To some, like Owen Lovejoy, the truly distinct human traits are involved in reproduction. Females are receptive to mating even when they are not ovulating, couples mate privately, and they often mate only with one partner for a long stretch of time and even for life. This is a striking contrast to reproductive behaviors of baboons and chimpanzees, for instance, which share many similarities with humans in biology, sociality, and intelligence.
Female baboons and chimpanzees usually only mate when they are either in estrous, which is a segment of their ovulatory cycle when their anogenital perineum engorges into a bright pink swelling to signal the arrival of their fertile time. Often an abundance of food will trigger the females of a group to come into estrous and males reach heightened levels of competition around these females. Human females have conceived ovulation, which is hidden from males as well as from themselves. This situation opens them up for perpetual mating opportunities and may require males to find alternative ways to detect female ovulation, like, for instance, other visual or even olfactory (scent) cues. Additionally, concealed ovulation has the potential to place males in a state of perpetual competition.
Of course, there is much less privacy in the baboon and chimpanzee worlds, but they may mate secretly if codes of status and hierarchy are being breached. Also, for most primates, it is more often better to mate with more than one partner, not just in one mating cycle but throughout life. Most humans use the monogamous strategy, that is they form enduring pair-bonds (either serially or for life), but many humans are polygamous, forming long-lasting relationships between one male and multiple females instead. In either case, there is much less human male competition and also an unusually high level of male parental investment. Because of their extended period of growth and maturation, mostly dealing with the incredible amount of time required to grow the large human brain, human infants are altricial, which means that they are vulnerable and highly dependent on their parents after they are born. The opposite condition is to be precocious like newborn horses that can take off running soon after they touch the ground because they complete more growth in the womb.
Humans clearly have their own reproductive strategy that stands apart from even their closest relatives and we have the unique anatomy to show for it. H. sapiens show low levels of sexual dimorphism in body size, strength, physiology and are the least dimorphic out of all the great apes, but we are not as monomorphic as monogamous gibbons. Human males are larger and stronger and have higher metabolic rates than females. They have higher juvenile mortality rates and they attain sexual maturity later than females. Hair and fat distribution and abundance differentiate the sexes and so do the somewhat large and conspicuous sex organs. Humans fall between chimpanzees (large) and gorillas (small) in testes size for body weight. In other words, humans have somewhere between highly competitive sized (chimpanzees) and essentially competition-free sized (gorillas) testes. Males in a primate species where females mate regularly with more than one male tend to have elaborate penises that are either highly stimulatory or bristle-like for direct sperm competition inside the vagina. Those species where females tend to mate with only one male during an estrous cycle have much less elaborate anatomy and resemble the human form. However, the human penis is much larger and lacks a baculum (penis bone) which some suggest is the result of sexual selection by females, especially since bipedalism made the male genitals much more conspicuous.
Bipedalism may have helped shape the sexual anatomy of human females as well. The breasts are larger than expected when not lactat-ing and the distribution of fat on the hips, thighs, and buttocks lowers the waist-hip ratio (WHR). Both are probably fertility signals and fitness indicators. A WHR of 0.7 is nearly cross culturally considered the "ideal" female figure, whether the woman is thin or heavy. Women with that classic hourglass silhouette have optimal estrogen levels that is correlated to fertility. So although no males are literally measuring and calculating the WHR of potential mates, they are able to pick up on the cues from a woman's body that are linked to evolutionary fitness.
The australopiths had a high degree of sexual dimorphism in body and canine size, which was greater than that in humans so the human reproductive strategy had not evolved in australopiths. Early hominins were probably polygynous in their mating behaviors like modern chimpanzees but by the time H. erectus arrived the strategies changed. Fossil evidence indicates that sexual dimorphism greatly decreased by H. erec-tus. Plus, the species grew in body size but retained a small hip-breadth
(at par with australopiths) for reasons to do with bipedal efficiency as well as climate adaptations in body proportions.
An increase in brain size must accompany an increase in body size, so H. erectus could not have fit the same sized infant's brain and skull as expected for its body size through its narrow hips. That is, H. erectus may have shortened gestation (i.e., the period of fetal development in the uterus) to be physically capable of giving birth to larger brained babies through its relatively small birth canal. An earlier birth results in a more helpless, less developed, altricial infant. So it is probable, that with H. erectus, higher levels of parental investment, especially from the father (paternal investment), began to evolve. Selection would favor such a change if it fostered brain growth and development, especially if selection was acting strongly on brain size increase in the species. Other major changes are correlated, like the incorporation of meat into the diet, food-sharing, and the creation of home bases where males provisioned females and offspring and where females localized their large contribution to family diet from foraging and gathering (sexual division of labor). Once heavy paternal investment in offspring and the formation of pair-bonds became successful adaptations, females would have also chosen to mate and bond with males that could not only provide food and protection but that possessed dependable and fatherly attributes like kindness, generosity, and trustworthiness. Less time is spent foraging when the quality of the food goes up, so with the addition of meat and other high-quality foods into the diet, presumably more free time was spent at the home base. When this occurred, intellectual qualities like elaborate language, singing, music, wit, and even dancing would have become largely important in mate selection. There is no evidence that language evolved as early as H. erectus times, but singing, dancing, and humor cannot be ruled out in the early Pleistocene.
Menopause, or the termination of reproduction, is an evolutionary puzzle. How could natural selection favor it when it clearly prefers increased reproduction? Human females experience menopause around age fifty when their finite supply of eggs is exhausted and reproductive processes slow and eventually cease.
Some, including Kristen Hawkes and her colleagues, posit that menopause could be an adaptive trait. Females that stop reproducing at menopause, start helping their reproductively active daughters provide for their grandchildren. While her daughter is lactating (an energetically expensive process), the grandmother helps subsidize the diet of the non-nursing children by gathering additional food. In this way, grandmothers help their daughters have more children and also increase the children's survival rates. Presumably, a grandmother's genetic material, which includes the genes for living long past menopause, is passed onto her better surviving children and grandchildren.
Another explanation for menopause, put forth by James Wood and colleagues, does not consider it an adaptation, despite the benefits of grandmother care. According to Wood's hypothesis, menopause and postmenopausal life are not themselves beneficial—rather they evolved because of antagonistic pleiotropy, a process whereby genes that have harmful effects later in life can be actively selected for if they have beneficial effects earlier in life.
In young women, a process known as follicular atresia helps to maximize fertility, but also causes the reproductive system to speed through the finite supply of eggs. Menopause is therefore a compromise, not an adaptation, because women give up the ability to reproduce later in life in order to experience high levels of fertility earlier in life.
Perhaps we should not ask why menopause evolved, but instead investigate when women started living beyond their reproductive years and why females, in comparison to males, have limited gamete production in the first place.
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