HP-LCA foot not chimp-like?


Marc Verhaegen
 

The foot of the human-chimpanzee last common ancestor was not African ape-like:
a response to Prang (2019)
Morgan E Chaney ... CO Lovejoy 2021 JHE

keywords: Ardipithecus, Cuboid, Midfoot

1. Introduction
Huxley (1863) & Darwin (1871) speculated that African apes were humans’ closest living relatives,
so the notion that the HP-LCA was essentially a chimp has pervaded inferences about its locomotor behavior.
Robust refutations of these notions are now available (Machnicki & Reno 2020, Reno 2014, Sayers cs 2012, White cs 2015),
but relying only on an examination of foot-bone lengths from Ardipith.ramidus ARA-6/500, Prang (2019) argued that the HP-LCA foot was likely similar to modern African apes, although it exhibits a plethora of symplesiomorphies.
A.ramidus lacks advanced grasping-related adaptations evident in the feet of extant great apes, e.g.
- decreased dorso-plantar heights of their metatarso-cuneiform joint surfaces,
- proximo-distal (PD) shortening of their mid-foot tarsals (Lovejoy cs 2009a):
Prang (2019: 8) concluded: “modern humans evolved from an ancestor with an African ape-like foot, ass.x terrestrial plantigrady & vertical climbing.”
We contend here that data do not support this assertion over earlier conclusions that the HP-LCA was a predom.above-branch clambering quadruped (Lovejoy cs 2009c).
Prang’s (2019) analysis is insufficient for 3 reasons:
1) his results support only the simple conclusion that the HP-LCA relative forefoot bone lengths were similar to extant African apes.
2) intrinsic foot proportions are at least partially reflective of body mass. Because the body size of ARA-VP-6/500 & the HP-LCA was likely similar to chimps, their proximity to the latter in Prang’s (2019) phylomorpho-space plots is unsurprising & uninformative.
3) numerous primitive soft-tissue retentions in the human foot are uniquely derived in Pan & Gorilla (Lovejoy cs 2009).
...

(the HP-LCA had a flat foot? --mv)

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