Conclusions

Comparative studies of mammalian reproduction and insights gained from anthropology and evolutionary psychology support a number of conclusions concerning the origins of human sexuality. These are listed in Table 9.1, and summarized below.

1. It is likely that Homo sapiens evolved from a primarily polygynous non-human primate precursor, and that the earliest members of the genus Homo were to some degree polygynous. In this context, the absence of marked sexual dimorphism in canine tooth size in the australop-ithecines and members of the genus Homo is perplexing, as larger male canines would be expected to occur in association with polygyny. It is possible that modest canine size represents part of the overall changes in the shape and size of the dental arcade which occurred in these hominids, for selective reasons which have not been fully explained. Whatever factors affected the evolution of hominid canine tooth size, the occurrence of multiple sexually dimorphic traits in H. sapiens (in body size, shape and composition, facial traits, secondary sexual adornments, the larynx and vocal pitch, and age at reproductive maturity) all indicate that human ancestors were polygynous. Fossil evidence for the occurrence of pronounced body size sexual dimorphism in some of the aus-tralopithecines is far from conclusive. Thus, further research is required to clarify whether these hominids were sexually dimorphic, as is posited here to be the case for the direct ancestors of the genus Homo.

Sexual dimorphism in extant anthropoids which have multi-male/multi-female mating systems is not strong evidence for the occurrence of such a system in the australopithecine precursor of the genus Homo, or in those forms (e.g. H. ergaster; H. heidelber-gensis) which preceded H. sapiens. This is because comparative studies of reproductive anatomy and patterns of copulatory behaviour consistently indicate that H. sapiens did not evolve directly from a multi-male/multi-female precursor (see below: points 3 and 4).

The propensity for men and women to form long-lasting relationships for reproductive purposes is an ancient trait, probably present in the earliest members of the genus Homo. The occurrence of such relations between the sexes is universal across extant human populations, and its existence in ancestral forms provided the biological foundation for the later emergence of marriage. Lengthy periods of human infancy, childhood, and adolescence require considerable parental investment by both sexes if offspring are to survive. These selective pressures may have contributed to the formation of long-term relationships between men and women early in human evolution. Nowadays, as throughout recorded history, marriages might be either polygynous or monogamous, depending upon the culture considered. Most human cultures allow polygynous marriage, but for the vast majority of people, monogamy is the accepted norm. During evolution of the genus Homo, a shift towards paired relationships and monogamy has occurred. In those modern cultures which allow polygynous marriage, it tends to be prevalent among men of higher status, and those who control most resources. Nonetheless, a contributory factor to occurrences of extra-pair copulations by men in primarily

Table 9.1 Status of selected traits and mating systems, during three phases of human evolution.

Common ancestor Of Pan and Homo

Australopithecine precursor of Homo

H. ergaster; H. erectus

Mating system Male philopatry? Sperm competition pressure Incest avoidance? Male genitalic complexity Copulatory posture Copulatory pattern* Female sex skin? Oestrus occurs? Female orgasm? Copulatory frequency Sexual dimorphism Body size ^>2? Larynx ^>2?

Secondary sexual adornments

Males

Females Hirsuteness

Multi-male/multi-female

High

Pronounced

DV;VV;FS

Swelling likely

High

Moderately so Moderately so

Not pronounced

Sexual skin? Pronounced

Polygyny Yes Low Yes

Reduced

DV;VV;FS

Unlikely

Yes: major Yes: major

Pronounced e.g. capes and crests of hair ?

Reduced?

Monogamy > polygyny Yes

Very low

Minor

VV;FS;DV

Absent

Yes: major

Pronounced e.g. facial hair Breasts, low WHR Body hair lost: sweat glands well developed

'Definitions of these copulatory patterns are provided in Table 5.1.

