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Human evolution: initial steps toward an hourglass figure in the female
The female hourglass figure obviously reflects sexual selection. However, sexual selection acts on male-female differences to start with. Therefore, what prompted shape differences in the first place? Boguslaw Pawlowski and Marzena Grabarczyk have written a paper on this, and it is addressed here.
The paper refers to the center of mass of a body, which is the point around which the mass of the body is equally distributed. For instance, most of the mass of a human body lies away from a finger tip. Therefore, the center of mass of a human body is not going to be in the fingers, but somewhere in the abdominal region.
The authors have proposed that when our ancestors started walking upright, walking became more challenging for females in the later stages of pregnancy and those carrying babies (in the front) as these conditions shift the center of mass upward and forward, and the women would surely have to walk to obtain food. This challenge can be eased by adding more mass to the hips (more protruding buttocks, wider hips) and thighs (e.g., more fat deposition on the thighs), as well as reducing upper body mass, thereby shifting the center of mass toward the rear and downward. Hominid females with relatively bigger hips for a given waist size and somewhat lesser upper body mass than others would be at an advantage with respect to foraging during the later stages of pregnancy and when carrying infants in front, and the males more attracted toward such females would enjoy better reproductive success. This would lead to a greater frequency of Hominid females shifted toward the modern human female form as well as a greater proportion of hominid males with a preference for females having bigger hips relative to waist size and reduced upper body mass in the form of a smaller musculoskeletal upper body frame. Modern human male preference for large female breasts is not inconsistent with the advantage of reduced upper body mass to the ancestral Hominid females because the prominence of female breasts is due to fat, which doesn’t weigh much; a reduction of skeletal and muscular mass would compensate for an increase in breast size many times over.
To assess the feasibility of the proposal above, the authors obtained various body measurements, including center of mass, in a sample of young women and assessed the correlations between the measurements. A low center of mass, adjusted for height, corresponded to narrower shoulders, a lower waist-to-hip ratio and greater thigh circumference, but not the following variables: trunk length, chest circumference, lower limb length, hip width, hip circumference, waist circumference and body mass. Therefore, the position of the center of mass was correlated with three variables relevant to a feminine look and consistent with the authors’ proposal above, namely shoulder width, waist-to-hip ratio and thigh circumference. Although hip width is relevant to an hourglass/feminine look, since the region where hip width is measured is very close to the center of mass, the lack of correlation should not be surprising, but one sees correlations with parts of the body distant from the center of mass, namely shoulders and thighs. In other words, the data are consistent with the authors’ contention.
Pawlowski B, Grabarczyk M. Center of body mass and the evolution of female body shape. Am J Hum Biol. 2003 Mar-Apr;15(2):144-50. (zip)
Abstract: Among primates, the genus Homo has a unique sexual dimorphism in general body shape. The stenotypic female "hourglass figure" has often been attributed to sexual selection. Sexual dimorphism both in shape and in position of the center of body mass (CoM) emerges during puberty and is related to hormonal influences. These are only the proximal and not the ultimate causes of this feature. This article explores the hypothesis that the evolutionary (i.e., ultimate) reason for female body shape and male preference for a lower waist-to-hip ratio (WHR) is due to the acquisition of bipedal locomotion and different biomechanical constraints on each sex. The demands of pregnancy and subsequently carrying infants may have more tightly constrained CoM in females than in males. A lower-position of CoM relative to height (RCoM=(CoM/height)*100%) would contribute to better stability during pregnancy and infant carrying. Using body measurements from 119 female students, we show that RCoM correlates negatively with only maximal thigh circumference and positively with only WHR and shoulder width. The relationship between RCoM and traits that best characterize female body shape seems to confirm a hypothesis of biomechanical selection pressure that may have acted on Homo female morphology, thus contributing to sexual dimorphism.