PREGNANCY THROUGH HUMAN EVOLUTION
- López, B. & Rueda, A.
- 17 dic 2019
- 2 Min. de lectura
The Obstetrician Dilemma
The obstetrician dilemma is a hypothesis that tries to explain why childbirth in modern humans is more complicated than in non-human primates. Also, it tries to explain altriciality (species that have a very immature and helpless offspring that require care for some time) in modern humans. It claims that the enlargement of the brain and the reduction of the birth canal due to a bipedal posture made giving birth more difficult. This idea was proposed in 1960 but it has since been criticized.
In 2012 a metabolic hypothesis for human altriciality was proposed: no evidence for the idea that the length of human gestation is a compromise between hip width and brain size was found in a study directed by Holly Dunsworth of the University of Rhode Island. In the study it was concluded that the timing of childbirth occurs when women’s bodies can no longer keep up with the energy demands of pregnancy.
During the study they analyzed the gestation length, they decided to compare gestation length to maternal body size. It was concluded that our pregnancies are 37 days longer than it should be for primates our size. When comparing it to the gestation length of chimpanzees, gorillas and orangutans it was observed that our pregnancy is longer.
The idea that the broadening of the hips would increase the force needed by hip muscles to walk and run making locomotion more energy demanding has been accepted for in the scientific field. In the study it was estimated the a three-centimeter increase would be needed for humans were born with the same brain development as chimps. Actually, women’s hips already vary by three or more centimeters.
They suggested that gestation length might be limited by the mother’s energy. During pregnancy mammal females reach the limit of the amount of energy they can provide to the baby, known as the “metabolic ceiling”. In the study it is concluded that women reach the metabolic ceiling by their sixth month of pregnancy and that but by the ninth month the energy she provides to the fetus surpasses the limit. This hypothesis was named “EGG hypothesis” (energetics of gestation and growth hypothesis).
Some other studies have also contradicted the Obstretician Dilemma or its bases (the narrowing of the hips for more efficient motility):
- Females that give birth to babies with a large head possess a larger birth canal: Fischer, Barbara; Mitteroecker, Philipp (2015). "Covariation between human pelvis shape, stature, and head size alleviates the obstetric dilemma". PNAS. 112 (18): 5655–5660. Bibcode:2015PNAS..112.5655F. doi:10.1073/pnas.1420325112. PMC 4426453. PMID 25902498
- A broader birth canal or pelvis does not make walking less efficient in humans: Warrener, A.G. et al, 2015. A wider pelvis does not increase locomotor costs in humans with implications for the evolution of childbirth. PLoS One 10(3): e0118903.
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