Metabolic hypothesis for human altriciality

Abstract

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 ...

Relaxed genetic control of cortical organization in human brains compared with chimpanzees

“…Humans have evolved a secondary altricial pattern of development compared to the more precocial pattern that characterizes other living primates (34); this developmental pattern might be related to obstetrical (35) or metabolic constraints (36). Regardless of the initial causal factor, an altricial pattern of development, once established, may have provided fundamental selective advantages through the opportunity for postnatal maturation and associated increased learning abilities to allow human offspring to incorporate cultural information through social-transmission mechanisms.…”

Developmental evidence for obstetric adaptation of the human female pelvis

“…There is general agreement that the female pelvis is under obstetric selection to be adequately capacious for childbirth. However, the exact nature of selective pressures and developmental mechanisms yielding female and male pelvic phenotypes is still largely unknown, and whether obstetric adaptations involve trade-offs with other aspects of pelvic function, such as locomotor efficiency and abdominal stabilization, continues to be debated (12,13).…”

“…Over the past years, the OD hypothesis has been reexamined extensively and has been challenged on various grounds (10)(11)(12)(13)(19)(20)(21)(22). The energetics of gestation and growth (EGG) hypothesis (12,20,23) provides a new perspective, proposing that the timing of birth is constrained by the limited metabolic output of the mother rather than by spatial limitations of her pelvis.…”

Extraordinary intelligence and the care of infants

“…Humans may have solved this dilemma by having young and incapable neonates and decreasing gestation time because pelvis size could not increase further. Alternatively, the metabolic costs of gestation may have constrained the maximum size child that mothers could support (39). Either case results in a maximum allowable size at birth.…”