Homo Ancestors or Pan and Gorilla?

THE EVOLUTION OF PAN AND GORILLA

Paleoanthropologists of course always hope that any fossil they find may be a human ancestor. The strange thing is that there are no fossils (or almost none) for the ancestors of Gorilla or Pan. The automatic assumption therefore is that if it’s clearly a bipedal ape, then it must have been a human ancestor, but convergent evolution tells us that a species can resemble another simply by occupying the same environmental niche or challenges, rather than being a direct species. All ape/hominin species were already orthograde and if they had been living in waterside or swamp/forest environments (which much of Africa was at the time) then they are likely to have been more bipedal then extant apes are in Africa today.

PAN (Chimpanzee and Bonobo species)

Traditional paleoanthropologists observe the Homo line as starting with Austrolapithecus africanus, a species which lived in South Africa 3.3 million years ago. However, the brain casing of this species shows that they had relatively small brains which were organised in a way more similar to chimpanzees.

According to Wikipedia:

"A. africanus was similar in many traits to A. afarensis, a bipedal hominin with arms slightly longer than the legs (a physical trait also found in chimpanzees). It has slightly human-like, advanced cranial features (seen in the crania of Mrs. Ples and STS 71), but also presents primitive features including ape-like, curved fingers adapted to tree climbing.

Instead of it being a direct ancestor of later hominins and thereby of humans, some researchers believe that A. africanus evolved into Paranthropus, and specifically that P. robustus descended from A. africanus. Both P. robustus and A. africanus crania seem very alike despite the more heavily built (robust) features of P. robustus, which, like those of the modern gorilla, are adaptations for heavy chewing. A. africanus, on the other hand, had a cranium which quite closely resembled that of a modern chimpanzee; yet both brains measure about 400 cc to 500 cc and probably presented an ape-like intelligence.[10] A. africanus had a pelvis that would enable more efficient bipedalism than that of A. afarensis. A 2015 study of hand bones in A. africanus indicated the species had a "human-like trabecular bone pattern in the metacarpals consistent with forceful opposition of the thumb and fingers typically adopted during tool use". Such a morphology would support an earlier time for making and using tools than previously had been thought likely.[17]"

Making tools is of course another trait that until recently we have assumed could only belong to the Homo species, but this has been disproved and we know now that Chimpanzees also use and make tools. Were these oldest stone tools made by a Homo species, or by a Chimpanzee ancestor?

Also, the skeletons of these animals were robust, the tooth enamel was thinner, they had longer canines, longer iliac blades with less flaring, and smaller braincases. All of this is more similar to Pan than to Homo.

Australopithecus sediba (1.98 Ma) may have been an off shoot of A. africanus, leading to Homo (misnamed, should be: Pan) naledi, more likely a closer cousin to chimpanzees than to humans.

Neoteny also tells us that we can often see the process of a species’ evolution unfolding in utero, and in the case of chimpanzees, this would tell us that at one stage, chimpanzee ancestors had feet similar to ours:

"The embryo of a chimpanzee at one stage has a foot resembling that of man in that its great toe points forward for walking rather than backward for grasping. Only as it approaches its birth size does its foot acquire the appearance of a hand. At no stage of its development does the human foot resemble that of an adult ape."
SC Coon 1958 "The story of man" p.12

Australapithecus africanus, therefore, between 3.3 and 3.1 Ma, may have been followed by Preanthropus robustus (1.8 – 1.2 Ma) which may have actually been the ancestors of today’s chimpanzee and bonobo species. The fact that chimpanzees and bonobo today have reverted to knuckle-walking reflects their return to the trees and away from the water, although in the case of Bonobo, they do continue to swim, wade and walk on two legs when required.

A possible evolutionary lineage for Pan.

GORILLA

Molecular evidence suggests that the Gorilla line split from the Homo/Pan line approximately 8-10 Ma, when many Miocene hominids existed in Tethys (pre Mediterranean) swamp forests and coastal lagoons. The European swamp ape, Oreopithecus bambolii, could be a hypothetical candidate. Fossil evidence suggests that this ape was at least partly bipedal although more likely a fully orthograde aquarboreal ape which spent a lot of time feeding in the water. We could hypothesise that Oreopithecus was a competent swimmer, and some of these individuals may have spread out across these proto mediterranean islands, with one group eventually making its way into northern Africa.

Whether or not Gorilla broke from the Homo/Pan line in Europe or in Africa, the first candidate to pick up the story in the African continent would be Sahelanthropus tchadensis, dated to about 7 million years ago and found in Chad (where at the time there was a paleo megalake or inland sea: Mega-Chad. Many other species followed the waterways from Libya to this great inland sea, so it’s possible that a missing link between Oreopithecus and Sahelanthropus did too). The species’ placement in our family tree, however, is still not clear or defined, with some believing it to be an ancestor of Homo/Pan and others as a forebear of Gorilla.

Wikipedia:

"Sahelanthropus tchadensis ...may have been ancestral to both humans and chimpanzees (which would place it in the Hominini tribe), or alternatively an early member of the Gorillini tribe.

The braincase, being only 320 cm3 to 380 cm3 in volume, is similar to that of extant chimpanzees and is notably less than the approximate human volume of 1350 cm3.

The teeth, brow ridges, and facial structure differ markedly from those found in Homo sapiens. Cranial features show a flatter face, u-shaped dental arcade, small canines, an anterior foramen magnum, and heavy brow ridges.

With the sexual dimorphism known to have existed in early hominins, the difference between Ardipithecus and Sahelanthropus may not be large enough to warrant a separate species for the latter.[6]"

"A further possibility is that Toumaï (the name given to the fossilised skull found in Chad) is not ancestral to either humans or chimpanzees at all, but rather an early representative of the Gorillini lineage. Brigitte Senut and Martin Pickford, the discoverers of Orrorin tugenensis, suggested that the features of S. tchadensis are consistent with a female proto-gorilla. Even if this claim is upheld the find would lose none of its significance, because at present, very few chimpanzee or gorilla ancestors have been found anywhere in Africa. Thus if S. tchadensis is an ancestral relative of the chimpanzees or gorillas, then it represents the earliest known member of their lineage. And S. tchadensis does indicate that the last common ancestor of humans and chimpanzees is unlikely to closely resemble extant chimpanzees, as had been previously supposed by some paleontologists.[19][20]"

A possible evolutionary lineage for Gorilla species