Australopithecus sediba: is she or is she not a human ancestor?

English: Malapa Hominin 1 (MH1) left, Lucy (AL...
Australopithecus sediba 1 (MH1) left, Au. afarensis( AL 288-Lucy) centre and Au. sediba 2 (MH2) right. (credit: L. R. Berger, University of the Witwatersrand, via Wikipedia)

The remarkable find of two well-preserved skeletons of a 2 Ma hominin in a South African cave in 2008 and publication of their preliminary analysis in 2011  seemed set to shake up human origins research. There was a more or less complete hand – indeed an entire arm and shoulder – a lower leg with ankle bones, a near-complete head and lots more besides. Most was from one female individual, but significant bits from two others that allowed a well-supported reconstruction of the new species Au. sediba. The discoverer, Lee Berger of The University of the Witwatersrand, South Africa (well he initiated the dig, but his young son found the first critical material) is so excited 5 years on that he uses hip-hop phraseology, she ‘got swag’, presumably assuming that means pretty cool (Gibbons, A. 2013. A human smile and funny walk for Australopithecus sediba. Science, v. 340, p. 132-133), but on the street there are other meanings and attitudes towards the phrase and unwary use is not advised.

More details now have emerged in a special issue  of Science introduced by Berger  in less fulsome language (Berger, L.R. 2013. The mosaic nature of Australopithecus sediba. Science, v. 340, p. 163). As the title suggests, the surprise lies in almost every critical part of the species. Although the spine shows curvature (lordosis) needed for an animal evolved from a quadruped to bipedality in order to balance when upright, the ankle bone is unlike the flat-based human one, being pointed as is that of chimpanzees. As a result walking would have involved an unusual and perhaps unsteady gait; the individuals did fall over into a death pit and one commentator thought the gait might have seemed ‘provocative’. An unusual knee bone is thought to be an evolved countermeasure to such exaggerated mincing.  Despite the very human-like hand, extremely long arms and shoulders remarkably like those of the favoured jacket of a star of the BBC series The Dragons’Den point to habitual clambering in trees. Authors of a report on dentition suggest a close similarity to that of the Au. africanus, living at the same time and also found in the same system of fossil-rich caves north-west of Johannesburg, South Africa. Controversially, the tooth team suggests a closer similarity of both to early Homo species than to earlier australopithecines in East Africa, which would shift the focus of human origins to southern Africa. Counter to that view is a find of 400 ka-older, putative human remains in Ethiopia. Yet they take the form of a lower jaw that resembles that of Au. sediba.

The emerging, more detailed picture is not tidy, as suspected from early examination of the Malapa hominins. One thing is for sure, the South African caves are being swarmed over, which paid dividends in 2011 just 15 km from the Malapa cave with another embarrassment of riches at Sterkfonein in the form of abundant foot bones of a currently un-named species of roughly the same age. Things are beginning to take on an element of national pride, with ‘The Birthplace’ at stake: Kenya, Tanzania, Ethiopia or South Africa?

Hominin evolution becoming a thicket

Scientific American is renowned for its eminently readable reviews of both emerging and perennial topics. Its February 2013 issue takes on one that is guaranteed to run and run; the evolutionary course that produced us (Harman, K. 2013. Shattered ancestry. Scientific American, v. 308 (February 2013), p. 36-43). Since its launch Earth Pages has covered much of the new science in the field but did not anticipate the depth of the stir towards which it has led.

Australopithecus afarensis reconstruction
Australopithecus afarensis reconstruction (credit: Wikipedia)

For a decade it has become increasingly clear that anatomically modern humans are unique in one respect: they are the first species in perhaps 4 million years to be the sole extant member of the cladistic tribe Hominini. As recently as 30 ka Homo sapiens shared the planet with Neanderthals, Denisovans, H. erectus and H. floresiensis. At the time the genus Homo emerged around 2.0-2.5 Ma ago there were at least four other fossil groups that shared the major characteristic of upright gait, all australopithecines in ‘robust’ and ‘gracile’ guises.

As time goes by there will likely be more fossil discoveries that show important anatomical signs of other novel evolutionary divergence, which therefore warrant new species. Pliocene-Pleistocene time is becoming crowded, and the more diversity in its fossil record the less likely it is that some clear evolutionary pathways can be devised to explain just what was going on. Katherine Harmon of Scientific American’s editorial team touches on the thorny issues of upright walking and gait, tree climbing, precise use of the fingers and thumb, and brain size that are raised by 22 species; 2 living and 20 extinct.

