There is no \u27obstetrical dilemma\u27: Towards a braver medicine with fewer childbirth interventions

Humans give birth to big-brained babies through a bony birth canal that metamorphosed during the evolution of bipedalism; they have a tighter fit at birth between baby and bony birth canal than do our closest relatives the chimpanzees; and they are incapable of grasping onto caregivers as early as infant chimpanzees develop the skill. Since the mid-20th century, these observations and more have been linked together into the obstetrical dilemma (OD): human babies are helpless because they are born early to escape before they outgrow the mother\u27s pelvis, the expansion of which is prevented by natural selection for bipedalism. The OD continues to be a popular idea, often expressed as incontrovertible fact, but it no longer deserves its popularity. There are better explanations for gestation length, childbirth difficulty, and the developmental biology of newborns than mainly or only because of natural selection\u27s constraints on women\u27s hips. And humans are not born early either, as is widely assumed. This all-too-powerful human evolutionary narrative deserves our skeptical consideration. Bias from OD thinking is likely amplifying the perceived risk of cephalopelvic and fetopelvic disproportion during labor—contributing, even if slightly, to medicine\u27s underestimation of women\u27s bodies and over-implementation of childbirth interventions

Origin of the Genus \u3ci\u3eHomo\u3c/i\u3e

The origin of the genus Homo in Africa signals the beginning of the shift from increasingly bipedal apes to primitive, large-brained, stone tool-making, meat-eaters that traveled far and wide. This early part of the human genus is represented by three species: Homo habilis, Homo rudolfensis, and Homo erectus. H. habilis is known for retaining primitive features that link it to australopiths and for being the first stone tool makers. Little is known about H. rudolfensis except that it had a relatively large brain and large teeth compared to H. habilis and that it overlapped in time and space with other early Homo. Our understanding of the paleobiology and evolution of the larger-brained H. erectus is enhanced due to its rich fossil record. H. erectus was the first obligate, fully committed biped, and with a body adapted for modern striding locomotion, it was also the first in the human lineage to disperse outside of Africa. The early members of the genus Homo are the first to tip the scale from the more apish side of our evolutionary history toward the more human one

The Evolution of Difficult Childbirth and Helpless Hominin Infants

Because of the implications for behavioral, social, and cultural evolution, reconstructions of the evolutionary history of human parturition are driven by two main questions: First, when did childbirth become difficult? And second, does difficult childbirth have something to do with infant helplessness? Here we review the available evidence and consider answers to these questions. Although the definitive time frame remains unclear, childbirth may not have reached our present state of difficulty until fairly recently

Investigating the Genetic Basis for Hominoid Taillessness

Investigating the Genetic Basis for Hominoid Taillessness: A Comparative Genetic Approach Across Ten Catarrhine Taxa Samantha Tickey-McCrane1,2, Johanna E. Wegener2, and Holly Dunsworth1 Honors Thesis Abstract Written by Samantha Tickey-McCrane, Departments of Anthropology & Biology Advisor: Dr. Holly Dunsworth, Department of Anthropology How did hominoid tail loss occur? My goals are to test phylogenetic and adaptive hypotheses for tail length variation among macaques, and use those insights to reconstruct the evolution of hominoid taillessness. Further, I aim to ultimately uncover which candidate genes or pathways may be responsible for catarrhine tail loss, and what other traits may be affected by these developmental and genetic pathways. I explored published catarrhine vertebral counts and phylogenies in the literature. I also collected data from 95 Macaca and Papio individuals in the collections at the American Museum of Natural History, NY. Based on known mechanisms of tail formation in embryos, I identified the genes that might be responsible for the interruption of tail development. I took these candidates to the annotated whole genomes of catarrhine primates and used a comparative approach across 10 taxa. I also focused on cis-regulatory regions 1,000 base pairs upstream of the candidate genes, that may have been involved in gene regulation. Regarding the skeletal data, there appears to be a pattern where tail length variation is determined by factors of 3-4 caudal vertebrae, suggesting a segmental basis for the genetic factors involved. My preliminary genomic analyses indicate that comparing candidate genes is valuable, but is only a first step because regulatory non-exonic elements associated with these genes are more likely to be involved in taillessness. Investigating the developmental and genetic bases of tail variation among Macaca holds great promise for reconstructing the evolutionary history of hominoid taillessness and its consequences. Future studies continuing to probe whole genomes and the expansion of available primate genomes will make this possible. If we can discover the underlying genetic mechanisms for taillessness, we can reconstruct the evolution of this significant feature that we share with apes, and that may have been a necessary precursor to bipedalism. Acknowledgements Thank you to Johanna E. Wegener and Dr. Holly Dunsworth of the University of Rhode Island for their continual mentorship in a new area of study for me, collaboration, and support

