The Scars of Human Evolution

Krogman, Wilton Marion

doi:10.1038/scientificamerican1251-54

Cited by 81 publications

(64 citation statements)

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“…Large brains and bipedal walking in modern humans have made parturition an especially difficult task [1,2]. Researchers have argued that the process of human birth is unique among primates and mammals in three aspects: the mechanism of birth, difficulty of labour, and behaviour during and after labour [3][4][5].…”

Section: Introductionmentioning

confidence: 99%

Mechanism of birth in chimpanzees: humans are not unique among primates

et al. 2011

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Researchers have argued that the process of human birth is unique among primates and mammals in that the infant emerges with its face oriented in the opposite direction from its mother (occiput anterior) and head rotation occurs in the birth canal. However, this notion of human uniqueness has not been substantiated, because there are few comparative studies of birth in non-human primates. This paper reports the mechanism of birth in chimpanzees (Pan troglodytes) based on the first clear, close-up video recordings of three chimpanzee births in captivity. In all three cases, the foetus emerged with an occiput anterior orientation, and the head and body rotated after the head had emerged. Therefore, these characteristics are not uniquely human. Furthermore, in two of the three cases, the chimpanzee newborns landed on the ground without being guided from the birth canal by the mother. The fact that the human newborn emerges with an occiput anterior orientation has thus far been taken as evidence for the necessity of midwifery in modern humans, but this view also needs revision. Our observations raise the need to reconsider the evolutionary scenario of human birth.

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Section: Introductionmentioning

confidence: 99%

Mechanism of birth in chimpanzees: humans are not unique among primates

et al. 2011

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“…However, it is more commonly proposed that human gestation length and altriciality are a solution to the uniquely human "obstetrical dilemma" (10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20), also called the "obstetric dilemma" and, henceforth, referred to as OD. The OD hypothesis proposes that Pleistocene hominins faced antagonistic selection pressures on locomotor requirements and encephalization.…”

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confidence: 99%

Metabolic hypothesis for human altriciality

Dunsworth

Warrener

Deacon

et al. 2012

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

297

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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“…The shape of the human pelvis is assumed to be a compromise solution. Human pelvises are shaped for upright walking, but at the same time, they must remain wide enough for giving birth to large-headed neonates (1,2,(9)(10)(11)(12). Upright walking evolved at least 4-5 million years ago and required major skeletal adjustments (9,13,14).…”

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confidence: 99%

Covariation between human pelvis shape, stature, and head size alleviates the obstetric dilemma

Fischer

Mitterœcker

2015

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

104

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Compared with other primates, childbirth is remarkably difficult in humans because the head of a human neonate is large relative to the birth-relevant dimensions of the maternal pelvis. It seems puzzling that females have not evolved wider pelvises despite the high maternal mortality and morbidity risk connected to childbirth. Despite this seeming lack of change in average pelvic morphology, we show that humans have evolved a complex link between pelvis shape, stature, and head circumference that was not recognized before. The identified covariance patterns contribute to ameliorate the "obstetric dilemma." Females with a large head, who are likely to give birth to neonates with a large head, possess birth canals that are shaped to better accommodate large-headed neonates. Short females with an increased risk of cephalopelvic mismatch possess a rounder inlet, which is beneficial for obstetrics. We suggest that these covariances have evolved by the strong correlational selection resulting from childbirth. Although males are not subject to obstetric selection, they also show part of these association patterns, indicating a genetic-developmental origin of integration.

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The Scars of Human Evolution

Cited by 81 publications

References 0 publications

Mechanism of birth in chimpanzees: humans are not unique among primates

Mechanism of birth in chimpanzees: humans are not unique among primates

Metabolic hypothesis for human altriciality

Covariation between human pelvis shape, stature, and head size alleviates the obstetric dilemma

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