Genetics clearly indicates that our nearest living relatives belong to two species in the genus Pan(chimpanzees and bonobos). It has been generally assumed that the common ancestor of this extant kinship some 8 Ma back was chimp-like, and that evolutionary divergence from its habits and anatomy produced the growing ‘bramble patch’ of hominin evolution. That assumption is based on the principle of parsimony, i.e. the simplest view of the evidence – what there is now and fragments from the past eight million years. The trouble is there is a dearth of fossils that can be said to be en route to chimps in some way.

In fact today’s chimps and bonobos are more or less restricted to clambering in tropical forest habitats, for which they are well-adapted. Maybe they are the survivors of evolutionary vagaries just as complex as those leading to us. For one thing, almost embarrassingly, their brain size is substantially larger than those of quite a few fossil hominins; and why not? How they behave socially may possibly have arisen as part of their specialisation too, of which more shortly. Our big difference from them is being supreme generalists, as well as consciousness.

All the fossils classed as hominins show some signs of being able to walk upright, classically the forward position of the foramen magnum at the base of the skull where it joins to the backbone, but in some cases merely the geometry of the hip joint to the pelvis for that is all that has been found. Yet that anatomical likelihood glosses over the vital detail of the actual gait – heel-to-toe like us (Australopithecus afarensis),  on the outside edge of the foot akin to chimps (Ardepithecus ramidus) or differently again but possible as efficient as us (Au. sediba). Then there is the matter of arboreal abilities: chimps are masters despite their bulk, but every hominin whose foot bones have been found shows some evidence of grasping with the big toe. Indeed humans are pretty nimble climbers but do not brachiate from branch to branch.

As regards the hands, an interesting point is that while chimpish knuckle walking is not seen in fossils, Ardipithecus probably could walk on all fours with hands flat on the ground but had fingers quite capable of precise manipulation, an ability shown spectacularly well by 2 Ma old Au. sediba. Upright walking may have evolved more than once, and it is even possible that chimps evolved specifically for climbing in forestlands, their highly adapted grasping hands only capable of knuckle walking on the ground.

English: Fossil of Oreopithecus bambolii, an e...
Oreopithecus bambolii from the Upper Miocene of northern Italy(credit: Wikipedia)

The late-Miocene of Africa – the likely time range for the Pan-Homo common ancestor – is a fossil desert as regards primates. Yet its Italian equivalent has yielded a fascinating and well-preserved creature; Oreopithecus bambolii has skeletal features compatible with an upright posture and bipedal locomotion. Until the African Miocene yields something more appropriate, Oreopithecus is a candidate for a common ancestor, and interesting in another respect. Its dentition does not include prominent canine teeth that in the predominantly vegetarian, though occasionally carnivorous, Pan species serve well in their aggression-based, hierarchical social systems, as they do in the even more spectacular baboons.

Christopher Boehm, primate behaviouralist cum anthropologist, in his recent book Moral Origins (2012 Basic Books, ISBN-13: 978-0465020485) uses the principle of parsimony to reconstruct the social system of the Miocene Pan-Homo common ancestor from those of chimps and surviving human hunter-gatherers. His thesis is that it was centred on the hierarchical dominance of ‘alpha’ males, as is that of chimps. Prolonged social selection in hominin evolution largely tempered such a ‘Big Man’ tendency through a variety of strategies directed by majorities. Social punishments, including capital punishment, evolved to combat free-loading, theft and individual dominance in favour of cooperative egalitarianism. Such measures developed increasingly conscious self-suppression of such traits that eventually manifested themselves as what we now regard as human morals. Boehm considers that this psychological trend in evolution accelerated once Homo sapiens began hunting of large prey animals that added substantially to diet.

Aggressive male chimpanzee (Credit: Daily Mail)
Aggressive male chimpanzee (Credit: Daily Mail)

There is a major problem for this view: like Oreopithecus every well-preserved hominin species, even the earliest Sahelanthropus tchadensis, do not have prominent canines irrespective of whether they show evidence of at least partial meat-eating or pure vegetarianism. For some species with many fossil members, such as Au. afarensis, there are signs of sexual dimorphism – larger males than females – but that does not necessarily signify hierarchical social behaviour. With the appearance of H. erectus that difference wanes to the present slight differences between modern male and female humans.

