The evolution of easier births means slower walking and pelvis issues

Hip Width and its Complex Role in Human Evolution, Childbirth, and Health

The breadth of human hips, a trait influenced by the interplay between evolving larger brains and the adoption of upright walking, is the focus of a comprehensive new study. This research, the most extensive of its kind, sheds light on the intricate evolutionary trade-offs shaping human anatomy and health outcomes like birth complications and back pain.

The Obstetrical Dilemma: Balancing Brain Size and Pelvis Size

“The increasing size of the brain during human evolution presented a biomechanical challenge as the pelvis simultaneously adapted for bipedal locomotion, becoming narrower,” explains Vagheesh Narasimhan from the University of Texas at Austin. “This evolutionary tug-of-war, where brain enlargement and pelvic narrowing occur concurrently, creates an inherent conflict.”

This concept, initially proposed in the 1960s, is termed the obstetrical dilemma. More recently, researchers have proposed that susceptibility to pelvic floor disorders also contributes to this evolutionary balancing act. The pelvic floor, a network of muscles supporting internal organs, can weaken or tear, potentially leading to incontinence and difficulties during childbirth.

Conflicting Results in Prior Pelvis Research

“The obstetrical dilemma has spurred lively debate within the scientific community,” notes Narasimhan. Numerous prior investigations have explored the relationship between pelvis structure and factors like walking speed and efficiency. However, these earlier studies, often involving smaller participant groups, have yielded inconsistent and sometimes contradictory findings, according to Narasimhan.

Large-Scale Study Utilizing UK Biobank Data

In a significant advancement, Narasimhan and his team leveraged data from the UK Biobank, analyzing information from 31,000 men and women. Using dual-energy X-ray absorptiometry scans, the researchers meticulously measured various aspects of pelvic structure. They then examined correlations between these measurements, genetic variations, and health indicators, including the necessity for emergency cesarean sections.

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Key Findings: Pelvis Width, Health Risks, and Benefits

The study’s findings suggest that a wider pelvis is associated with a reduced likelihood of birth complications. However, it also correlates with slower walking speed and an elevated risk of pelvic floor conditions and hip osteoarthritis. Conversely, a narrower pelvis may facilitate faster walking but increases the risk of birth complications, back pain, and knee osteoarthritis.

Pelvis Width and Pregnancy Duration

Previous theories suggested a link between narrower hips and earlier births as a mechanism to mitigate birth complications. Contrary to this, the current research found no correlation between pelvis width and the duration of pregnancies. “This aligns with other research indicating that human infants are not born prematurely compared to other great ape offspring,” Narasimhan clarifies.

Natural Selection and Infant Head Size

The research team did identify a connection between pelvic width and infant head circumference at birth. “Individuals predisposed to giving birth to babies with larger heads tend to possess wider pelvises,” Narasimhan explains. “This phenomenon is attributable to natural selection, which has historically favored individuals exhibiting this correlation.” However, a 2016 study proposed that the prevalence of C-sections might be diminishing this selective pressure.

Pelvic Asymmetry and Handedness

An additional noteworthy discovery is the prevalence of minor pelvic asymmetry among individuals, linked to their dominant hand. Handedness typically dictates leg dominance, influencing gait and pelvic development. This interplay can contribute to slight asymmetries in pelvic structure as individuals mature, according to Narasimhan.

Expert Perspectives on the Study’s Significance

Scott Simpson at Case Western Reserve University in Cleveland, Ohio, hails the study as “an extraordinary contribution to a fundamental aspect of human evolutionary biology.” He emphasizes the study’s integrated approach, stating, “By synthesizing anatomical, genetic, clinical, and behavioral data, the authors have provided profound insights into this uniquely human adaptation.”

Nicole Webb from the University of Zurich in Switzerland, whose prior work has shown that chimpanzee birth canals are surprisingly similar in width to those of humans, acknowledges the value of large datasets. However, Webb also points out a limitation: the UK Biobank dataset comprises individuals over 40 of UK origin. “Conducting this research on younger, more diverse populations could potentially yield even more compelling results,” she suggests.