Human Pelvis and Long Bones Reveal Differential Preservation of Ancient Population History and Migration Out of Africa

Abstract

One of the main events in the history of our species has been our expansion out of Africa. A clear signature of this expansion has been found on global patterns of neutral genetic variation, whereby a serial founder effect accompanied the colonization of new regions, in turn creating a within-population decrease in neutral genetic diversity with increasing distance from Africa. This same distinctive pattern has also been described for cranial and dental morphological variation in human populations distributed across the globe. Here, we used a data set of postcranial linear measurements for 30 globally distributed human populations, and a climatic data set of minimum annual temperature, maximum annual temperature, and precipitation in order to separate for the first time the relative effect of neutral demographic processes and climatic selection on four long (limb) bones (femur, tibia, radius, and humerus) versus the pelvic bones of the human appendicular skeleton. We implemented a stepwise regression procedure in which phenotypic variance is assumed to be affected by the iterative founder events that accompanied human expansion from Africa, as well as by climate. This model included, as independent factors, geographic distance from central Africa, the three climatic variables, and all possible interactions between the three climatic variables. We excluded all nonsignificant factors by backward stepwise elimination with the aim of identifying the minimal model significantly explaining variation in the phenotypic data. Our results indicate a sharp difference in the way the pelvis and the limb bones reflect the neutral signature of the out-of-Africa expansion. Consistent with previous analyses of the cranium and dentition, pelvic shape variation shows a significant within-population decrease with increasing distance from Africa. However, no such pattern could be found in the long bones. Rather, in the case of both the tibia and the femur, a significant relationship between population-level variance and minimum temperature was demonstrated. Hence, in the case of these limb bones, it is probable that the effects of climatic selection have obliterated the demographic signature of human dispersal from Africa. Our finding that pelvic variation exhibits the neutral effects of demographic history suggests that consideration of this skeletal element might be used to shed light on factors of human population history, just as the cranium has done. KeywordsSerial Founder Effect, Out Of Africa, Anatomically Modern Humans, Pelvis Cover Page FootnoteWe are grateful to Brendon Billings of the Medical School of the University of Witwatersrand and Dr. Ogeto Mwebi of the National Museum of Kenya for allowing access to collections and general assistance. We are also especially grateful to Dr. Benjamin Auerbach for generously making his data freely available. We thank Franz Manni and three anonymous reviewers for constructive comments. This study was funded by Sigma Xi, The Abstract One of the main events in the history of our...

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

“…As discussed in §3, a wider body conserves heat, whereas a narrower body increases heat loss [8,9]. In a series of exhaustive studies, Ruff has demonstrated that pelvic bi-iliac breadth (intercristal diameter) varies with latitude among modern humans [2,12,13,115], and fossil hominins show the same pattern, with those from warmer climates, including African earliest Homo, having smaller pelvic breadths than those from colder climates [2,12,13,116] (although see [18]). …”

“…The relationship between body size and shape, overall pelvic size and shape, and birth canal size and shape in modern humans is complex [3,15,17,18,79,[138][139][140][141]. With the evolution of the modern pelvis, the size of the birth canal was essentially decoupled from the size of the body: birth canal proportions in modern humans do not necessarily correlate with body proportions [138,141,142].…”

“…Evolutionary theory would suggest that because 'selection intensity and phenotypic variability are inversely related' ( [143], p. 59), there should be sexual dimorphism in the variability of pelvic morphology, with variation in the size and shape of the female birth canal limited by obstetric demands. However, the nature and extent of sexual dimorphism in the modern pelvis are particularly complex, and a large number of studies have produced no clear indication that obstetric constraints produced between-sex differences in variability [3,18,138,139,[141][142][143]. To further complicate matters, work by Betti and co-workers [3,18,141] has suggested that among modern humans, neutral evolutionary processes reflecting demographic history have also been important in determining variation in pelvic form (see also [17]).…”

“…Through the evolution of the uniquely shaped birth canal, modern humans have been able to birth larger-brained infants, while maintaining the flexibility to adapt body breadth to varying habitats and at the same time improving locomotor performance. Endnotes 1 Betti and co-workers [3,18] have shown that in addition to selection based on thermoregulatory and locomotor constraints, neutral evolutionary processes have also had a significant effect on pelvic shape variation among modern humans (see §3). However, the fossil record is insufficient to allow investigation of the extent of neutral evolution of the pelvis in extinct humans.…”

Population differences in the postcrania of modern South Africans and the implications for ancestry estimation

“…In contrast to crania, postcrania are frequently labelled as exceedingly variable and unreliable for assessing population affinity, owed primarily to the extreme plasticity exhibited by the appendicular skeleton [24][25][26]. For example, long bone lengths and epiphyseal breadths have been correlated to climate, specifically minimum average temperatures [27]. Similarly, stature is frequently correlated with socioeconomic status (SES), where lower SES typically results in shorter stature [28,29].…”