Sexual Selection: A Mechanism for Human Upright Posture?
Sexual selection, inter- and intra-, was possibly a factor in the evolution of bipedalism, an upright posture, and perhaps the refinement of modern skull characteristics. The combination of sexual selection processes (1, 2, 3) and those for supernormal stimuli (4, 5) could have directed the change in much the same way as possibly occurred for peacock tails.
Hominids moved to a vertical posture over many generations; however, the selective advantages of an upright posture are still unclear. Speculations about our posture include its allowing us to detect prey or predators further away, to move with greater agility, carry tools and children,(6) or to survive intermittent floods.(7) These options of see further, run nimbly, throw further, carry kids, or drown later each have advantages but must be weighed against the homeostatic costs incurred with the decision to move a horizontal design to a vertical one.
The costs include those of increased spinal stress, rededication of existing abdominal straps that held our gut to our backbone, a narrowing of the birth canal, prolonged human infancy, and a narrower base of structural support. We also had to reprogram 2 limbs to perform the job of 4, a developmentally more complex task and, because of its novelty, perhaps more vulnerable to disruption. There was also a requirement that we shed muscle thickness because of our need to lose heat from a greater body mass if we were to maintain a relatively high metabolic rate.
Sexual selection is thought to produce rapid, unidirectional changes in physical traits even in the absence of environmental pressures and perhaps to have been an important mechanism in the enlargement of our brain.(3) However, sexual selection is usually described as the product of genetic models, particularly "linkage disequilibrium" (8) in which a "gene for" a trait is located near a presumed "gene for an attraction to" that same trait.
There is no means to choose between the sexual selection option and those mentioned earlier; indeed, it may be that all of them made their contribution to reproductive fitness at different times. Sexual selection, however, has the unusual characteristic of providing a consistent influence on reproductive choices during mating rather than at more distal moments.
However, sensory mechanisms could reduce or eliminate the role of a "gene for an attraction to" whatever trait is under consideration.(3, 9) Von Bekesy (10) identified a mechanism for lateral inhibition in touch, auditory, and visual systems. Lateral inhibition between sensory units increases the contrast between adjacent stimulus fields -- those of greater amplitude are perceptually enhanced, those of lesser are diminished further. Both are maximum at the boundary between stimuli. A functionally similar process has been identified in hypothesized reward systems in which greater than expected amounts of reward are correlated with dopamine release.
Regardless of physiological details, stimuli that have survival relevance and are larger, livelier, or more intense are often more salient in directing behavior. This process could underly many male sexual displays, at least to the extent that such are driven by female mating preferences. The same mechanisms may also operate in the case of displays between males; again, rapid changes in intensity and frequency are more easily detected and more apt to elicit changes in behavior outcomes. Thus, similar behavior sequences could impress both females and males.
Posture could respond to exactly the same principles. Our preference for greater height is seen in several ways. The preference appears in linguistic uses such as "Look up to," "working our way up," or even our putting "Heaven" above us. In contrast, "Looking down on," "getting demoted" and "lower than low" have the opposite denotations. These preferences appear to be transcultural -- the gods almost never live "down" in the valley and Satan almost never resides on the mountain top -- and perhaps evolved to the same extent as some of our other features that have also been shown to occur in fellow primates and to be universal among humans.
Females have also become taller alongside the males while maintaining sexual dimorphism; perhaps reflecting similar causes. Men and women share many behavioral traits and a preference for greater height may be one of them. Differences in height may actually result in command of greater resources directly or be mediated primarily through social deference. Either way, there could be enhanced reproductive success and resulting changes in gene frequencies. The actual mechanisms for such changes could be sensory but remain obscure as do the initial events in such preferences. Certainly, not all possible changes are equally probable; it is usually easier to make existing structures -- legs, vertebrae -- longer by modulating a regulator gene than to create new structures.
Attaining a vertical posture may have facilitated other mechanical changes such as cranial shape and skull alignment. Shortening the sphenoid bone (the central bone of the cranial base from which the face is levered outward), possibly resulted in facial receding back below the brow, reduction of the supraorbital crests, rounding of the braincase, and possibly lowering the larynx.(11) These changes also appear consistent with balancing a larger head on top of the spinal column while needing less help from surrounding muscles. These are expensive changes in terms of yielding a structure that could be more vulnerable to trauma; however, other species such as the peacock have made equally costly investments and perhaps for similar reasons.
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11) Lieberman, D. (1998) Sphenoid shortening and the evolution of modern human cranial shape. Nature, 393, 158-162.