Marc R. Meyer1, Scott A. Williams2,3, Michael P ...

Lucy plus one: Intermingling of a second individual in the spinal column of A.L. 288-1 1Department

Marc R.

1 Meyer ,

Scott A.

2,3 Williams ,

Michael P.

4 Smith ,

Gary J.

4 Sawyer

of Anthropology, Chaffey College 2Center for the Study of Human Origins, Department of Anthropology, New York University 3New York Consortium in Evolutionary Primatology, 4American Museum of Natural History

INTRODUCTION

DISCUSSION

In the description of postcranial material associated with the A.L. 288-1 partial skeleton popularly known as Lucy, Johanson et al. (1982:432) noted that the A.L. 288-1am thoracic neural arch fragment stood out from the other fossils because it was “polished or even ‘waterworn’ and is the only specimen from A.L. 288 with these characters.” This observation led us to reexamine the attribution of this fossil to Lucy.

We demonstrate that this vertebra exhibits characters absent in hominoids but common in baboons, explaining its anomalous fit within Lucy’s axial skeleton. This vertebra is best attributed to the genus Theropithecus, which today is only represented by the gelada baboon, but was the most abundant cercopithecoid in the KH-1s level where A.L.288-1 was discovered. Quantitative analyses also situate this vertebra among baboons. A.L. 288-1 was discovered in the KH-1s deposit, a sheet of sandstone between the Kada Hadar Tuff and Confetti Clay characterized as a sinusoidal stream deposit and crevasse splay deposit that spilled out across the floodplain. We suggest the pattern of erosion unique to this fossil amongst the A.L. 288-1 assemblage is the result of water displacement, and that the A.L. 288-1am vertebra washed into the site. Our morphological comparisons of mammalian taxa represented in the KH-1 submember at Hadar of similar size to A.L. 288-1am (e.g., Hystrix) excludes all except Theropithecus. Although no corresponding fossil vertebrae are in existence for Theropithecus darti, it would be the most likely candidate for A.L. 2881am as this species peaks in frequency in the KH-1 level, and at 91% of all cercopithecoids, T. darti represents the dominant large monkey at Hadar in SH-1 to KH-1 submembers (Eck, 1993). Although one partial vertebra was erroneously attributed to Lucy, aspects of vertebral morphology are particularly similar in humans and baboons, explaining why it escaped notice for 40 years. However, we confirm that other A.L.2881 vertebrae were correctly attributed to Lucy.

MATERIALS & METHODS Morphological and linear dimensions of A.L. 288-1am were compared to those of Papio sp. (N=62), Theropithecus gelada (N=6), Theropithecus brumpti (N=2), Pan troglodytes (N=22), Gorilla gorilla (N=20), Homo erectus (N=2), Australopithecus sediba (N=2), Australopithecus africanus (N=1), Australopithecus afarensis (N=1), and Homo sapiens (N=74). Linear measurements were collected with digital calipers and angles and areas were calculated on scaled photographs using ImageJ (NIH). The overall size of the A.L. 2881am partial vertebra was calculated as the geometric mean of six linear dimensions: lamina superoinferior height and dorsoventral thickness, total pars interarticularis width, total craniocaudal articular process height, and superior articular interfacet and inferior articular interfacet maximum transverse widths. The geometric mean was used as a proxy of body size to compare each measurement across taxa. We also carried out principal component and discriminant function analyses in an attempt to assess the taxonomic attribution of A.L. 288-1am. To this end, qualitative observations of neural arch characters were also collected and compared among taxa.

RESULTS A.L.288-1am most closely resembles a hominin T2 vertebra; however, it falls well below the expected intra-individual size pattern for a given spinal column in adult (N=61) and subadult humans (N=13), African apes (N=42), and fossil hominins (N=5) when compared to the other A.L. 288-1 vertebrae. Consequently, this vertebra does not appear to belong to Lucy and must belong to a second, smaller individual. We show that the erosion, possibly from carnivore ingestion and partial digestion, is minimal and likely inconsequential to our analyses.

FIGURE 1 (above): Geometric mean of six dimensions from the pars interarticularis and articular facets. A.L. 288-1am falls below the expected intraindividual size relationship while Lucy’s other thoracic vertebrae follow the human pattern.

FIGURE 2 (above): (a) Composite split image of A.L. 288-1am vs. human and A.L. 288-1am vs. Papio. The inset magnification highlights morphology shared by A.L. 288-1am and Papio but absent in hominins. (b) Dorsal view of A.L. 288-A.L. 288-1am and Papio. A.L. 288-1am and Papio exhibit the superior laminar trigon, the triangular squama descending the laminae contiguous with the spinous processes of Papio and Theropithecus but absent in hominoids.

A. L. 288-1am

ACKNOWLEDGMENTS We thank Getachew Senishaw, Yonas Yilma, Tomas Getachew, and Yared Assefa of the National Museum of Ethiopia and Authority for Research and Conservation of Cultural Heritage, for allowing and facilitating access to the Hadar material, Bruce Latimer, Tim White, and Bill Kimbel for their keen insight and advice, and Chris Gilbert for sharing photographs of Theropithecus brumpti.

Reconstruction of fossil Theropithecus

Johanson, D.C., Lovejoy, C.O., Kimbel, B., White, T.D., Ward, S.C., Bush, M.E., Latimer, B.M., Coppens, Y., 1982. Morphology of the Pliocene partial hominid skeleton (A.L. 288-1) from the Hadar Formation, Ethiopia. Am. J. Phys. Anthropol. 57, 432-451. Eck, G.G., 1993. Theropithecus darti from the Hadar Formation, Ethiopia, in: Jablonski, N.G. (Ed.), In Theropithecus: The rise and fall of a primate genus. Cambridge University Press, pp. 15-83.

A.L.288-1 “Lucy”