The missing segment of the autopod 1st ray: new insights from a morphometric study of the human hand

Abstract

Whether the 1st segment of the human autopod 1st ray is a ‘true’ metapodial with loss of the proximal or mid phalanx or the original basal phalanx with loss of the metacarpal has been a long‐lasting discussion. The actual knowledge of the developmental pattern of upper autopod segments at a fetal age of 20–22 weeks, combined with X‐ray morphometry of normal long bones of the hand in the growing ages, was used for analysis of the parameters, percentage length, position of epiphyseal ossification centers and proximal/distal growth rate. The symmetric growth pattern in the fetal anlagen changed to unidirectional in the postnatal development in relation to epiphyseal ossification formation. The percentage length assessment, the distribution of the epiphyseal ossification centers, and differential proximal/distal growth rate among the growing hand segments supported homology of most proximal segment of the thumb with the 2nd–5th proximal phalanges and that of the proximal phalanx of the thumb with the 2nd–5th mid phalanges in the same hand. Published case reports of either metanalysis of ‘triphalangeal thumb’ and ‘proximal/distal epiphyseal ossification centers’ were used to support the applied morphometric methodology; in particular, the latter did not give evidence of growth pattern inversion of the proximal segment of the thumb. The presented data support the hypothesis that during evolution, the lost segment of the autopod 1st ray is the metacarpal.

Trabecular variation in the first metacarpal and manipulation in hominids

“…While developmental signals are certainly present in bone, bone functional adaptation also occurs in the mature skeleton (Ruff et al, ) and to some extent during senescence (Homminga et al, ). However, it should be noted that the Mc1 head develops from a pseudoepiphysis whereas the base arises from a true secondary ossification center (Haines, ) reflecting a different evolutionary history from the other metacarpals (Pazzaglia et al, ). This developmental difference may also potentially affect trabecular architecture (Lazenby et al, ).…”

Covariation of proximal finger and toe phalanges inHomo sapiens: A novel approach to assess covariation of serially corresponding structures

“…In the first two hypotheses, the thumb is a paramorph to the other fingers (Wagner, 2014). Insights from morphometric studies support the hypothesis that the lost segment of the first ray is the true metacarpal, and the metacarpal is the true proximal phalanx (Bondioni et al, 2020; Pazzaglia et al, 2018). Nevertheless, from an evolutionary perspective, the data suggest that the first metapodial is not a true phalanx (Reno et al, 2013).…”

Movable Surface Models of Hand Joints Using Visible Korean

“…In addition, the first proximal phalanx and the second to fifth middle phalanges were similar, in terms of length assessment (Figs. 1 to 5) (Pazzaglia et al, 2018). Second, there were similarities in terms of the distribution of the epiphyseal ossification centers and differential proximal-distal growth rates (Pazzaglia et al).…”