Bipedal locomotion in zoo apes:
Revisiting the hylobatian model for bipedal origins
Kyle H Rosen cs 2022 Evol.Hum.Sci. doi org/10.1017/ehs.2022.9

BP locomotion is a hallmark of being human.
Yet the body form from which BPism evolved remains unclear.
The positional behaviour (ortho- vs pronograde) & the length of the lumbar spine (long-mobile vs short-stiff) of the Afr.ape & human LCA require further investigation.
Fossil evidence would be the most conclusive, but the paucity of hominid fossils 5–10 Ma makes this field of research challenging.
In their absence, extant primate anatomy & behaviour may offer some insight into the ancestral body form from which BPism could most easily evolve.
Here we quantify the frequency of BPism in 496 of zoo-housed hominoids & cercopithecines.
Results:
each studied species of ape & monkey can move bipedally, but hylobatids
- are significantly more BP,
- engage in BP locomotion more frequently & for greater distances than any other primate sampled.
These data support hypotheses of an orthograde, long-backed & arboreal LCA, this is consistent with hominoid fossils from the middle-to-late Miocene.
If true,
- KWing evolved in parallel in Pan // Gorilla,
- the human body form (long lower back & orthogrady) is conserved.

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IOW, they don't revisit, but confirm our view:
Mio-Pliocene Hominoidea were already BP in coastal & swamp forests:
wading upright + climbing arms overhead in the branches above the water, google "aquarboreal".
This also implies:
BP australopiths were related to Gorilla or Pan (E & S.Afr.apiths resp.), e.g. Hum.Evol.9:121-139,1994 & Hum.Evol.11:35-41,1996, rather than to Homo:
Pliocene Homo was thenn on his way to Java (Mojokerto early-Pleistocene), e.g. see my book p.299-300, google "GondwanaTalks Verhaegen".

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