Oreopithecus – Earth-logs

Charles Darwin famously suggested that humans evolved from apes, and since great apes (chimpanzees, bonobos and gorillas) live in Africa he reckoned it was probably there that the human ‘line’ began. Indeed, the mitochondrial DNA of chimpanzees (Pan troglodytes) is the closest to that of living humans. Palaeoanthropology in Africa has established evolutionary steps during the Pleistocene (2.0 to 0.3 Ma) by early members of the genus Homo: H. habilis, H. ergaster, H. erectus; H. heidelbergensis and the earliest H. sapiens. Members of the last three migrated to Eurasia, beginning around 1.8 Ma with the individuals found at Dmanisi in Georgia. The earliest African hominins emerged through the Late Miocene (7.0 to 5.3 Ma): Sahelanthropus tchadensi, Orrorin tugenensis and Ardipthecus kadabba. Through the Pliocene (5.3 to 2.9 Ma) and earliest Pleistocene two very distinct hominin groups appeared: the ‘gracile’ australopithecines (Ardipithecus ramidus; Australopithecus anamensis; Au. afarensis; Au. africanus; Au. sediba) and the ‘robust’ paranthropoids (Paranthropus aethiopicus; P. robustus and P. boisei). The last of the paranthropoids cohabited East Africa with early homo species until around 1.4 Ma. Most of these species have been covered in Earth-logs and an excellent time line of most hominin and early human fossils is hosted by Wikipedia.

All apes, including ourselves, and fossil examples are members of the Family Hominidae (hominids) which refers to the entire world. A Subfamily (Homininae) refers to African apes, with two Tribes. One, the Gorillini, refers to the two living species of gorilla. The other is the Hominini (hominins) that includes chimpanzees, living humans and all fossils believed to be on the evolutionary line to Homo. The Tribe Hominini is defined to have descended from the common ancestor of modern humans and chimps, and evolved only in Africa. As the definition of hominins stands, it excludes other possibilities! The Miocene of Africa before 7.2 Ma ‘goes cold’ as regards the evolution of hominins. There are, however fossils of other African apes in earlier Miocene strata (8 to 18 Ma) that have been assigned to the Family Hominidae, i.e. hominids, of which more later.

Much has been made of using a ‘molecular clock’ to hint at the length of time since the mtDNA of living humans and chimps began to diverge from their last common ancestor. That is a crude measure at it depends entirely on assuming a fixed rate at which genetic mutation in primates take place. Many factors render it highly uncertain, until ancient DNA is recovered from times before about 400 ka, if ever. The approach suggests a range from 7 to 10 Ma, yet the evolutionary history of chimps based on fossils is practically invisible: the earliest fossil of a member of genus Pan is from the Middle Pleistocene (1.2 to 0.8 Ma) of Kenya. Indeed, we have little if any clue about what such a common ancestor looked like or did. So the course of human evolution relies entirely on the fossil sequence of earlier African hominins and comparing their physical appearances. Each species in the African time line displays two distinctive features. All were bipedal and had small canine teeth. Modern chimps habitually use knuckle walking except when having to cross waterways. As with virtually all other primates, fossil or living, male chimps have large, threatening canines. In the absence of ancient DNA from fossils older than 0.4 Ma these two features present a practical if crude way of assessing to when and where the hominin time line leads.

In 2002 a Polish geologist on holiday at the beach at Trachilos on Crete discovered a trackway on a bedding plane in shallow-marine Miocene sediments. It had been left by what seems to have been a bipedal hominin. Subsequent research was able to date the footprints to about 6.05 Ma. Though younger than Sahelanthropus, the discovery potentially challenges the exclusivity of hominins to Africa. Unsurprisingly, publication of this tentative interpretation drew negative responses from some quarters. But the discovery helped resurrect the notion that Africa may have been colonised in the Miocene by hominins that had evolved in Europe. That had been hinted at by the 1872 excavation of Oreopithecus bambolii from an Upper Miocene (~7.6 Ma) lignite mine in Tuscany, Italy – a year after publication of Darwin’s The Descent of Man.

