A shift toward birthing relatively large infants early in human evolution

DeSilva, Jeremy M.

doi:10.1073/pnas.1003865108

Cited by 141 publications

(101 citation statements)

References 71 publications

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“…As bipedalism was refined in conjunction with an evolutionary increase in neonate and adult brain sizes, the morphology of the birth canal constrained the size and shape of the neonate (30)(31)(32). Although exactly when during hominin evolution the obstetric dilemma arose has been a subject of debate (33,34), this dilemma is especially severe in humans because of their large-headed (and relatively large-brained) neonates and relatively constricted birth canals.…”

Section: Discussionmentioning

confidence: 99%

Metopic suture of Taung ( Australopithecus africanus ) and its implications for hominin brain evolution

Falk

Zollikofer

Morimoto

et al. 2012

Proc. Natl. Acad. Sci. U.S.A.

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The type specimen for Australopithecus africanus (Taung) includes a natural endocast that reproduces most of the external morphology of the right cerebral hemisphere and a fragment of fossilized face that articulates with the endocast. Despite the fact that Taung died between 3 and 4 y of age, the endocast reproduces a small triangular-shaped remnant of the anterior fontanelle, from which a clear metopic suture (MS) courses rostrally along the midline [Hrdlicka A (1925) Am J Phys Anthropol 8:379-392]. Here we describe and interpret this feature of Taung in light of comparative fossil and actualistic data on the timing of MS closure. In great apes, the MS normally fuses shortly after birth, such that unfused MS similar to Taung's are rare. In humans, however, MS fuses well after birth, and partially or unfused MS are frequent. In gracile fossil adult hominins that lived between ∼3.0 and 1.5 million y ago, MS are also relatively frequent, indicating that the modern human-like pattern of late MS fusion may have become adaptive during early hominin evolution. Selective pressures favoring delayed fusion might have resulted from three aspects of perinatal ontogeny: (i) the difficulty of giving birth to large-headed neonates through birth canals that were reconfigured for bipedalism (the "obstetric dilemma"), (ii) high early postnatal brain growth rates, and (iii) reorganization and expansion of the frontal neocortex. Overall, our data indicate that hominin brain evolution occurred within a complex network of fetopelvic constraints, which required modification of frontal neurocranial ossification patterns.brain size evolution | virtual endocast | fossil hominins | frontal cortex | obstetrics

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Section: Discussionmentioning

confidence: 99%

Metopic suture of Taung ( Australopithecus africanus ) and its implications for hominin brain evolution

Falk

Zollikofer

Morimoto

et al. 2012

Proc. Natl. Acad. Sci. U.S.A.

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“…One possible interpretation of human altriciality is that human mothers invest less in fetal brain and body development than other primates, forgoing the last bit of maternal investment during gestation so neonates can negotiate the uniquely constrained birth canal. However, human neonates are large compared with other mammals (14,27,28), indicating that they receive more, not less, maternal investment during gestation than expected.…”

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confidence: 98%

Metabolic hypothesis for human altriciality

Dunsworth

Warrener

Deacon

et al. 2012

Proc. Natl. Acad. Sci. U.S.A.

288

253

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The classic anthropological hypothesis known as the "obstetrical dilemma" is a well-known explanation for human altriciality, a condition that has significant implications for human social and behavioral evolution. The hypothesis holds that antagonistic selection for a large neonatal brain and a narrow, bipedal-adapted birth canal poses a problem for childbirth; the hominin "solution" is to truncate gestation, resulting in an altricial neonate. This explanation for human altriciality based on pelvic constraints persists despite data linking human life history to that of other species. Here, we present evidence that challenges the importance of pelvic morphology and mechanics in the evolution of human gestation and altriciality. Instead, our analyses suggest that limits to maternal metabolism are the primary constraints on human gestation length and fetal growth. Although pelvic remodeling and encephalization during hominin evolution contributed to the present parturitional difficulty, there is little evidence that pelvic constraints have altered the timing of birth.bipedalism | EGG hypothesis | energetics | metabolic crossover hypothesis | pregnancy E utherian mammals vary widely in their degree of development at birth. Altricial species (e.g., rodents and some carnivores) are characterized by a large number of littermates and short gestation lengths, resulting in relatively undeveloped brains, a lack of specialization in corporal development, and feebleness at birth. Altricial neonates are usually hairless and dependent on external sources for warmth, and their sensory organs are often closed. In contrast, precocial species (e.g., bovids, equids, cetaceans) are born when they are highly developed with fully open and operating sensory organs. Immediately after birth, precocial neonates begin behaving similarly to adults in movement, sensory perception, and communication. Neonate development is thought to reflect each species' evolved maternal investment strategy, as well as environmental pressures, such as resource availability and predation risk (1-3).Humans differ from other primates in terms of neonatal development. Our neonates are born with the least-developed brains of any primate, with brains less than 30% of adult size (4). As a result, although human newborns are precocial in other respects, our neonates are neurologically and behaviorally altricial. Portmann (5) coined the term "secondary altriciality" to describe the distinct state of human neonates compared with the kind of primary or primitive altriciality experienced by other mammals and derived with respect to primate precociality. He estimated that instead of 9 mo, a gestation period of 18-21 mo would be required for humans to be born at neurological and cognitive developmental stage equivalent to that achieved by a chimpanzee neonate (see also ref. 6).Human altriciality has long been seen as an important hominin trait, not just because of its departure from the other primates but because of the reproductive and social strategies that vulnerable ...

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“…Work by WallScheffler and co-workers [69,70] suggests that a broad pelvis may provide the additional important benefit of increasing speed flexibility relative to locomotor cost, especially in short-legged individuals. Furthermore, Wall-Scheffler and co-workers have found that a wider pelvis reduces the cost of carrying an infant, which would have been an important energy drain for early female bipeds [69,71,72]. …”

Section: Pelvic Evolution In Early (Non-homo) Homininsmentioning

confidence: 99%

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

Gruss

Schmitt

2015

Phil. Trans. R. Soc. B

141

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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.

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A shift toward birthing relatively large infants early in human evolution

Cited by 141 publications

References 71 publications

Metopic suture of Taung ( Australopithecus africanus ) and its implications for hominin brain evolution

Metopic suture of Taung ( Australopithecus africanus ) and its implications for hominin brain evolution

Metabolic hypothesis for human altriciality

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

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