Ontogeny of hallucal metatarsal rigidity and shape in the rhesus monkey (Macaca mulatta) and chimpanzee (Pan troglodytes)

Life history variables including the timing of locomotor independence, along with changes in preferred locomotor behaviors and substrate use during development, influence how primates use their feet throughout ontogeny. Changes in foot function during development, in particular the nature of how the hallux is used in grasping, can lead to different structural changes in foot bones. To test this hypothesis, metatarsal midshaft rigidity [estimated from the polar second moment of area (J) scaled to bone length] and cross-sectional shape (calculated from the ratio of maximum and minimum second moments of area, Imax /Imin ) were examined in a cross-sectional ontogenetic sample of rhesus macaques (Macaca mulatta; n = 73) and common chimpanzees (Pan troglodytes; n = 79). Results show the hallucal metatarsal (Mt1) is relatively more rigid (with higher scaled J-values) in younger chimpanzees and macaques, with significant decreases in relative rigidity in both taxa until the age of achieving locomotor independence. Within each age group, Mt1 rigidity is always significantly higher in chimpanzees than macaques. When compared with the lateral metatarsals (Mt2-5), the Mt1 is relatively more rigid in both taxa and across all ages; however, this difference is significantly greater in chimpanzees. Length and J scale with negative allometry in all metatarsals and in both species (except the Mt2 of chimpanzees, which scales with positive allometry). Only in macaques does Mt1 midshaft shape significantly change across ontogeny, with older individuals having more elliptical cross-sections. Different patterns of development in metatarsal diaphyseal rigidity and shape likely reflect the different ways in which the foot, and in particular the hallux, functions across ontogeny in apes and monkeys

Metabolic hypothesis for human altriciality

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

Human Races Are Not Like Dog Breeds: Refuting a Racist Analogy

In 1956, evolutionary biologist J.B.S. Haldane posed a question to anthropologists: “Are the biological differences between human groups comparable with those between groups of domestic animals such as greyhounds and bulldogs…?” It reads as if it were posted on social media today. The analogy comparing human races to dog breeds is not only widespread in history and pop culture, but also sounds like scientific justification for eschewing the social construction of race, or for holding racist beliefs about human nature. Here we answer Haldane’s question in an effort to improve the public understanding of human biological variation and “race”—two phenomena that are not synonymous. Speaking to everyone without expert levels of familiarity with this material, we investigate whether the dog breed analogy for human race stands up to biology. It does not. Groups of humans that are culturally labeled as “races” differ in population structure, genotype–phenotype relationships, and phenotypic diversity from breeds of dogs in unsurprising ways, given how artificial selection has shaped the evolution of dogs, not humans. Our demonstration complements the vast body of existing knowledge about how human “races” differ in fundamental sociocultural, historical, and political ways from categories of nonhuman animals. By the end of this paper, readers will understand how the assumption that human races are the same as dog breeds is a racist strategy for justifying social, political, and economic inequality

Remnants of an ancient forest provide ecological context for Early Miocene fossil apes

The lineage of apes and humans (Hominoidea) evolved and radiated across Afro-Arabia in the early Neogene during a time of global climatic changes and ongoing tectonic processes that formed the East African Rift. These changes probably created highly variable environments and introduced selective pressures influencing the diversification of early apes. However, interpreting the connection between environmental dynamics and adaptive evolution is hampered by difficulties in locating taxa within specific ecological contexts: time-averaged or reworked deposits may not faithfully represent individual palaeohabitats. Here we present multiproxy evidence from Early Miocene deposits on Rusinga Island, Kenya, which directly ties the early ape Proconsul to a widespread, dense, multistoried, closed-canopy tropical seasonal forest set in a warm and relatively wet, local climate. These results underscore the importance of forested environments in the evolution of early apes