Tracing hominin evolution further back

The earliest hominin known from Africa is Sahelanthropus tchadensis, announced in 2002 by Michel Brunet and his team working in 7 Ma old Miocene sediments deposited by the predecessor to Lake Chad in the central Sahara Desert. Only cranial bones were present. From the rear the skull and cranial capacity resembled what might have been regarded as an early relative of chimpanzees. But its face and teeth look very like those of an australopithecine. Sadly, the foramen magnum – where the cranium is attached to the spine – was not well preserved, and leg bones were missing. The position of the first is a clue to posture; forward of the base of the skull would suggest an habitual upright posture, towards the rear being characteristic of knuckle walkers. Some authorities, including Brunet, believe Sahelanthropus may have been upright, but others strongly contest that. The angle of the neck-and-head ball joint of the femur (thigh bone), where the leg is attached to a socket on the pelvis to form the hip joint is a clue to both posture and gait. The earliest clear sign of an upright, bipedal gait is the femur of a fossil primate from Africa – about a million years younger than Sahelanthropus, found in the Tugen Hills of Kenya. Orrorin tugenensis was described from 20 bone fragments, making up: a bit of the other femur, three hand bones; a fragment of the upper arm (humerus); seven teeth; part of the left and right side of a lower jawbone (mandible). Apart from the femur that retains a neck and head and signifies an upright gait, only the teeth offer substantial clues. Orrorin has  a dentition similar to humans apart from ape-like canines but significantly smaller in size – all known hominins lack the large canines, relative to other teeth. Despite being almost 2 Ma older than Ardipithecus ramidus, the first clearly bipedal hominin, Orrorin is more similar to humans than both it and Australopithecus afarensis, Lucy’s species.

Oreopithecus_bambolii_1
Near-complete skeleton of Oreopithecus bambolii from Italy (credit: Wikipedia Commons)

DNA differences suggest that human evolution split from that of chimpanzees about 12 Ma ago. Yet the earlier Miocene stratigraphy of Africa has yet to provide a shred of evidence for earlier members of either lineage or a plausible last common ancestor of both. In 1872, a year after publication of Charles Darwin’s The Descent of Man parts of an extinct primate were found in Miocene sediments in Tuscany and Sardinia, Italy. In 1950 an almost complete skeleton was unearthed and named Oreopithecus bambolii (see Hominin evolution becoming a thicket, January 2013). Despite dozens of specimens having been found in different localities, the creature was largely ignored in subsequent debate about human origins, until 1990 when it was discovered that not only could Oreopithecus walk on two legs, albeit differently from humans, it had relatively small canine teeth and its hands were like those of hominins, capable of a precision grip. Dated at 7 to 9 Ma, it may lie further back on the descent path of hominins; but it lived in Europe not Africa. Now the plot has thickened, for another primate has emerged from a clay pit in Bavaria, Germany (Böhme, M. and 8 others 2019. A new Miocene ape and locomotion in the ancestor of great apes and humans. Nature, online publication; DOI: 10.1038/s41586-019-1731-0).

Danuvius
Bones from 4 Danuvius guggenmosi individuals. Note the diminutive sizes compared with living apes (Credit: Christoph Jäckle)

Danuvius guggenmosi lived 11.6 Ma ago and its fossilised remains represent four individuals. Both femurs and a tibea (lower leg), together with the upper arm bones are preserved. The femurs and vertebrae strongly suggest that Danuvius could walk on two legs, indeed the vertebral shapes indicate that it had a flexible spine; essential for balance by supporting the weight of the torso over the pelvis. It also had long arms, pointing to its likely hanging in and brachiating through tree canopies. Maybe it had the benefit of two possible lifestyles; arboreal and terrestrial. Its discoverers do not go that far, suggesting that it probably lived entirely in trees using both forms of locomotion in ‘extended limb clambering’. It may not have been alone, another younger European primate found in the Miocene of Hungary, Rudapithecus hungaricus, may also have had similar clambering abilities, as might have Oreopithecus.

There is sure to be a great deal of head scratching among palaeoanthropologists, now that three species of Miocene primate seem – for the moment – to possess  ‘prototype specifications’ for early entrants on the evolutionary path to definite hominins. Questions to be asked are ‘If so, how did any of them cross the geographic barrier to Africa; i.e. the Mediterranean Sea?’, ‘Did the knuckle-walking chimps evolve from a bipedal common ancestor shared with hominins?, ‘Did bipedalism arise several times?’. The first may not have been as difficult as it might seem (see Africa_Europe exchange of faunas in the Late Miocene, July 2013). The Betic Seaway that once separated Iberia from NW Africa, in a similar manner to the modern Straits of Gibraltar, closed during the Miocene after a ‘mild’ tectonic collision that threw up the Betic Cordillera of Southern Spain. Between 5.6 and 5.3 Ma there was a brief ‘window of opportunity’ for the crossing, that ended with one of the most dramtic events in the Cenozoic Era; the Zanclean Flood, when the Atlantic burst through what is now the Straits of Gibraltar cataclysmically to refill the Mediterranean .

See also: Barras, C. 2019. Ancient ape offers clues to evolution of two-legged walking. Nature, v. 575, online; Kivell, T.L. 2019. Fossil ape hints at how walking on two feet evolved. Nature, v. 575, online; DOI: 10.1038/d41586-019-03347-0

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