Abbreviations: DV = Dorso-ventral; VV = ventro-ventral; FS = female superior mating position.

monogamous relationships may be an underlying tendency towards polygyny. Chapais (2008) rejects the notion that joint parental care acted as the selective force to promote pair formation and monogamy during human evolution. He proposes instead that the development of weapons during human evolution made inter-male competition for access to mates more equal, and potentially more lethal. Under these conditions, most males may have opted for monogamous rather than polygynous mating strategies. Contrary to this view, there is some evidence from molecular biological studies that polygyny may have been more widespread until relatively recently in H. sapiens. Dupanloup et al. (2003) suggest (on the basis of studies of Y-chromosome diversity) that a recent shift from polygyny to monogamy has occurred in humans, coincident with the rise of agriculture and expansion of settled populations. However, the evidence presented and reviewed throughout this book and arguments advanced by Chapais

(2008) support the conclusion that the transition from polygyny to monogamy began much earlier, in taxa that were ancestral to H. sapiens.

The occurrence of a polygynous mating system in human ancestors does not mean that the social system necessarily comprised spatially separate 'one-male units', such as are found in modern day gorillas. Larger social groups with a multilevel social system seem more probable, especially in early hominids such as the australopithecines, which inhabited more open savannah/woodland habitats. Some extant monkeys, such as the hamadryas baboon, live in large groups composed of many one-male units. It is possible that the australopithecine ancestors of the genus Homo formed groups in which individual males associated with several females for mating purposes. Based upon such a polygynous system, a transition to monogamy as well as polygyny would have characterized the evolution of mating systems within the genus Homo.

2. Because the human species is intellectually and culturally sophisticated, it is easy to overlook the contribution made by ancient traits and adaptations to our sexual lives. The anthropoid primates are par excellence visually orientated creatures, and we have inherited a rich repertoire of nonverbal communication from non-human primate ancestors. There may be a very good reason why cues serving for visual communication and facial recognition are stored in the human memory and retrieved more effectively than linguistic labels. How often, for example, have we heard someone say 'I never forget a new face, but I can't always remember a new name?' All human cultures share the propensity to exhibit particular facial expressions in the same emotional contexts (Ekman 1973) and homologues of these have been identified in the great apes. Fundamental aspects of human sexual communication such as eye contact and other nuances of facial communication derive from periods of evolution before the emergence of language. Likewise, our assessments of sexual attractiveness relate, initially at least, to physical cues involving the body and face of members of the opposite sex. These physical cues provide information about age, health, reproductive condition, and a host of variables which were (and still are) important for mate choice. Sexual selection, as well as natural selection, has affected the evolution of traits such as masculine somato-type, female waist-to-hip ratios, and facial cues in both sexes. Moreover, these outcomes of sexual selection predated the more complex decision making facilitated by language, e.g., concerning matters such as a potential partner's kindness or sense of humour. Such complex abstractions represent more recent developments in human evolution, and they may be less important in people's initial rankings of mate preferences than is currently recognized.

3. Darwin was apparently unaware that the primary genitalia and patterns of copulatory behaviour have also been influenced by sexual selection during the evolution of many animals, including the primates. The more recent concepts of sperm competition and cryptic female choice provide rich sources of comparative information concerning the evolution of genitalic complexity in the anthropoids, including H. sapiens. Genitalic complexity is, in turn, strongly linked to patterns of sexual behaviour. The information considered throughout this book does not support the conclusion that sperm competition has played a significant role in the evolution of anatomical or physiological specializations of the human reproductive system. By implication, therefore, sperm competition is unlikely to have exerted a significant effect on the evolution of human sexual behaviour. Whatever the role of sexual selection in evolution of the human brain (Miller 2000), it is unlikely that sperm competition has impacted the evolution of neurological mechanisms which facilitate human mate choice and copulatory patterns. Nor, unfortunately, can the evolution of such neurological mechanisms be addressed by analyses of overall brain size. As an example of this approach, Pitnik, Jones, and Wilkinson (2006) reported that mating systems and brain sizes are correlated in bats; those species which engage in sperm competition have smaller brains than those in which females mate primarily with a single male. Applying this line of reasoning to human evolution and the importance of human sperm competition posited by evolutionary psychologists, one might expect human brain size to be reduced, compared to that of more exclusively monogamous or polygynous primates. Plainly this is not the case. Nor is it useful to engage in this type of speculation, because so many correlations between brain size and behavioural traits have been reported in mammals and birds. In their review of fifty such studies, Healy and Rowe (2007) concluded that overall measures of brain size are too crude to facilitate meaningful correlations with complex behavioural traits. Even the cautious statements I have made in this book concerning brain sizes in fossil hominids, with respect to likely tool use or language acquisition, should be viewed with more caution. Of brain size and fossil hominid mating systems nothing of value may be said.