Agressive male humans, note gumshields (credit: John_Scone via Flickr)

If it is valid – and who knows? – for morphology to give clues to social behaviour, then it is equally likely that the beginnings of the hominin evolutionary thicket may well have involved a trend in social behaviour towards cooperative action; 8 million years ago. For generally small, gracile creatures with habits no more threatening to the big predators of the African savannahs that that of the porcupine, there would have been a powerful selection pressure towards a united front. Of course, in the last ten thousand years since the shift to economic strategies based on storable surpluses and their expropriation, hierarchical social systems with violence at their heart emerged among modern humans. Judging by the body shapes and dentition of extant ‘alphas’, as in capital’s boardrooms and among the frontbenchers at Westminster, anthropology clearly is in need of some refinement…

Eats barks leaves nuts and fruits

English: The Malapa site valley, looking North...
The Malapa valley South Africa, where Australopithecus sediba was found. (Credit: Lee R. Berger via Wikipedia)

The first stone tools and bones that had been cut by them, found in rocks  dated at 2.5-2.6 Ma in the Bouri area of Ethiopia’s Afar Depression, have generally been taken as a sign that their invention was connected with more easily accessing meat for food. A corollary of this idea is that it was the introduction of meat into the hominin diet that helped ‘fuel’ the growth of their brains: meat-tools-brain interrelated in an evolutionary sense. There is a spatial link between such  tools and fossils of Australopithecus, but direct attribution of the tools to these australopithecines  has not been widely accepted. It is more generally accepted that a link to tools can be made with Homo habilis, but they lived, at the earliest, 200 to 300 ka later. The wear patterns on their teeth and association with animal bones bearing cut marks has been taken to indicate that at least part of their diet was meat.

Another approach to diet is to analyse the proportions of stable carbon isotopes (13C and 12C) in tooth enamel, which can discriminate between the ultimate plant source in their diet, i.e. between grasses that use  the C4 photosynthetic pathway and the C3 version used by woody and herbaceous plants. The isotopic ‘signature’ of plants is also passed on to animals, depending on what vegetation they eat, and so up the food chain to predators and the scavengers that depend on their leavings. South African Au. africanus of around 2.5 Ma ago show a definite  C4 preference as do local paranthropoids (‘robust’ australopithecine-like creatures) from around 1.8 Ma. The early humans H. habilis and H. ergaster also show the C4 isotopic proportions, which in both cases may be from a meaty diet or from a vegetarian component. The main point from these similar results, whatever the plant-meat proportions being consumed, is that these hominins were very different from chimpanzees in their eating habits, and probably as regards their habitats: savannah rather than woodlands respectively.

There are no reports of C-isotope research on Au. garhi teeth, but results from 2 Ma old Au. sediba found in South Africa have just been published (Henry, A.G and 8 others 2012. The diet of Australopithecus sediba, Nature, v. 487, p. 90-93) along with plant materials from dental plaque and tooth wear patterns. Au. sediba is notable for its very modern-looking hands and other ‘advanced’ features. Some believe it to have been closer to the direct line of human descent than a number of other hominin species, including the poor quality remains of H. habilis. So, did sediba eat meat? The forensic evidence suggests something unexpected. The C-isotope data points towards food dominated by C3 plants – less grasses and sedges, and more shrubbery. Tooth wear suggests bark was eaten, while plant remains from plaque indicate fruit leaves and wood. This is a feeding pattern more like that of chimpanzees than Homo species, Au. africanus and the paranthropoids  that are roughly contemporary with Au. sediba. Ecological analysis of the sediments which buried the hominin specimens suggest a seasonal climate and savannah biome with abundant C4 plants that supported grazing herds, mixed with possibly some denser woodland along drainages. This is a pattern familiar from living savannah chimpanzee bands.

English: The hand and forearm of Australopithe...
The hand and forearm of Australopithecus sediba (Credit: Peter Schmid, courtesy Lee R. Berger via Wikipedia)

So, despite being an ‘advanced’ hominin, by carrying clear signs of foods that were not consumed by meaty potential prey animals Au. sediba probably was not a meat eater. Yet species with strong C4 ‘signatures’ cannot be assigned to carnivory on C-isotope  evidence alone. One has to decide from other data, such as tooth-wear and plaque, whether this or that hominin ate grasses, those that clearly did not becoming candidates for dominantly meat-eating. How to detect a tool-using species with a mixed diet, akin to more modern humans, is a tough nut to crack.