Lignites in Tuscany and Sardinia have since yielded many more specimens, so the species is well documented. Oreopithecus could walk on two legs, its hands were capable of a precision grip and it had relatively small canines. Its Wikipedia entry cautiously refers to it as ‘hominid’ – i.e. lumped with all apes to comply with current taxonomic theory (above). In 2019 another fascinating find was made in a clay pit in Bavaria, Germany. Danuvius guggenmosi lived 11.6 Ma ago and fossilised remains of its leg- and arm bones suggested that it could walk on two legs: it too may have been on the hominin line. But no remains of Danuvius’s skull or teeth have been found. There is now an embarrassment of riches as regards Miocene fossil apes from Europe and the Eastern Mediterranean (Sevim-Erol, A. and 8 others 2023. A new ape from Türkiye and the radiation of late Miocene hominines. Nature Communications Biology, v. 6, article 842.; DOI: 10.1038/s42003-023-05210-5). A number of them closely resemble the earliest fossil hominins of Africa, but most predate the hominin record there by several million years.

Phylogenetic links between fossils assigned to Hominidae found in Africa and north of the Mediterranean Sea. (Credit: Sevim-Erol et al. 2023, Fig 5)

Ayla Sevim-Erol of Ankara University, Turkiye and colleagues from Turkiye, Canada and the Netherlands describe a newly identified Miocene genus, Anadoluvius, which they place in the Subfamily Homininae dated to around 8.7 Ma. Fragments of crania and partial male and female mandibles from Anatolia show that its canines were small and comparable with those of younger African hominins, such as Ardipithecus and Australopithecus. But limb bones are yet to be found. Around the size of a large male chimpanzee, Anadoluvius lived in an ecosystem remarkably like the grasslands and dry forests of modern East Africa, with early species of giraffes, wart hogs, rhinos, diverse antelopes, zebras, elephants, porcupines, hyenas and lion-like carnivores. Sevim-Erol et al. have attempted to trace back hominin evolution further than is possible with African fossils. They compare various skeletal features of different fossils and living genera to assess varying degrees of similarity between each genus, applied to 23 genera. These comprised 7 hominids from the African Miocene, 2 early African hominins (Ardipithecus and Orrorin) and 10 Miocene hominids from Europe and the Eastern Mediterranean. They also assessed similarities with 4 living genera, Homo, orang utan (Pongo), gorilla and chimp (Pan).

The resulting phylogeny shows close morphological links within a cluster (green ‘pools’ on diagram) of non-African hominids with the African hominins, gorillas, humans and chimps. There are less-close relations between that cluster and the earlier Miocene hominids of Africa (blue ‘pool’) and the possible phylogeny of orang utans (orange ‘pool’). Sevim-Erol et al. note that African hominins are clearly more similar and perhaps more closely related to the fossils of Europe and the Eastern Mediterranean than they are to Miocene African hominids. This suggests that evolution among the non-African hominids ceased around the end of the Miocene Epoch north of the Mediterranean Sea. But it may have continued in Africa. Somehow, therefore, it became possible late in Miocene times for hominids to migrate from Europe to Africa. Yet the earlier, phylogenetically isolated African hominids seem to have ‘crashed’ at roughly the same time. Such a complex scenario cannot be supported by phylogenetic studies alone: it needs some kind of ecological impetus.

The Mediterranean Basin at the end of the Miocene Epoch when the only water was in the deepest parts of the basin. (Credit: Wikipedia, Creative Commons)

Following a ‘mild’ tectonic collision between the African continent and the Iberian Peninsula during the late Miocene connection between the Atlantic Ocean and the Mediterranean Sea was blocked from 6.0 to 5.3 Ma. Except for its deepest parts, seawater in the Mediterranean evaporated away to leave thick salt deposits. Rivers, such as the Rhône, Danube, Dneiper and Nile, shed sediments into the exposed basin. For 700 ka the basin was a fertile, sub-sea level plain, connecting Europe and North Africa over and E-W distance of 3860 km. There was little to stop the faunas of Eurasia and Africa migrating and intermingling, at a critical period in the evolution of the Family Hominidae. One genus presented with the opportunity was quite possibly the last common ancestor of all the hominins and chimps. The migratory window vanished at the end of the Miocene when what became the Strait of Gibraltar opened at 5.3 to allow Atlantic water. This resulted in the stupendous Zanclean flood with a flow rate about 1,000 times that of the present-day Amazon River. An animation of these events is worth watching

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.

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.

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)

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.

Scrum — 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…

Tag: Oreopithecus

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