Brain expansion: the obstetric dilemma (Evolution)

by David Turell , Monday, July 06, 2020, 23:49 (1387 days ago) @ David Turell

A real problem for hominin and homo species at every level of development, and is still present:

The Obstetric Dilemma: An Ancient Game of Russian Roulette, or a Variable Dilemma Sensitive to Ecology?

"ABSTRACT: The difficult birth process of humans, often described as the ‘‘obstetric dilemma,’’ is commonly assumed to reflect antagonistic selective pressures favoring neonatal encephalization and maternal bipedal locomotion. However, cephalo-pelvic disproportion is not exclusive to humans, and is present in some primate species of smaller body size. The fossil record indicates mosaic evolution of the obstetric dilemma, involving a number of different evolutionary processes, and it appears to have shifted in magnitude between Australopithecus, Pleistocene Homo, and recent human populations. Most attention to date has focused on its generic nature, rather than on its variability between populations. We reevaluate the nature of the human obstetric dilemma using updated hominin and primate literature, and then consider the contribution of phenotypic plasticity to variability in its magnitude. Both maternal pelvic dimensions and fetal growth patterns are sensitive to ecological factors such as diet and the thermal environment. Neonatal head girth has low plasticity, whereas neonatal mass and maternal stature have higher plasticity. Secular trends in body size may therefore exacerbate or decrease the obstetric dilemma. The emergence of agriculture may have exacerbated the dilemma, by decreasing maternal stature and increasing neonatal growth and adiposity due to dietary shifts. Paleodemographic comparisons between foragers and agriculturalists suggest that foragers have considerably lower rates of perinatal mortality. In contemporary populations, maternal stature remains strongly associated with perinatal mortality in many populations. Long-term improvements in nutrition across future generations may relieve the dilemma, but in the meantime, variability in its magnitude is likely to persist. (my bold)

"The notion that maternal pelvic dimensions are subject to powerful competing demands from reproduction and locomotion is widely accepted in the biomedical and anthropological literature. The maternal pelvis is frequently considered to be subject to two counteracting evolutionary forces: decreased height and increased mediolateral breadth in order to optimize the biomechanics of locomotion, and increased anteroposterior dimensions in order to enable birth of the unusually encephalized human infant. The compromise imposed by these antagonistic demands manifests as a difficult passage of the fetal head through the birth canal, resulting in the birth process being a more complex and lengthy procedure in humans than in closely related species of ape. The antagonistic interaction of bipedalism and encephalization has been assumed to have followed the emergence of the large Homo brain within the last 2 million years. (my bold)

"In addition to shaping the unusual mechanism of birth, however, the ‘‘tug-of-war’’ exerted on maternal pelvic dimensions by bipedalism and encephalization is widely assumed to have increased the risks of delivery. Krogman (1951) described human birth as a ‘‘scar’’ of our evolutionary history, while Washburn (1960) coined the term ‘‘obstetric dilemma,’’ which has been widely used subsequently."

Comment: This obstetric dilemma is a major evolutionary issue as this review makes quite clear, and even has major obstetric importance today. I don't believe dhw's 'smart cooperating' cells could solve the problems by themselves, but God could easily. This is the website source in one of my accounts. I don't know if it will allow a download of the whole huge article to read it all:

https://www.academia.edu/2123423/The_Obstetric_Dilemma_An_Ancient_Game_of_Russian_Roule...