First steps of bipedality in hominids: evidence from the atelid and proconsulid pelvis

Upright walking absent a bent-hip-bent-knee gait requires lumbar lordosis, a ubiquitous feature in all hominids for which it can be observed. Its first appearance is therefore a central problem in human evolution. Atelids, which use the tail during suspension, exhibit demonstrable lordosis and can achieve full extension of their hind limbs during terrestrial upright stance. Although obviously homoplastic with hominids, the pelvic mechanisms facilitating lordosis appear largely similar in both taxa with respect to abbreviation of upper iliac height coupled with broad sacral alae. Both provide spatial separation of the most caudal lumbar(s) from the iliac blades. A broad sacrum is therefore a likely facet of earliest hominid bipedality. All tailed monkeys have broad alae. By contrast all extant apes have very narrow sacra, which promote “trapping” of their most caudal lumbars to achieve lower trunk rigidity during suspension. The alae in the tailless proconsul Ekembo nyanzae appear to have been quite broad, a character state that may have been primitive in Miocene hominoids not yet adapted to suspension and, by extension, exaptive for earliest bipedality in the hominid/panid last common ancestor. This hypothesis receives strong support from other anatomical systems preserved in Ardipithecus ramidus.

145 we distinguish "deliberate climbing" as a form of relatively cautious arboreal locomotion, based on the 146 relatively large body mass of several Miocene specimens that would render the leaping and acrobatic 147 behavior seen in smaller primates hazardous because of substrate reactive elasticity in the arboreal 148 canopy. We distinguish "vertical climbing" (sensu stricto, as opposed to generally climbing upward) as 166 Examination of their pelvic and sacral morphology therefore may provide indirect evidence of the 167 potential pathway toward lordosis in earliest hominids.

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In order to address this we have reviewed lumbar and pelvic form and function in primates.
169 First, we predict that the ability to generate lordosis should facilitate the attainment of more erect hind 170 limb postures. To address this, we report observations of terrestrial behavior of northern muriquis 171 (Brachyteles hypoxanthus) and utilize opportunistic observations of bipedal locomotion in Ateles.
172 Second, we hypothesize that lordosis will be accompanied by features associated with caudal lumbar 173 emancipation. As such, we investigate iliac height, lower lumbar ligamentous support, and sacral width 174 in relevant extant and fossil anthropoids. Finally, we provide a possible scenario for the role of pelvic 175 and sacral form in the evolution of bipedality.
241 2). Nevertheless, the latter produced a slightly broader ala than indicated in their scaled drawing (see 254 activities had also diversified to include both essential ones (e.g., drinking, traveling across gaps in the 255 forest, and feeding) and nonessential ones (e.g., socializing, including mating, resting, and traveling in 256 areas where arboreal alternatives were available). Increasing terrestriality has continued to be present 257 in this group and in the other three muriqui groups in our study population (Strier, Unpublished). In  Manuscript to be reviewed 336 to be a reasonable minimum because the strong mediolateral angulation (about 45°) of the specimen's 337 auricular surface requires the anterior surfaces of its sacral alae to be as least as large as the ones both 338 Ward and we reconstructed. Admittedly these are crude estimates (theirs at 17.7 mm and ours at 18.7 339 mm), but when plotted in Figure 6 (see Supplemental Table 1), the specimen falls above the regression 340 line defined by monkeys and not with that representing the extant African apes.

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Such a reconstruction, although seeming quite reasonable, should be considered here mainly as 342 a means of illustrating a hypothesis, which will hopefully be tested by and the recovery of more 343 complete specimens. In any case, these data do indicate that the hominoid sacrum did not undergo 344 substantial reduction in breadth simply due to tail elimination.

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The potential parallelisms between atelids and hominoids with respect to suspensory 348 locomotion have long been observed (Erickson 1963;Larson 1998). However, the lumbar column, 349 thorax, and pelvis of spider, muriqui, woolly, and howler monkeys differ substantially from those of 350 hominoids in many ways, especially since monkeys retain external tails. The presence of a tail is also 351 shared with Old World monkeys, although those of atelids are more massive, highly innervated, and  Manuscript to be reviewed 368 (Okada 1985). Comparisons between studies shows there is some variation between achieved hip 369 angles, particularly in smaller bodied spider monkeys and gibbons as Yamazaki observed gibbons 370 attaining greater limb extension (Yamazaki 1985). With the caveat that it is difficult to know what 371 motivates individual primates to attain a particular range of extension during short bouts, it is 372 noteworthy, that larger bodied chimpanzees do not achieve more extended postures, despite the 373 greater kinematic pressures to do so (Biewener 1989). Regardless, the observations of Okada (1985) 374 approximately match the levels of extension observed in the spider monkey in Figure 4. This is despite

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It is important to note that relatively low iliac height and spinal invagination also occur in 386 Alouatta as in other atelids (Figures 3 & 4). In combination with potential parallel evolution of Manuscript to be reviewed