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…