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Developmental identity versus typology: Lucy has only four sacral segments.

Allison L Machnicki1, C Owen Lovejoy2, Philip L Reno1

  • 1Department of Anthropology, Pennsylvania State University, University Park, PA.

American Journal of Physical Anthropology
|April 22, 2016
PubMed
Summary

The "Lucy" sacrum has four segments, not five, aligning with early hominid vertebral evolution. This finding supports the "long-back" model for the last common ancestor of chimpanzees (Pan) and humans (Homo).

Keywords:
A.L. 288-1AustralopithecusHoxsacrumvertebral formula

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Area of Science:

  • Paleoanthropology
  • Vertebral Morphology
  • Hominoid Evolution

Background:

  • Vertebral counts are influenced by developmental patterning mechanisms like Hox genes.
  • The sacrum of A.L. 288-1 ("Lucy") has been debated, with some suggesting five sacral vertebrae despite apparent fossilization damage.
  • Understanding sacral segmentation is crucial for reconstructing hominoid evolutionary history.

Purpose of the Study:

  • To re-evaluate the sacral segmentation of the "Lucy" specimen (A.L. 288-1).
  • To investigate sacral/coccygeal variation in extant hominoids.
  • To contextualize these findings within mammalian vertebral development.

Main Methods:

  • Updated descriptions of the "Lucy" sacrum.
  • Analysis of sacral/coccygeal variation in a large sample of extant hominoids.
  • Comparison with developmental variation in the mammalian vertebral column.

Main Results:

  • Fossilization damage did not alter the transverse processes of the "Lucy" sacrum's fifth segment.
  • Sacral hiatus variability in apes and hominids limits its utility for determining segment identity.
  • Vertebrae at regional boundaries often exhibit a mix of sacral and coccygeal features.

Conclusions:

  • The sacral/caudal boundary is cranially displaced in early hominids compared to extant African apes and humans.
  • A four-segmented sacrum in "Lucy" is consistent with the ancestral condition for Miocene hominoids.
  • This finding supports the "long-back" model for the Pan/Homo last common ancestor.