Thoracic vertebral count and thoracolumbar transition in Australopithecus afarensis

Carol V. Ward; Thierra K. Nalley; Fred Spoor; Paul Tafforeau; Zeresenay Alemseged

doi:10.1073/pnas.1702229114

Edited by Bruce Latimer, Case Western Reserve University, Cleveland, OH, and accepted by Editorial Board Member C. O. Lovejoy March 26, 2017 (received for review February 8, 2017)

Significance

The discovery of a 3.3 million-year-old partial skeleton of Australopithecus afarensis, from Dikika, Ethiopia, preserved all seven cervical (neck) vertebrae and provided the only known evidence for the presence of 12 thoracic (rib-bearing) vertebrae in hominins prior to 60,000 years ago. This skeleton has seven cervical and only 12 thoracic vertebrae like humans, rather than 13 like African apes. However, the anatomical transition from thoracic to lumbar (lower back) vertebral form occurs at the 11th thoracic segment. This distinctive pattern of vertebral segmentation, rare in modern apes and humans, is also seen in the three other early hominins for which this area is known, with the Dikika skeleton providing the earliest and most complete example.

Abstract

The evolution of the human pattern of axial segmentation has been the focus of considerable discussion in paleoanthropology. Although several complete lumbar vertebral columns are known for early hominins, to date, no complete cervical or thoracic series has been recovered. Several partial skeletons have revealed that the thoracolumbar transition in early hominins differed from that of most extant apes and humans. Australopithecus africanus, Australopithecus sediba, and Homo erectus all had zygapophyseal facets that shift from thoracic-like to lumbar-like at the penultimate rib-bearing level, rather than the ultimate rib-bearing level, as in most humans and extant African apes. What has not been clear is whether Australopithecus had 12 thoracic vertebrae as in most humans, or 13 as in most African apes, and where the position of the thoracolumbar transitional element was. The discovery, preparation, and synchrotron scanning of the Australopithecus afarensis partial skeleton DIK-1-1, from Dikika, Ethiopia, provides the only known complete hominin cervical and thoracic vertebral column before 60,000 years ago. DIK-1-1 is the only known Australopithecus skeleton to preserve all seven cervical vertebrae and provides evidence for 12 thoracic vertebrae with a transition in facet morphology at the 11th thoracic level. The location of this transition, one segment cranial to the ultimate rib-bearing vertebra, also occurs in all other early hominins and is higher than in most humans or extant apes. At 3.3 million years ago, the DIK-1-1 skeleton is the earliest example of this distinctive and unusual pattern of axial segmentation.

Footnotes

↵¹To whom correspondence should be addressed. Email: WardCV{at}missouri.edu.

Author contributions: C.V.W. and Z.A. designed research; C.V.W., T.K.N., F.S., P.T., and Z.A. performed research; C.V.W., T.K.N., F.S., and Z.A. analyzed data; and C.V.W., T.K.N., F.S., P.T., and Z.A. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission. B.L. is a guest editor invited by the Editorial Board.
Data deposition: The DIK-1-1 fossil is housed at the National Museum of Ethiopia. Requests to study it should be made to the museum and to Z.A. (alemseged{at}uchicago.edu). Scan data are archived at the University of Chicago and the European Synchrotron Radiation Facility (requests for these data should also be made to Z.A.).

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