Evolution of vertebral numbers in primates, with a focus on hominoids and the last common ancestor of hominins and panins
JHE 179,103359 doi org/10.1016/j.jhevol.2023.103359

The primate vert.column has been extensively studied, with a particular focus on hominoids & the H/P-LCA.
The nr of vertebrae in hominoids (up to & incl. the H/P-LCA) is subject to considerable debate,
but few formal ancestral state reconstructions exist,
none include a broad sample of primates, or account for the correlated evolution of the vertebral column.
Here we conduct an ancestral state reconstruction, using a model of evolution that accounts for homeotic (changes of 1 type of vertebra to another) + meristic (addition or loss of a vertebra) changes.
Our results suggest:
- ancestral primates had 29 pre-caudal vertebrae:
- most commonly: 7 cervical, 13 thoracal, 6 lumbar, 3 sacral vertebrae,
- extant hominoids evolved tail-loss & a reduced lumbar column via sacralization (homeotic transition at the last lumbar vertebra),
- the ancestral hylobatid: 7 C, 13 Th, 5 L, 4 S,
- the ancestral hominid: 7 C, 13 Th, 4 L, 5 S.
The H/P-LCA likely either retained this ancestral hominid formula, or was characterized by an additional sacral vertebra: a homeotic shift at the sacro-coccygeal border?
Our results support the ‘short-back’ model of hominin vert.evolution: hominins evolved from an ancestor with an Afr.ape-like numerical composition of the vert.column.

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Hominoid sacralisation = adaptation to vertical wading-climbing in swamp forests?
Google "aquarboreal".

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