The ‘African ape model’ of human evolution in 2023

Monday, January 23, 2023 13:01

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Williams et al. 2023 argue
“that an “African ape model” (referring largely to the postcranial anatomy and positional behaviors exhibited by extant African apes, Pan and Gorilla) for the origins of human bipedalism is a well-justified inference, the best-supported model given available evidence, and is currently and will remain testable against the fossil record. Given the long history of this debate, we have no illusion our conclusions about model building and testing will be universally accepted, but we argue that rejecting an African ape model requires significantly better evidence than currently exists.”

‘Significantly better evidence’ does exist (Fig 1) in a lineage of gibbons leading to humans. This line was recovered in a cladogram that minimizes taxon exclusion: the LRT.

Figure 1. Above: foot series from the LRT. Below: Foot series from Williams et al 2023 who cherry-picked taxa vs another the minimizes taxon exclusion (the LRT). Ausralopithecus is not shown because workers don’t have one complete Australopithecus foot skeleton at present.

Williams and other anthropologists follow a tradition
that posits the African ape model. Often it includes Australopithecus (Fig 3), an African biped.

By contrast, the large reptile tree
(LRT, 2206 taxa) recovers a different tree topology (Fig 3) in which bipedal running was ?invented by gibbons (Hylobates) and retained by Ardipithecus, Oreopithecus and Homo. In the LRT Australopithecus (Fig 3) nests with the great apes (Pongo, Pan, Gorilla) as a biped by convergence.

Figure 2. Cladogram from Mongle et al. 2019 depicting the phylogenetic ancestors of humans. Here Australopithecines are derived from chimps and gorillas.

Williams et al. 2023 continue:
“The ‘molecular revolution’ revealed a close relationship and recent (late Miocene, 5–7 Ma) divergence of humans and chimpanzees. Many morphologists remained skeptical of a hominine (Gorilla-Pan-Homo) clade or supported a Pan-Gorilla clade to the exclusion of hominins. Some morphological studies did successfully recover a Pan-Homo clade but arguably most came to this conclusion well after the molecular relationships were established.”

“At the same time, many paleoanthropologists continue to advance ideas that the LCA was not African ape-like in its body plan and positional behavioral repertoire despite the fact that such standpoints require increasingly more complex scenarios and greater instances of homoplasy to justify them.”

Unfortunately the ‘molecular revolution’ too often recovers untenable results based on continental areas (e.g. Afrotheria) rather than morphology. And fossils are off the table.

Figure 3. The gibbon lineage leading to humans. At right is Australopithecus, a bipedal ape by convergence with humans.

Taking another tack, Almécija et al 2021 wrote,
“There is no consensus on the phylogenetic positions of the diverse and widely distributed Miocene apes. This has led some authors to exclude known Miocene apes from the modern hominoid radiation. Early hominins likely originated in Africa from a Miocene LCA that does not match any living ape (e.g., it might not have been adapted specifically for suspension or knuckle walking). Despite phylogenetic uncertainties, fossil apes remain essential to reconstruct the “starting point” from which humans and chimpanzees evolved. Bipedalism would have emerged because of the selection pressures created by the progressive fragmentation of forested habitats and the need for terrestrial travel from one feeding patch to the next.”

A fact apparently omitted from all prior bipedal studies: gibbons run bipedally (Fig 3).

“We need more fossils because we are likely missing vastly more than what we have.”

The LRT does not require more fossils. What the authors are missing is largely due to omission: taxon exclusion.

“Habitual bipedalism is reflected in several traits across the body (e.g., foramen magnum position and orientation; pelvic, lower-back, and lower-limb morphology), present (or inferred) in the earliest hominins Darwin linked the origin of bipedalism with an adaptive complex related to freeing the hands from locomotion to use and make tools (replacing large canines), leading to a reciprocal feedback loop involving brain size, cognition, culture, and, eventually, civilization.”

Ironically, gibbons depend even more on their hands for locomotion AND they are bipedal runners. Tall grass may have been the reason for bipedal locomotion in Australopithecus (Fig 3), but not in gibbons.

“Notably, Keith developed a scenario in which a “hylobatian” brachiating stage preceded an African ape-like creature: a knuckle-walking “troglodytian” phase immediately preceding bipedalism.”

Keith could have skipped the knuckle-walking African ape-like creature step.
Keith did not report bipedalism in gibbons. That omission continues widespread to this day.

“Keith (1902, 1923) initially coined “orthograde” to describe the positional behavior of hylobatids: “The body is held, in all movements, upright to the plane of progression.”

Williams et al reported,
“Focused on Keith’s “hylobatian” stage, Morton proposed that the “vertically suspended posture” of a small-bodied hylobatid-like ancestor caused the erect posture of human bipedalism. Gregory, another prominent “brachiationist,” supported similar views. Morton argued that knuckle walking did not represent an intermediate stage preceding bipedalism but rather a reversion toward quadrupedalism in large-bodied apes specialized for brachiation. At that time, “brachiation” was used for any locomotion in which the body was suspended by the hands. Now, it refers to the pendulum-like arm-swinging locomotion of hylobatids”.

Williams et al concluded,
“Thus, in the absence of evidence to the contrary (which currently does not exist), our phylogenetic position within the African ape clade dictates that interpretation.”

The gibbon lineage hypothesis of the LRT now needs
confirmation, refutation or modification with a taxon lists that include Oreopithecus, Homo floresiensis (Fig 3). Perhaps someday someone will add taxa to the Mongle et al. cladogram (Fig 2) to see if it becomes a little more bushy.

Everyone agrees that knuckle-walking
was not part of the evolution of bipedal humans, yet both the Mongle et al 2019 cladogram and the Williams et al 2023 foot series (Fig 1) indicate that three successive knuckle-walkers (Pongo, Pan and Gorilla) are in the lineage of humans, contra the LRT.

Adding taxa will solve this issue, as it solved so many other problems earlier.

References
Almécija S et al 2021. Fossil apes and human evolution. Science, 372, eabb4363.
Morton DJ 1926. Evolution of man’s erect posture (preliminary report). J. Morphology 43: 147–179. 10.1002/jmor.1050430108
Gregory WK 1027. How near is the relationship of man to the chimpanzee-gorilla stock? Q. Rev. Biol. 2, 549–560. 10.1086/394289
Keith A 1902. The extent to which the posterior segments of the body have been transmuted and suppressed in the evolution of man and allied primates. Journal of Anatomy and Physiology, 37, 18–40.
Keith A 1923. Man’s posture: Its evolution and disorders. British Medical Journal, 1, 451–454.
Mongle CS, Strait DS and Grine FE 2019. Expanded character sampling underscores phylogenetic stability of Ardipithecus ramidus as a basal hominin. Journal of Human Evolution 131:28–39.
Peters D 1991. From the Beginning – The Story of Human Evolution Wm. Morrow.
Tuttle RH 1981. Evolution of hominid bipedalism and prehensile capabilities. Philosophical Transactions of the Royal Society of London B, 292, 89–94.
Williams SA et al (4 co-authors) 2023. African apes and the evolutionary history of orthogrady and bipedalism. Am J Biol Anthropol. 2023;1–23.