Was the Swamp Ape Bipedal?

by Marcel F. Williams 

Tuscany is renowned for its beautiful cities of Florence and Siena, and is  historically famous as the birthplace of the Italian Renaissance. But amongst the paleontological community, Tuscany is also known as the birthplace of an  extinct Late  Miocene ape-- that some paleontologist controversially believe may have been the earliest primate to walk predominantly on just two legs-- and possibly the earliest bipedal ancestor of humankind.

Oreopithecus bambolii first emerged on an ancient island bioprovince known as Tuscany-Sardinia (Tusco-Sardinia) sometime after 9 million years ago where it existed as the sole primate on the island until approximately  7 million years ago. Tuscany-Sardinia was  isolated from the rest of the Italian peninsula  and from  the rest of  continental Europe by the marine waters of the Late Miocene Mediterranean.

Skull of Oreopithecus bambolii

Between 11 to 9 million years ago, sea levels fell during a glacial period caused by the  deposition of  ice and snow on polar land masses. The lower sea levels allowed various mammals from  North Africa and Southern Europe  to enter the Tuscany region.   Small mammals from Europe apparently entered the isolated island region through rafting or swimming. But fauna from North Africa appears to have entered the region via ephemeral land bridges.

The mostly likely ancestor of Oreopithecus was a small African ape know as Mabokopithecus which  appears in the fossil record in Africa about 15 million years ago.  Oreopithecus and Mabokopithecus were both folivores (planet eaters) that shared a unique dental attribute in their upper molars: a well-defined hypocone-metaconule crest on the upper molars--  a unique characteristic that has never been found in any other monkey or ape  living or extinct.

Because  its fossil remains are strongly associated with the wetland freshwater environments that existed on Tuscany-Sardinia at that time, Oreopithecus  is frequently referred to as the 'swamp ape'. Dryer areas did exist on the island, but Oreopithecus was rare in those deposits while being abundantly represented in the swampy wetland coastal environments on the island.

It is interesting that the African oreopithecine ancestor, Mabokopithecus, existed in an ecological niche largely restricted to riparian woodland areas. This is a niche similar to that of the extant folivorous primate Colobus guereza (the Colobus monkey).  Although the Colobus monkey is  highly arboreal, it is known to come down from the trees in order to feed on aquatic plants in nearby swamps.

Oreopithecus bambolii has long  been at the center of controversy principally because of its remarkable cranio-dental and post cranial  similarity to African hominins (human ancestors) which was first fully noted by Johannes Hurzeler as far back as the 1950s.   Further adding to the controversy is  the argument that Oreopithecus may have also been the earliest obligatory bipedal primate and the first ape to walk exclusively on just two legs. 

Oreopithecine remains display a significant  number of post cranial characteristics that could be associated with either arboreal or  bipedal locomotion. However, there is one post cranial characteristic that is clearly related to bipedality and that's lumbar lordosis, a curvature of the spine that is unique to the hominins (humans and their bipedal ancestors).   However, a recent paper by Russo and Shapiro has question the existence of lumbar lordosis in Oreopithecus, arguing that  the supposed features  may be simply  be an artifact of the skeleton's distortion caused by  its compression during fossilization. Russo and Shapiro  also argue that the arboreal three toed sloth (Bradypus) would be a better convergent model for the skeletal anatomy and  locomotive behavior of the extinct oreopithecines rather than the bipedal hominins.

Three-toed sloth (Bradypus)

While slow climbing suspensory behavior has long been argued as an explanation for some of the anatomical attributes of Oreopithecus, such locomotive behavior appears to be contradicted by the exceptionally robust metatarsals of the oreopithecine foot along with the proportions of the foot's entocuneiform which was proximally-distally short and dorso-ventrally high. The length-height index of the oreopithecine  entocuneiform related to the mass placed on the hind limbs and is most similar to that of the gorilla. The gorilla's low entocuneiform length-height index is related to the large amount of mass placed on the hind limbs due to its large body size. Oreopithecus, however, was a very small ape, with males typically weighing about 32 kilograms and females approximately half that size. The average Gorilla male weighs 175 kg with average female gorillas weighing about 85 kilograms.    So in order to explain, the gorilla-like entocuneiform index, in Oreopithecus, the swamp ape must have been carrying its entire body weight on just its hind limbs.   Kohler and Moya- Sola also noted that the  power armload arm ratio strongly suggest that Oreopithecus carried its entire body weight on its hind limbs.

There's also strong evidence in the oreopithecine feet of a significantly reduced arboreal ability. The Oreopithecus foot had robust metatarsals plus a non helical ankle joint, characteristics typically associated with terrestrial catarrhines. Kohler and Moya-Sola have also noted that the mobility and grasping ability of the oreopithecine foot had been appreciably reduced relative to arboreal primates and even more so than in terrestrial baboons.

It would also be difficult to understand why Oreopithecus would expend the energy and the risk of  being a highly arboreal tree living primate on an island where there were no terrestrial predators. But even on the ground, why would  Oreopithecus abandon terrestrial quadrupedalism for bipedalism? The answer may come from its food source.

Chimpanzee wading bipedally with a stick

Orangutan wading bipedally

Gorilla wading bipedally

The coastal wetlands that Oreopithecus preferred were rich in aquatic plants. Harrison and Rook   have suggested that Oreopithecus may have specialized in feeding  on aquatic plants which were abundant in the wetland coastal areas of the island such as: sedges, water lilies, reeds, cattail, pond- weeds, horestails, and stoneworts which were abundantly represented in the fossil pollen spectrum.

Extant primates that feed on aquatic plants, usually wade bipedally in the shallow waters to access the food resource. Bipedal wading in primates has been observed in: baboons, macaques, the gorilla, orangutan, and in the chimpanzee. Such aquatic bipedalism in order to access aquatic plants  comprises as much as 27% of the feeding behavior of the Western Gorilla.

Additionally,  freshwater mollusk and turtles were also abundant in the wetland areas of Tuscany-Sardinia island-- along with predatory crocodiles. Turtle and crocodile eggs may have served as a good source of protein for Oreopithecus.

Marine biologist, Alister Hardy, hypothesized that the power precision grip in humans originally evolved as an adaptation for picking up benthic invertebrates. Wading bipedally in shallow water for food resources such as shellfish and aquatic plants is, of course, common behavior in many hunter-gatherer human populations. But such behavior has also been observed in non-human primates such as baboons, macaques,  guenons, and capuchin monkeys.

Curiously, Oreopithecus also had a hominin-like power precision grip. Such manual dexterity could have evolved in Oreopithecus as a feeding adaptation for picking up   the shelled invertebrates that existed in the wetland areas of Tuscany-Sardinia while it was also feeding on aquatic plants.

As the sole primate on the ancient island of Tuscany-Sardinia for nearly two million years, it seems unlikely that Oreopithecus would have avoided the exploitation of freshwater aquatic food resources   that were so rich in carbohydrates and proteins. 

Sea levels in the Mediterranean began to fall sometime after 7.4 million years ago and the island's isolation from Europe and Africa appears to have ended sometime between 6.9 to 7.2 million years ago. By 6.1 million years ago, global sea levels fell to such an extent that the Mediterranean Sea became completely isolated from  the inflow of marine waters from the Atlantic Ocean. During this period of lowering sea levels, land bridges were created between Europe and North Africa.

 The  earliest African hominin,  Sahelanthropus, appeared in the fossil record in North Africa sometime between 6.8 and 7.2 million years ago. While not much is known about the postcranial remains of Sahelanthropus, its craniodental morphology is remarkably similar to that of Oreopithecus bambolii.

Marcel F. Williams


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