1. BACKGROUND INFORMATION The main functions of the rib cage are to protect the important internal organs of the thorax and to be flexible and allow for expansion during breathing 1 . The human thorax differs from that of African great apes in many aspects (see Fig 3 and Table 1), and some of these differences are probably related to upright posture and bipedal locomotion in humans, and to knuckle-walking in chimpanzees and gorillas. For example, the dorso-ventrally flatter human torso and the deeper spinal invagination into the thorax helps position the upper body more directly over the center of gravity during bipedal walking 1 . Works Cited 1 Aiello, L. and C. Dean. 2002. An Introductions of Human Evolutionary Anatomy. Elsevier Academy Press, London. [Ch 15: The Hominoid Thorax and Vertebral Column] 2 Haile-Selassie et al. 2010. An early Australopithecus afarensis postcranium from Woranso-Mille, Ethiopia. Proceedings of the National Academy of Sciences 107(27): 12121-6. 3 Kagaya, Miyuki, Naomichi Ogihara, and Masato Nakatsukasa. 2008. Morphological study of the anthropoid thoracic cage: scaling of thoracic width and an analysis of rib curvature. Primates 49: 89-99. 4 Jellema, L.M., B. Latimer, and A. Walker. 1993. The rib cage [Chapter 13]. In The Nariokotome Homo erectus Skeleton . A. Walker and R. Leakey, eds. Cambridge: Harvard University Press. 5 Latimer, B. and C.V. Ward. 1993. The thoracic and lumbar vertebrae [Chapter 12]. In The Nariokotome Homo erectus Skeleton . A. Walker and R. Leakey, eds. Cambridge: Harvard University Press. 6 Todd, T.W. 1912. The descent of the shoulder after birth. Anatomischer Anzeiger 41(14): 385-95. FURTHER RESEARCH POSSIBILITIES ~Posture & locomotion: What relationship does KSD’s torso shape have to its posture and mode of locomotion as a biped? 2,3 ~Growth & development: How does the rib cage change in humans during puberty as the kyphotic curve of the thoracic spine changes? Can this help us understand the evolutionary changes that have taken place in the spine and rib cage? 4,5 ~Shoulder movement: How does upper thorax shape impact the position and range of motion of the clavicle and scapula? 6 RESULTS ~Transverse Diameter: A clear separation between humans and apes is evident, in that the upper ribs (2-3) are wider in humans and the lower ribs (7-12) begin to get narrower, indicating the barrel shape. The KSD fossil rib 2 is also wide like humans; however, its lower ribs seem to widen more like apes. This seems to suggest a more “bell-shaped” thorax, wide at the top and flaring out again at the bottom. ~Rib Declination: Human ribs are clearly more declines than ape ribs, as can be seen by measurements of both vertical distance and declination angle. In addition, while declination seems to steadily increase from ribs 1-10 in apes, it peaks at rib 8 in humans and then decreases again for ribs 9 and 10. ~Spinal Invagination (Fig 6): Humans have deeper invagination than apes, as measured by an invagination index (see Fig 7). The KSD ribs fall within the human range, indicating a more human-like placement of the spine closer the center of gravity during bipedal posture and locomotion. PROJECT OVERVIEW As thin, fragile bones, ribs are rare in the human ancestor fossil record, and when found are often poorly preserved 1 . Thus, the five fossil ribs preserved as part of the 3.6 million year old Kadanuumuu skeleton (KSD-VP-1/1) from Ethiopia provide a unique opportunity to study aspects of the Australopithecus afarensis thorax 2 . By comparing the morphology of the Kadanuumuu ribs to that of human and extant ape ribs, we can begin to understand the evolution of the human thorax and hypothesize about the thorax of the chimpanzee-human last common ancestor. Fig. 1 : Elements of the KSD-VP-1/1 partial skeleton 2 Fig. 2 : Woranso-Mille field site in Ethiopia RESEARCH QUESTIONS ~How can rib shape be used to accurately describe thorax shape, and to differentiate between ape-like and human-like morphology? ~Do the ribs of Kadanuumuu indicate a torso shaped more like humans or more like apes? ~What does KSD’s torso shape imply about its posture and locomotion? METHODS To best approximate the torso shape, three main measurements were taken: transverse diameter, rib declination, and spinal invagination. First, two consecutive ribs were articulated with their respective thoracic vertebrae using stand clamps (Fig 4). Then, a microscribe was used to take three-dimensional spatial coordinates of a series of points on the ribs and vertebrae. Using these points, calculations using coordinate geometry yielded a selection of length and angle measurements that could be used to describe features of the torso shape at each rib. By normalizing these measurements by body size to account for allometric affects, they could be compared to show the differences between extant apes and humans. Kadanuumuu was measured using fossil casts. Unfortunately, there are no thoracic vertebrae fossils, so the casts were aligned using human vertebrae as an approximation. The fossil measurements were then normalized against the range of body size as reported in Haile-Selassie et al. 2010, and plotted onto the graphs. Fig. 4 : Ribs set up for measurement Fig. 5: Fossil ribs of KSD-VP-1/1 include a left 2 nd and 11 th rib, one right upper rib (5?), two right lower ribs (7 & 9?), and one rib fragment 2 Fig. 3 : Sketches of human and chimpanzee torsos Fig. 6 : Graph showing spinal invagination measurements of humans, apes, and Kadanuumuu Drawing by Adolph Schultz, from Zinjanthropus.wordpress.com View of one part of the Woranso-Mille site, © CMNH, from blog.cleveland.com/metro Table 1 : Features of the human & ape thorax 1 Fig. S14 from Haile-Selassie et al. 2010 Rib torsion (obliquity) Rib declination Spinal invagination Lower thorax Upper thorax Overall thorax shape Total # of ribs Traits: No Yes Less declined More declined Less invaginated More invaginated Widens Narrows Narrower Wider Funnel-shaped Barrel-shaped 13 12 African Great Apes Humans The Ribs and Torso of Kadanuumuu, an Australopithecus afarensis partial skeleton from Ethiopia Judith Mendelsohn, Adopt-A-Student Intern Dr. Yohannes Haile-Selassie and Dr. Bruce Latimer, Advisors Lyman Jellema, Collections Manager www.cmnh.org © CMNH; Photo Credit Y. Haile-Selassie, L. Russell, CMNH. From www.cmnh.org Barrel-shaped Funnel-shaped A B Invagination Index = A/B Fig. 7 : Measurement dimensions for spinal invagination