Human evolution theory utilizing concepts of neoteny & female sexual selection
An etiology of neuropsychological disorders such as autism and dyslexia, and the origin of left handedness.

 

 Library of Excerpts

Sexual Dimorphism as Evidence of Heterochronic Patterns


"Different prehuman species suggest different levels of sperm competition. Judging from its teeth, Australopithecus africanus was certainly a vegetarian. Like gorillas, the vegetarian australopithecines showed a bigger difference in body size between sexes than men and women do today, suggesting that they were sperm-competition avoiders. Although no one, of course, knows for sure, Robert Smith speculates that physically imposing males bossed relatively sexually faithful females in australopithecine harems. These ancestors would have been very sexist, but the males, violently intolerant of promiscuity, would not have developed large genitals. This sultanlike breeding behavior could well have undergone radical change with the evolution of Homo habilis, "handy man": Smith postulates that subordinate habilis males, scavenging meat and offering pieces of it in exchange for sex, upset the earlier breeding system. The cooperative hunting groups that began with Homo erectus --- our most recent evolutionary predecessor --- ushered in relatively high levels of sperm competition. Homo erectus males were not much larger than Homo erectus females. Homo erectus was a communal species who not only gathered edible plants but hunted mammoths and used fire. Eating and sleeping together in groups --- the sort of cooperative groups needed to hunt --- may have made them far more social, more talkative, and better barterers than their sexually dimorphic australopithecine ancestors. And more promiscuous. It was with Homo erectus, Smith suggests, that people developed their relatively large male genitals." (Margulis, L & Sagan, D. (1991) Mystery Dance, On the Evolution of Human Sexuality: Summit Books, New York pp. 51)

“Chimpanzees are reproductively extraordinary among the great apes. They are not strikingly dimorphic for size as are the other two species (Fig 2), but male chimpanzees have enormous scrotal testes, proportionately about 5 and 10 times larger than Pongo and Gorilla, respectively, and a specialized penis more than twice as long as that on the much larger gorilla. Short (1981) has calculated that the testes of an averave chimpanzee can sustain sperm production at a level that will produce at least four full-strength ejaculates/day, each containing several times the number of sperm in an average gorilla or orangutan ejaculate.” (Robert L. Smith (1984) Human Sperm Competition. in Sperm Competition and the Evolution of Animal Mating Systems pp. 619)

The reduction in sexual dimorphism form Homo habilis to Homo erectus was acually an increase in females size, this size increase may have been driven primarily by selection for a wider pelvic outlet to allow the birth of larger-brained babies." (Miller, Geoffrey F. (1994) Evolution of the human brain through runaway sexual selection: the mind as a protean courtship device. unpublished thesis. pp. 25)

"The human brain also demonstrates gross anatomical features of sexual dimorphism. A recent neuropathological study showed the anterior commissure and the massa intermedia to be larger in females than in males (Allen & Gorski, 1991)." (Small S.L., Hoffman G.E. (1994) Neuroanatomical lateralization of language: sexual dimorphism and the ethology of neural computation. Brain and Cognition 26: 308)

"Bones of australopithecines, Australopithecus afarensis, were unearthed near Hadar, Ethiopia. The males of some of these ancestors were nearly twice as large as the females: on our evolutionary lineage, physical domination by males and feminine submission may be ancient phenomena." (Margulis, L & Sagan, D. (1991) Mystery Dance, On the Evolution of Human Sexuality: Summit Books, New York pp. 14)

"Male afarensis are much larger than females. One thing we know with confidence form other primates is that differences in body size between the sexes indicates a polygynous mating system (Clutton-Brock and Harvey 1976). Males evolve larger because big bodies help them in male-male competition. Male and female members of mongamous primate systems, in which there is little male-male competition, tend to be about the same size. Thus Lucy was probably not living in a monogamous social system. She may have been in a harem situation, but it's equally possible that she had access to several males and that they had access to her." (Small, Meredith F. (1993) Female choices: Sexual behavior of female primates. Cornell Univ. Press, Ithaca, 1993 pp. 190)

