Philos Trans R Soc Lond B Biol Sci. 2015 Mar 5; 370(1663): 20140063.
The evolution of the human pelvis: changing adaptations to bipedalism, obstetrics and thermoregulation
The fossil record of the human pelvis reveals the selective priorities acting on hominin anatomy at different points in our evolutionary history, during which mechanical requirements for locomotion, childbirth and thermoregulation often conflicted. In our earliest upright ancestors, fundamental alterations of the pelvis compared with non-human primates facilitated bipedal walking. Further changes early in hominin evolution produced a platypelloid birth canal in a pelvis that was wide overall, with flaring ilia. This pelvic form was maintained over 3–4 Myr with only moderate changes in response to greater habitat diversity, changes in locomotor behaviour and increases in brain size. It was not until Homo sapiens evolved in Africa and the Middle East 200 000 years ago that the narrow anatomically modern pelvis with a more circular birth canal emerged. This major change appears to reflect selective pressures for further increases in neonatal brain size and for a narrow body shape associated with heat dissipation in warm environments. The advent of the modern birth canal, the shape and alignment of which require fetal rotation during birth, allowed the earliest members of our species to deal obstetrically with increases in encephalization while maintaining a narrow body to meet thermoregulatory demands and enhance locomotor performance.
Science. 2008 Nov 14;322(5904):1089-92. doi: 10.1126/science.1163592.
A female Homo erectus pelvis from Gona, Ethiopia.
Department of Anatomy, Case Western Reserve University School of Medicine, Cleveland, OH 44106-4930, USA.
Analyses of the KNM-WT 15000 Homo erectus juvenile male partial skeleton from Kenya concluded that this species had a tall thin body shape due to specialized locomotor and climatic adaptations. Moreover, it was concluded that H. erectus pelves were obstetrically restricted to birthing a small-brained altricial neonate. Here we describe a nearly complete early Pleistocene adult female H. erectus pelvis from the Busidima Formation of Gona, Afar, Ethiopia. This obstetrically capacious pelvis demonstrates that pelvic shape in H. erectus was evolving in response to increasing fetal brain size. This pelvis indicates that neither adaptations to tropical environments nor endurance running were primary selective factors in determining pelvis morphology in H. erectus during the early Pleistocene.
PMID:19008443 DOI: 10.1126/science.1163592
Osteoarthritis – a consequence of evolution?
For Homo sapiens, the female pelvis is the single skeletal element that conveys information about the two most peculiar traits of human evolution. These are upright gait and an ultra-large brain. It shows both the adaptations that occurred to facilitate a permanent bipedal gait, and at the same time the adjustments required to accommodate the birth of a large-brained foetus.7,8 Using such an evolutionary perspective, two human hip disorders can be considered – FAI and DDH. Both feature frequently in current orthopaedic practice.
DNA evidence dates the shared ancestor of chimpanzees and humans to approximately between six and seven million years ago.9 Since then extensive changes have occurred in the pelvis (Fig. 2) and, by comparison, the morphological changes in the hip have been quite minor. The last 50 years have yielded spectacular fossil finds that have helped map hominid evolution. The restructuring of the pelvis is best described as a compacting of the pelvis, with transition from a nearly two-dimensional to three-dimensional form.
The main feature of this compacting8 has been a marked shortening of the ilium, while the sacrum enlarged in all dimensions and came lower to lie opposite the pubis. The result has been a bony birth canal that can cause trouble during childbirth. In addition, the sacrum moved forward (ventrally) and tilted, while the lumbar spine lengthened. The number of lumbar vertebrae increased, from three or four in the chimpanzee to five, sometimes six, in Homo sapiens.This facilitated the development of a lumbar lordosis, thereby positioning the spine more centrally and bringing the centre of gravity of the upper body closer to the hip joints in the sagittal (lateral) plane.
The human ilium may have become shorter, but it also arches further forward (ventrally), creating prominent anterior superior iliac spines. This forward-arching ilium repositions the gluteal muscles over the hip joint. In the large apes (orang-utan, chimpanzee, gorilla) these muscles are almost entirely posterior to the hip joint, which is why they function mainly as hip extensors. Meanwhile, human gluteal muscles are posterior, directly above and anterior to the hip joint, making them true hip abductors.
Early human ancestors (hominids) first began walking upright and only later developed a large brain. Evidence for this comes from Australopithecus afarensis of 3.2 million years ago, that was well-adapted to a permanent upright gait11 but still had a body and brain size similar to a chimpanzee.7 In the subsequent three million years, body size approximately doubled while brain size tripled. This brain enlargement thus happened when the pelvis, in evolutionary terms, had already undergone extensive restructuring to facilitate a true upright gait. There had also been a remarkable elongation of the lower limbs.
From approximately three to 0.5 million years ago only anteroposterior deepening of the pelvis appears to have taken place through relative growth of the pubic bones while the relative width of the pelvis decreased. This may be because of the importance of an efficient abductor mechanism for the now permanent bipedal gait of early humans. To keep required abductor work within limits, the lever arm of bodyweight should also be kept within limits.12 Indeed, the distance between the midline of the pelvis and the centre of the femoral head has been said to be larger for human females than males.13
A large foetal brain and long legs may present serious problems at childbirth. Today’s rate of Caesarean section is approximately 20% in developed countries.14 Meanwhile, obstetric problems have never been documented in the large apes.15 Nevertheless, difficulties with childbirth are not exclusive to humans, as bovids and smaller primates such as the macaque are known to have birthing problems.