It has become commonplace to use comparisons of human and great ape testes sizes as evidence that sperm competition has influenced human evolution (the argument often presented is that human testes are larger than those of the polygynous gorilla, but smaller than those of the chimpanzee; thus sperm competition played a significant role in human evolution). However, the occurrence of relatively small testes sizes in human males nests within an extensive array of traits shared by those mammals which exhibit little evidence of sperm competition. These traits include facets of sperm morphology, the structures and functions of the reproductive ducts and accessory glands, phallic morphology, and patterns and frequencies of copulatory behaviour. It is regrettable that traits such as penile morphology are often misrepresented in the scientific and popular literature as being exceptional in human beings. Quantitative comparisons of primate genital morphology and mating systems place H. sapiens consistently among those species which have low relative testes sizes, small accessory sexual glands, a less muscular vas def-erens, and monogamous or polygynous mating systems.

4. Evidence pertaining to sexual selection by cryptic female choice in mammals, although limited, is consistent with the conclusions reached from studies of the male reproductive system. The oviduct is elongated in mammals in which males have larger relative testes sizes and large sperm midpieces. The human oviduct, however, is relatively short, as would be expected, given the small testes and sperm midpiece volumes of men. The notion that female orgasm represents an adaptation to selectively transport or reject sperm of different male partners is based upon misinterpretations of false and limited evidence (Lloyd 2005).

5. Human copulatory postures, far from being unique, are derivable in evolutionary terms from those exhibited by other anthropoids. Ventro-ventral or female superior copulatory postures have been described in a number of ape species. These traits are probably primitive for the ape-human lineage, and would have been present in early hominids, including the australopithecines. Face-to-face mating postures are thus not a uniquely human specialization. Nor did their evolution have any connection with a requirement to enhance clitoral stimulation and to facilitate female orgasm. The dorso-ventral copulatory postures sometimes employed by humans have a still more ancient origin, for these are found throughout the primates, as well as in the majority of mammals.

6. The occurrence of extended periods of intromission during human copulation is likely to represent an effect of cultural evolution in various populations. The primitive human pattern involves attainment of ejaculation during an intromission of a few minutes in duration, accompanied by pelvic thrusting. The human copulatory pattern, like those of other primates, and mammals in general, may be classified using a modified version of the schema originally developed by Dewsbury (1972). This modified schema includes measures of the presence or absence of extended intromission as a component of the copulatory pattern. This is important because intromission duration has been subject to both natural and sexual selection during mammalian evolution. The human copulatory pattern, when viewed in comparative perspective, is unremarkable and there is no evidence that sperm competition or cryptic female choice would have impacted its evolution.

