of the later hominids, starting with H. antecessor). And considering that Homo most likely evolved from Australopithecus, it is of utmost importance to see where and how they differed.
The above table, which is more of an intuitive lumping together of the hominins into groups, tells us where Australopithecus (Au. spp.) is on the grand scale of things: an archaic hominin, and the picture below shows when they were living (Au. sediba is not featured, but it would be in a box above Au. afarensis).
Instead of going through them in chronological order, we will (to reflect my recent booming interest in the history of biology) look at them in order of discovery.

Australopithecus was first set up as a genus in 1925, following the 1924 discovery of a skull belonging to Au. africanus in South Africa, with other finds, also from South Africa, allowing us to determine that Au. africanus lived at least between 3 and 2.4 Ma. While we don’t have every single bone of the postcranial skeleton of Au. africanus, we do have at least one of each of the long bones (arms, legs), and they deliver a wealth of information on their own. We know that it was facultatively bipedal (i.e. it could walk if it wanted/had to), but was adapted to living in trees (arboreal). With its large chewing teeth, the skull resembles that of an ape, with the only major difference being the reduced canines – from overall size and cranial volume, it is strictly average (460 m³). Sexual dimorphism was also quite stark.

In 1959, a Tanzanian subcranium was found and described as a new genus and species Zinjanthropus boisei. This was apparently seen as controversial, as in 1960, it was transferred to Paranthropus (P. boisei) and later still, in 1967, it was named as Au. boisei – however, the general consensus nowadays seems to be that it is a Paranthropus, and thus will not be discussed further.

The first specimen of Au. afarensis, a mandible, was discovered in 1974 in Tanzania and described in 1978. It lived from 3.7 (4?) to 3 Ma and its remains, so far, have only been found in East Africa (mostly Ethiopia). We have many fragments and overall a more complete picture of what Au. afarensis looked like. It seemed to be able to walk – as testified by fossil footprints, although from a strict functional morphology perspective, Au. afarensis was not well suited for this locomotion type, as its shoulder and hands were more adapted for tree climbing and swinging. Its braincase had a volume between 400 and 550 cm³, while the dentition was somewhat varied, with incisors smaller than a chimp’s, but larger molars and premolars. As for its size, it was considerably shorter than modern humans and weighed between 30 and 45 kg.

The next discovered species was Au. anamensis from Kenya in 1994 and described in 1995. As with Au. afarensis, it has so far only been found in Eastern Africa, from sites ranging in age from 4.2 to 3.9 Ma. While we don’t have many remains, most of them being jaws and teeth, with some limbs, the main difference between it and Au. afarensis seems to be in the dentition – the limbs are more or less identical. This has led to some researchers placing Au. anamensis as a regional variety of Au. africanus, or a primitive version of Au. africanus rather than a separate species, a not-too-crazy idea. The only way to resolve this is, of course, to find more fossils (or, possibly, to take another, closer look at the fossils we already have!)

In 1999, Australopithecus garhi from a 2.5 Ma locality in Ethiopia was described from a cranium and teeth which are unusually large (hence ‘megalodont’). The status of this taxon is, however, in limbo at the moment, as new fossil finds from the same area hint at it being a P. boisei instead.

The latest australopithecine, Au. sediba, I already talked about in the beginning.

So to summarise them as a whole: in general, they were robust, with a small head. They had a broad, flaring pelvis from which the human pelvis is derived; it was sufficient to allow bipedal locomotion, but Australopithecus was still very much an arboreal genus. However, it is this pelvis that separates it from the other apes and makes it a distinct genus in the lineage that later gave rise to humans. They were short – the only Homos this short were members of the island dwarf species Homo floresiensis. The earliest Homos known are ~2.5 million years old (older than Au. sediba), disproving the hypothesis (that may or may not have been put forward, I don’t know) that Australopithecus became Homo anagentically – the existence of many cotenmporaneous Au. species is now undoubted, and it is from one of those species that Homo evolved from (see diagram below).
There is not much more to say about the australopithecines as such, but I do want to conclude with a note on species concept in palaeontology. As I already said at the start, we have tremendous difficulties in really separating different species in the fossil record, especially in a clade such as the hominins, where convergence and homoplasy run rampant and different subjective interpretations prevail, simply because all we have are preserved hard parts (for the vertebrates, that’s bones and teeth). That’s not much to go by, and using the regular biological species concept (species are different if they can’t breed and produce fertile offspring) is impossible, as is recognising sympatric taxa (those that differ behaviourally and thus don’t mate, even though they have the same morphology).

In palaeontology, in general we use three species concepts, each with subtle differences in methodology and underlying logic: for example, in the phenetic concept, all characters are given equal weight regardless of their percieved importance, whereas in the (nowadays much more used, and for good reason) phylogenetic species concept, each character is given a certain weight according to its status as an apomorphy (unique trait) or plesiomorphy (retained ancestral trait). The monophyletic species concept is rarely applied, as it needs knowledge that we don’t have, namely proof of our specimens’ monophyly (proof that is acquired by applying the two previous methods!)

So the eternal problem is how to recognise these apomorphies. What size threshold do you need to pass to go from an Austealopithecus to a Homo, for example. This is where subjectivity naggingly comes in, in the form of a researcher’s opinion on the relative importance of stasis in the fossil record. If one agrees completely with punctuated equilibrium, he will have more taxa in his taxonomy, since he will argue for ‘jumps’ from one character state to another. But a researcher who favours gradualistic evolution (through anagenesis) will recognise less species, as he will argue for slow transitions and fuzzy boundaries. None of them are wrong, of course, but they’re not right either. It’s a paradox of sorts, but it cannot be escaped – it’s the nature of palaeontology, most unfortunately.

And it is unfortunate, not only because it leads to often longly drawn out discussions that end nowhere, but also because it is very easy for the uneducated and plain idiotic to misunderstand. Cue the creationist dolt Harun Yahya, who in his book ‘The Evolution Deceit‘ has this to say: “Briefly, Australopithecines [sic] have no link with humans and they are merely an extinct ape species.” Yes, they are. And humans are merely an extant ape species. It is rather unfair of me to bring this guy up due to his lack of critical thinking faculties, but this is an oft-used argument (“Archaeopteryx isn’t a dinosaur, it’s a bird!”) that stems directly from this very basic problem of palaeontology, that laying a line between species is nigh impossible, when we zoom in to the individual specimen level.

Diagram Sources:
Berger, L. R., de Ruiter, D. J., Churchill, S. E., Schmid, P., Carlson, K. J., Dirks, P. H. G. M. & Kibii, J. M. 2010. Australopithecus sediba: A New Species of Homo-Like Australopith from South Africa. Science 328, 195-204.

Klein, R. G. 2009. Darwin and the recent African origin of modern humans. PNAS 106, 16007-16009.

Wood, B. 2010. Reconstructing human evolution: Achievements, challenges and opportunities. PNAS 107, suppl. 2, 8902-8909.

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