Another candidate for earliest, direct human ancestor

The cranium of Malapa Hominid 1, Holotype of A...
The head of Australopithecus sediba. Image via Wikipedia

In May 2010 EPN commented on a new find from the famous fossil-rich caves of north-eastern South Africa; a new hominin species called Australopithecus sediba. At least one of them fell into a deathtrap shaft, died and remained unchewed without bones being spread far and wide. Inevitably, near-complete skeletons of individual hominins are soon pored over by dozens of specialists in human evolution, as they were for the much older Ardepithecus ramidus found in sediments of Ethiopia’s Afar Depression (see Early hominin takes over Science magazine in the November 2009 issue of EPN). Now there are two near-complete, well-preserved skeletons of Au. sediba and the palaeoanthropological world is agog. Dating to about 1.98 Ma the specimens represent the same time as do far less impressive remains of H habilis from Tanzania that were found with associated rudimentary stone tools. The first hint (just a fragment of upper jaw) of any remains that might be tagged ‘Homo’ dates to 2.3 Ma and is from Ethiopia, as are the first undoubted stone tools going back as far as 2.5 Ma, though lacking association with a maker.

Five consecutive papers on Au. Sediba occupy 22 pages in the 9 September 2011 issue of Science and make for startling reading. The first concerns the shape of its brain case, and therefore crudely its brain, discerned by tomographic X-ray scanning (Carlson, K.J. et al. 2011. The endocast of MH1, Australopithecus sediba. Science, v. 333, p. 1402-1407). It isn’t any bigger than that of other members of the genus but shows ‘some foreshadowing of the human frontal lobes’ and other shifts from the basic ape model that the authors imply are en route to human features. The next considers the two pelvis regions (Kibii, J.M. et al. 2011. A partial pelvis of Australopithecus sediba. Science, v. 333, p. 1407-1411); again australopithecine-like in the small size of the birth canal but with a hint of the S-shape of humans. Most astonishingly well-preserved are the fragile bones of a complete hand (Kivell, T.L.  et al. 2011. Australopithecus sediba hand demonstrates mosaic evolution of locomotor and manipulative abilities. Science, v. 333, p. 1411-1417), which convincingly shows the long thumb and short fingers (for a primate) that characterise Homo and are essential for a precision grip and making things. Actually, the thumb is longer relative to fingers (60%) than in humans (54%), but Lucy’s (Au. afarensis) was a closer match. No tools that such a hand might have created and wielded were found with the fossils. Then there is the foot (Zipfel, B. et al. 2011. The foot and ankle of Australopithecus sediba. Science, v. 333, p. 1417-1420), which, again, mixes human and australopithecine features, giving ‘a unique form of bipedality and some degree of arboreality’. The fifth paper (Pickering, R. et al. 2011. Australopithecus sediba at 1.977 Ma and implications for the origins of the genus Homo. Science, v. 333, p. 1417-1420) is as remarkable for the precision of U-Pb dating of speleothem (cave carbonates), which at 1.977+0.002 Ma far exceeds the workhorse Ar-Ar method used for most other hominins, as it is for the absolute age that precedes that of undisputed remains of humans.

In short, for Australopithecus sediba there is an embarrassment of riches unmatched until those of the 1.5 Ma old H. erectus (‘Turkana Boy’) found at Nariokotome in NW Kenya. To some extent this throws a flock of peregrines in among the palaeoanthropology pigeons, as an account of a meeting earlier in 2011, at which the bones were grandstanded, shows (Gibbons, A. 2011. Skeletons present an exquisite paleo-puzzle. Science, v. 333, p. 1370). Naturally, the authors are making the most of their material especially, it seems, its finder Lee Berger of the University of Witwatersrand, South Africa, the last author in all the papers. Comparisons with more australopithecine remains were said to be needed. The soon-to-be-famous hand has been said to be essentially like others from the same genus. While the remains of the creature’s pelvis could imply that its evolution was more driven by a need for efficient upright walking than to birth big-headed babies, the ankle shows a primitive trait that would have forced Australopithecus sediba to walk strangely as the heel bone is small and angled unlike that in human feet, which is broad and flat. But all the species’s features are combined in two near-complete individuals, whereas for the rest of its contemporaries, predecessors and near successors in time speculation is based on fragments of several individuals, none more so than in the case of the earliest agreed human, near contemporaneous H. habilis, which barely stands up to taxonomic scrutiny (Gibbons, A. 2011. Who was Homo habilis – and was it really Homo?  Science, v. 332, p. 1370-1371). Some would say that it was only the associated stone tools that assigned ‘Handy Man’ to more elevated status than slightly large-headed australopithecine. The fact is; stone tools were around since 2.5 Ma, at least in Ethiopia, and this newly found being could have handled them and even made them with its palpable dexterity. Finding tools and skeletons together is almost as rare as hens with teeth…