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A typical vertebra, with the exception of the sacrum and coccyx, consists of a body, a vertebral arch, and seven different projections termed processes. The anterior portion of the vertebrae, the body, supports about half the body’s weight. The vertebral bodies progressively increase in size and thickness from the cervical region to the lumbar region of the vertebral column. The intervertebral discs present between the bodies of adjacent vertebrae firmly unites them, forming a continuous...
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"Lucy" (A.L. 288-1) had five sacral vertebrae.

Gabrielle A Russo1, Scott A Williams

  • 1Department of Anthropology and IDPAS, Stony Brook University, Stony Brook, NY, 11794.

American Journal of Physical Anthropology
|October 22, 2014
PubMed
Summary

The "Lucy" fossil sacrum likely had five sacral vertebrae, not four, challenging the "long-backed" hominin evolution theory. This finding suggests no significant change in sacral vertebrae count from the human-ape common ancestor.

Keywords:
A.L. 288-1coccygeal vertebraesacrumvertebral numbers

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Area of Science:

  • Paleoanthropology
  • Hominin Vertebral Evolution
  • Skeletal Morphology

Background:

  • The "long-backed" hypothesis proposes early hominins had six lumbar and four sacral vertebrae, inherited from the human-ape common ancestor.
  • This theory relies partly on the interpretation of the "Lucy" (Australopithecus afarensis) sacrum as having four sacral vertebrae.
  • Re-evaluating Lucy's sacrum is crucial for understanding hominin vertebral formulas and ancestral states.

Purpose of the Study:

  • To reassess the number of sacral vertebrae in the "Lucy" sacrum (A.L. 288-1an).
  • To compare the morphology of Lucy's sacrum with modern human and Australopithecus sediba sacra and coccygeal vertebrae.
  • To determine if Lucy's sacral count supports the "long-backed" hypothesis and provides insights into the human-ape common ancestor's vertebral formula.

Main Methods:

  • Detailed re-examination of the distal end of the A.L. 288-1an sacrum.
  • Comparative analysis with a sample of modern human sacra and first coccygeal vertebrae (Co1).
  • Morphological comparison with the sacrum of Australopithecus sediba (MH2).

Main Results:

  • The last vertebra of A.L. 288-1an exhibits inferiorly projecting cornua and a kidney-shaped articular surface, consistent with a human S5 (fifth sacral vertebra).
  • This morphology contrasts with human Co1 vertebrae, which lack cornua or have superiorly projecting cornua and rounder articular surfaces.
  • The sacral hiatus apex position in A.L. 288-1an aligns better with five-vertebra sacra (modern humans, A. sediba) than a four-vertebra sacrum.

Conclusions:

  • The "Lucy" sacrum (A.L. 288-1an) most likely possessed five sacral vertebrae, similar to modern humans.
  • This finding challenges the interpretation that Lucy's sacral count supports the "long-backed" hypothesis.
  • The sacral number in "Lucy" does not provide evidence for a directional change in vertebral count from the human-ape common ancestor.