Humans & hylobatids vs great apes

[DD'eDeN aka note/nickname/alas_my_loves]

Long lower back, long achilles tendon primitive hominoid traits.

Great apes and humans evolved from a long-backed ancestor
Allison L Machnicki & Philip L Reno

Did the H/P LCA have a short, stiff lumbar column, like gr.apes?
or a longer, flexible column, observed in generalized Miocene hominoids?
Beyond having only 4 segments, 3 additional features contribute to lumbar stiffening:
- the position of the transitional vertebra (TV),
- orientation of the lumbar spinous processes,
- entrapment of lumbar vertebrae between the iliac blades.
For gr.apes, these features would be
- homologous, if inherited from a short-backed LCA,
- (likely) functionally convergent through dissimilar phenotypes, if evolved from a long-backed LCA.

We quantitatively & qualitatively analyzed Hs, ape & monkey thoracic & lumbar vertebrae (3D surface scanning, osteol.measurements) to compare spinous process morphology & sacral depth.
We also used a large sample of hominoid vertebral counts, to assess variation in the position of the TV & lumbo-sacral boundary.

All extant hominoids modally place the TV at the ultimate thoracic, but
- Hs & orangutans place the TV at the 19th post-cranial vertebral segment,
- other apes place the TV at the 20th.
P, G & orangs each have distinct patterns of spinous process angulation & morphology, ass.x lumbar stiffening,
Hs spinous process morphology is similar to longer-backed gibbons, monkeys & Miocene hominoids Moroto- & Pierolapithecus.
Chimps are unique (vs other hominoids) with a greater sacral depth, facilitating lumbar entrapment,
and there are differences among Afr.apes in the mechanisms governing variation in the lumbar-sacral boundary.

These differences suggest:
- lumbar stiffening is convergent among gr.apes,
- Hs BPism evolved from a more generalized long-backed ancestor, more cons.x evidence of TV placement in Australopithecus