Probable pathways of lumbar reduction in African apes and hominids as deduced from extant vertebral formulae for each taxon. All axial formulae that exceed 10% of the total sample for each taxon are shown here, along with presumed modal formulae (those of highest probable frequencies) for the LCA and early hominids. A horizontal arrow indicates loss of a body segment (i.e. a reduction in the number of somites contributing to the pre-coccygeal vertebral column). A vertical arrow signifies changes in the positions of the anterior boundaries of Hox gene expression domains underlying the indicated transformations of vertebral identities. Note the differences in extant Pan species. For details, see McCollum et al. (2009). Axial formula data from Pilbeam (2004) and McCollum et al. (2009). (© M.A. McCollum).

Components of sacral breadth in African apes and early hominids. Scatter plot of log total sacral breadth versus log alar breadth. The findings of a strong correlation (r = 0.901) between sacral breadth and alar breadth, and an absence of any significant correlation between alar breadth and centrum area (figure 3) indicate that total sacral breadth in African apes and early hominids is largely a consequence of alar breadth. Note especially the exceedingly broad alae in the early hominid specimens. This is consistent with their having mediolaterally expanded the sacrum as an adaptation to free the most caudal lumbar from contact with the iliac crest, and fully accounts for the unusual platypelloidy in A.L.288-1 and Sts-14. Later, a caudally directed gradient of increasing centrum size and interfacet distance (see text) appears in Homo, and probably accounts for the unusually large human centrum. Note the much narrower alae in the two African apes compared with those of all hominids.

Components of sacral breadth in African apes and early hominids. Scatter plot of log centrum area (length × breadth) versus log alar breadth. For discussion, see legend of figure 2.

Hominid and pongid mechanisms of emancipation or fixation of the most caudal lumbar(s). A human pelvis (left) compared with that of a chimpanzee (right). Note the following numbered characters in each. The iliac isthmus (1) and the ilium itself (3) have both been greatly shortened in the human, so much so that a greater sciatic notch has been created (entirely absent in the chimpanzee), and there is no potential contact between the iliac crest and the most caudal lumbar. In the chimpanzee, the opposite change has occurred, i.e. the iliac isthmus (1) and the iliac blade (3) have both been superoinferiorly elongated, encouraging such contact. In addition, the sacral alae have been greatly broadened in the human and narrowed in the chimpanzee (cf. figure 2). As a consequence, there is now a very substantial horizontal distance between the iliac crest and the most caudal vertebra in the human (2), but a greatly narrowed bi-iliac gulf in the chimpanzee. In combination with (1) and (3), such narrowing in the chimpanzee (via reduced alar breadth; cf. figure 2) results in full restrictive contact between the iliac crest and the most caudal lumbar vertebra(e). In the earliest phases of this morphological shift in hominids, the pelvis became decidedly platypelloid, and the enhanced iliac breadth encouraged a more effective use of the anterior gluteal muscles as abductors during upright gait (Lovejoy 2005a; Lovejoy et al. 2009d). The latter was thus an exaptation, rather than the primary adaptation.

(a) Sacra of a chimpanzee, (b) A.L. 288-1 and (c) a modern human. Note the extremely narrow sacrum of the chimpanzee compared with the two hominids. Note also the much broader alae in A.L. 288-1 compared with its centrum. Compare this with the similar dimensions in the human specimen (figure 2).

Comparison of interfacet distances in the third lumbars and sacra of African apes and hominids. The drawing on the left demonstrates the comparison being made. In this drawing, the third lumbar has been rotated 180° from its normal anatomical position (its superior zygapophyses now point inferiorly) for comparison with those of the sacrum. Double-headed arrows indicate the interfacet distance in each specimen. (a) chimpanzee; (b) gorilla; (c) A.L. 288-1 and (d) human. Note that in (a) and (b) the interfacet distance is greater in the lumbar vertebra than it is in the sacrum, whereas the opposite is true of the two hominids. The increasing gradient of centrum size and interfacet distance in Homo may be an adaptation that facilitated increased lordosis and thereby enabled lumbar column reduction. The opposite gradient in chimpanzees is the likely source of reduction of the bi-iliac space. For discussion, see text. Redrawn from Lovejoy (2005a).