"The evidence is also clear regarding female chimpanzees' significantly more frequent preparation and use of tools in food getting (albeit, in the case of chimpanzees, for insect gathering). Male chimpanzees simply do not use tools as often to obtain food, a fact consistent with the lesser nutritional stress on males. Further, there is not question but what mother chimpanzees share food with offspring. That tool-using, plant-gathering early hominid mothers would do likewise is probable. Also of interest are data on sexual dimorphism. Several remains at Laetoli and Hadar exhibit notable size differences---for example, the footprints at Laetoli and the upper jaws of AL200 and AL199 at Hadar. The differences in size can be interpreted as due to sexual dimorphism (Johanson and White, 1979). These size differences appear greater than those between sexes for humans today. However, the canine teeth do not appear to exhibit as pronounced dimorphism as among most living primates (Johanson and White, 1979). Quite possibly, then, A. afarensis had already undergone some reduction of sexual dimorphism, and its descendants were to undergo even more. This is consistent with the selective process hypothesized in Chapter 7: The gathering innovation led to increased maternal investment, which, in turn, was associated with preferential female sexual selection of males who, among other things, appeared less physically frightening or threatening to the females." (Tanner, Nancy M. (1981) On Becoming Human: Cambridge University Press, Cambridge p. 190)

"Although others have mentioned that early hominids probably gathered much of their food, until quite recently the prevailing idea underlying reconstructions of social behavior has been that Austalopithecus was primarily a hunter. There are, however, considerable data that are anomalous from a hunting perspective. For their small size, all australopithecines --- even the early basal hominids and the so-called gracile line --- had powerful chewing and grinding capacities. This kind of chewing apparatus is best understood as an adaptation to a diet including a high proportion of tough, uncooked plant food. Futher, given their small size and still very simple technology, it is improbable that they persued, captured, and killed large dangerous animals with tools. The data on size, tools, and teeth do not fit a hunting model but make a great deal of sense when viewed within a gathering context." (Tanner, Nancy M. (1981) On Becoming Human: Cambridge University Press, Cambridge p. 193)

"This trend is emphasized even further among pygmy chimpanzees, in which male and female canine teeth can be distinguished only on the basis of canine breadth, and facial features and cranial capacity show essentially no sexual dimorphism." (Tanner, Nancy M. (1981) On Becoming Human: Cambridge University Press, Cambridge p. 202)

"In humans and other primates, many intersexual limb proportions are simple allometric correlates of smaller female body size (Wood, 1986). For example, males have longer legs relative to trunk length simply because males spend a longer time in the prepubertal growth phase, when legs grwo relatively faster than the trunk (Harrison et al., 1988). Many other female traits, the more gracile skeleton, narrower joints, and so on, are also simple correlates." (McKinney, M.L. & McNamara, K.J (1990) Heterochrony: The Evolution of Ontegeny: Plenum Press, New York p. 322)

"In their discussion of A. afarensis, Johanson and White (17,85) note that although this species shows "marked body size dimorphism, the metric and morphological dimorphism of the canine teeth is not as pronounced as in other extant, ground-dwelling primates. This implies a functional pattern different from that seen in other primates and may have significant behavioral implications." (6). The reduction and effective loss of canine dimorphism in early hominids therefore serves as primary evidence in favor of the proposed behavioral model (86)." (Lovejoy, C.O. (1981) The o rigin of man. Science 211: pp. 346)

"The pygmy chimpanzee is the least sexually dimorphic primate. Cramer (1977) compared the measurements of 20 cranial variables of male and female pygmy chimpanzees, and found little difference: the female's mean value is 98.6% of the male's." (Kano, T. (1992) The Last Ape: Pygmy Chimpanzee Behavior and Ecology; Stanford Univ. Press, Standord p. 32)

"Sexual selection requires (and provides a potential explanation for) a sexual dimorphism. For human cerebral function the most striking sex difference is in laterality (Bear et al, 1986) - men have a greater mean cerebral asymmetry than women (as noted by Crichton-Browne; see McGlone, 1980). This difference may account for the small but consistent differences that have been observed in the pattern of distribution of intellectual abilities - there is a mean difference for verbal fluency in favour of women and for spatial ability in favour of men (McGlone, 1980). The most striking sex difference if psychosis is earlier onset in men (Hafner et al, 1993; Lewine, 1991). Could the sex differences in cerebral organization (greater asymmetry and spatial ability in men; less asymmetry and greater verbal fluency in women) and the age of onset of psychosis be related? The following hypothesis (Crow, 1993a,b) (components of which are derived from earlier formulations as indicated in Table 3) relates them through Darwin's mechanism of sexual selection." (Crow TJ (1995) A Darwinian approach to the origins of psychosis. Br J Psychiatry 167(1):21)


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