7. Oestrus was reduced or lost in the anthropoid lineage long before human beings appeared, and would already have been lacking in early homin-ids such as the australopithecines. There has been no loss of oestrus during human evolution. Female Old World monkeys and apes share with H. sapiens, to varying degrees, the ability to dissociate their sexual behaviour from a rigid control by ovarian hormones secreted during the menstrual cycle. Very few New World monkeys exhibit menstruation. However, like the Old World anthropoids, female New World primates allow and invite copulation when conception is unlikely to occur. Thus, the anthropoids as a whole do not exhibit oestrus, whereas it is present in the prosimian primates (Kev-erne 1981; Dixson 1983; 1998a; Loy 1987; Goy 1992; Martin 2003). Measurements of female proceptivity, receptivity, and sexual attractiveness, as defined by Frank Beach (1976b), are far more informative when analyzing how hormones control female sexuality. Cyclical changes in female attractiveness and pro-ceptivity (sexual invitational behaviour) are more pronounced in monkeys and apes than are changes in receptivity (willingness to accept the male). Ovarian hormones, and oestrogens in particular, facilitate increases in attractiveness and/or proceptivity, during the follicular and peri-ovulatory phases of the cycle.

Effects of the menstrual cycle upon human sexual behaviour are relatively subtle, subject to individual variability, and are more difficult to quantify than is the case for monkeys and apes. Nonetheless, mid-cycle or follicular phase increases in women's sexual interest and proceptivity have been identified in some studies, as have increases in female attractiveness (e.g. in olfactory and facial cues). Likewise, some studies report that women in the fertile phase of the menstrual cycle may be more responsive to masculinized faces, and other sexually dimorphic traits indicative of male fitness.

Two points should be emphasized with respect to these findings. Firstly, although the effects of the menstrual cycle on women's behaviour are subtle, in some cases they follow the same pattern as the mid-cycle, oestrogen-dependent increases in proceptivity which occur in many non-human primates. Although considerable weight has been given in the clinical literature to the importance of androgens in regulating women's sexual interest, it is possible that oestrogen is more significant in this context than currently recognized. Secondly, the modest shifts in behaviour measured in some women during the fertile phase of the menstrual cycle are insufficient to justify hypotheses concerning the evolution of dual mating strategies in women. It has been argued, for example, that women in the fertile phase of the cycle may switch their mating preferences from long-term partners towards extra-pair copulations with men who possess more attractive traits, indicative of superior genes (Gangestad and Simpson 2000). This hypothesis is rejected here for a number of reasons. Investment by women in long-term relationships probably outweighs genetic benefits they might theoretically derive from extra-pair matings. More probable is the possibility that hormonal (putatively oestrogenic) facilitation of female sexual interest is part of the mechanism by which women are attracted to men, seek to establish enduring relationships, and reproduce successfully. From puberty onwards, young and un-paired women undergo repeated menstrual cycles, and initially many of these cycles are ano-vulatory. Women are potentially able to mate at any time, but periodic increases in their interest in men, though slight, may reinforce the development of their sexual relationships with particular partners. Whilst acknowledging the fact that women may engage in extra-pair copulations in a number of cultures, I think it unwise to ascribe an evolutionary basis to what is an artificially created dichotomy between long-term versus short-term mating strategies. What appear to be short-term mating choices may have their origins in the requirement for women to cultivate potential partners in case of problems involving a long-term mate, such as desertion, illness, death, and inability to provide or to procreate.

More important than the relatively small, cyclical changes in female attractiveness or behaviour that occur during menstrual cycles are the long-term effects of female reproductive condition and health upon attractiveness. Thus, women whose cycles are fertile, regular, and involve higher follicular phase levels of oestrogen tend also to be facially more attractive, and to have a more healthy distribution of body fat. It is these longer-term hormonal effects which have probably been of greatest importance in shaping the evolution of human mate choice, rather than the transitory shifts which may take place during the fertile phase of an individual menstrual cycle. Perhaps this is why women do not exhibit graded signals of attractiveness and fertility status during their menstrual cycles. This contrasts with the occurrence of prominent, oestrogen-dependent sexual skin swellings which act as graded signals in female chimpanzees and some other Old World anthropoids. It is therefore most unlikely that in the direct ancestors of H. sapiens, females would have had sexual skin swellings.

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