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Bipedalism is a highly specialized and unusual form of primate locomotion that is found today only in modern humans. [DD: Sifaka, Hylobatidea, All known Homo spp.] The majority of extinct taxa within the Hominini [DD: Hominoidea] were [DD: arboreal]
bipedal, but the degree to which they were bipedal remains the subject of considerable debate. The significant discoveries of fossil hominin remains in the last 40 years have resulted in this debate becoming increasingly focused on how bipedal certain
fossil taxa were, rather than on the overall process. Although the early hominin fossil record remains poor, evidence points to at least two distinct adaptive shifts. First, there was a shift to habitual [DD: arboreal] bipedalism, as typified by certain
members of Australopithecus, but possibly including earlier genera such as Ardipithecus and Orrorin. Such taxa were [DD: arboreal] bipedal, but also retained a number of significant adaptations to arboreal climbing. The second shift was to fully obligate
bipedalism [DD: Sifaka, Hylobatidae, Homo are all obligate bipeds.] and coincides with the emergence of the genus Homo [DD: No, of course not.]. By the Early Pleistocene, certain members of Homo had acquired a postcranial skeleton indicating fully
humanlike striding bipedalism [DD: No chin so they must have had a different striding gait & weight distribution than modern humans]. The final part of this chapter reviews why bipedalism was selected for. [DD: Bipedalism long preceded the advent of Homo
sapiens.] There have been many theoretical explanations, and the most robust remain those linked to the emergence of more varied habitats. Such an environmental shift would have involved strong selection for new behavioral strategies most likely linked
to the efficient procurement of food. [DD: Such as better access to arboreal fruits and nuts]
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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
H/T Jack Barnes at AAT.io
Jillery & Erika, parsimony is useful, but the data points must be well understood.
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