A Partial Pelvis of Australopithecus sediba

Abstract

The fossil record of the hominin pelvis reflects important evolutionary changes in locomotion and parturition. The partial pelves of two individuals of Australopithecus sediba were reconstructed from previously reported finds and new material. These remains share some features with australopiths, such as large biacetabular diameter, small sacral and coxal joints, and long pubic rami. The specimens also share derived features with Homo, including more vertically oriented and sigmoid-shaped iliac blades, greater robusticity of the iliac body, sinusoidal anterior iliac borders, shortened ischia, and more superiorly oriented pubic rami. These derived features appear in a species with a small adult brain size, suggesting that the birthing of larger-brained babies was not driving the evolution of the pelvis at this time.

Primate origins, human origins, and the end of higher taxa

“…[118][119][120] But there is conclusive evidence that a lot of supposedly human features were acquired in parallel in different lineages of Pleistocene hominins. Probable parallelisms include the increased basicranial flexion and other basicranial apomorphies seen in late robust Australopithecus species 53:194 ; molar reduction and humanlike changes in the pelvis and hand in A. sediba [121][122][123] ; brain enlargement in some habilines (KNM-ER 1470) and facial and dental reduction in others (KNM-ER 1813); and so on. If our family tree within the genus Homo is as bushy and speciose as some like to think, 124 then parallel evolution was also rampant throughout the later Pleistocene, with brain enlargement and correlated cranial changes taking place independently between 1.0 and 0.3 Mya in the local Homo populations of Africa, Java (Ngandong), Europe (Neandertals), and perhaps in other isolated demes.…”

The evolution of the human pelvis: changing adaptations to bipedalism, obstetrics and thermoregulation

“…Some researchers are convinced that the nature of locomotion in early hominins was close to that of modern humans, and differences between australopithecines and Homo reflect obstetric changes [11,20,21]. Other researchers take a different view and argue that there were major locomotor differences between australopithecines and Homo (see [7,46,84]), and counter that pelvic differences reflect both locomotor and obstetric needs. In this formulation, there were multiple phases of locomotor evolution in australopithecines, and obstetric concerns played a relatively minor role until the evolution of Homo species and possibly not until H. sapiens specifically.…”

“…These fossils have garnered attention because of their age (close to some of the oldest specimens of Homo erectus) and because of arguments that they may be ancestral to Homo [84]. Pelvic fossils from a male and a female have been recovered, and although conclusions must be tempered by the male's juvenile status, Kibii et al ([84], p. 1408) report that Au.…”

“…sediba is complex and differs from both earlier australopithecines and early Homo. Kibii et al [84] report that these specimens not only retain the wide interacetabular distance, long pubic rami and sacral shape of prior australopithecines, but also have Homo-like features including shorter, more vertical ilia and a shortened ischium with an everted ischial tuberosity. Their explanation for these features is complex, noting at some points that it is possible that the shortened ischium is due to pleiotropy associated with other pelvic changes, and at others arguing for a phyletic affinity with Homo, rather than novel locomotor or obstetric requirements.…”

“…Kibii et al also point out that Au. sediba's relatively M-L broad birth canal (and long femoral neck), but smaller degree of iliac flare, may present a challenge for Ruff's model of australopithecine hip biomechanics [10,84]. A conclusive interpretation of the complex and potentially mosaic features of the pelvis of Au.…”