Sexual adaptations in Homo sapiens

Breasts

The females of all mammals have milk glands and in the majority of terrestrial quadrupedal mammals they are located under the body, on the belly or chest. In K-selected animals which have fewer offspring they can be found towards the hind end of the body, such as in cows or goats; and in animals which tend towards large litters, they run the length of the belly, as in pigs and dogs. Monkeys and apes have developed paired nipples at the top end of the chest, near the pectoral muscles. This makes sense as apes and monkeys are more likely to sit upright when they feed, so an infant clinging to its mothers fur would find this the most comfortable and safest place to nurse.

During development the nipples on the human embryo move further downward and this continues for a while even after birth so that in mature human females the nipples are slightly lower than in other apes. Unlike other primates, however, humans have no fur for babies to cling on to and, as human newborns are relatively helpless during the first few months (at least on land), mothers have to hold their babies in their arms when they nurse. Having nipples that reach the babies' mouths when they sit, walk or wade in water would have provided an evolutionary advantage to a semi-aquatic hominid ancestor.

To make this even easier, human women have varying degrees of fat surrounding the mammary gland, extending and lowering the nipple even further and making it more adaptable in being able to reach the infant's mouth. As fat is 90% denser than water (fat tissue is ~95 %) human breasts tend to float, leaving the nipple at/above the surface if a woman is standing or sitting in chest high water. This would also have been advantageous for a semi-aquatic ancestor, who would have spent much of her time in shallow waters with her baby. Breasts full of milk stand erect, floating above the water surface in back-floating women. Some primitive populations today are described as having back-floating-lactating women and floating babies (while the mother dives for shells).

The vagina

Even before birth, the female human fetus is being structurally modified to adapt it to walking or wading on two legs instead of four. Several changes take place, including to the pelvis and to the spine, in order to support the weight of the internal organs. In most mammals, the vagina runs horizontally straight from the uterus and exits the body just under the tail, but in humans, it makes a right-angled turn towards the front of the body. This not only probably makes childbirth more difficult, it means that the male genital organ also had to adapt in order to achieve successful copulation.

A comparable design is also found in marine mammals and it is hypothesised that it protects the reproductive tract from infection during mating in water. It also exists in at least one other terrestrial animal, the elephant; the female's vagina is around 3m from start to finish - the longest of any land mammal. Even with his legendary penis length, also the largest of any terrestrial animal, a bull elephant never enters the female's vagina, as its opening is located 1.3m into her body. [1]

The labia

in all female primates the sex organs are covered by two external genital folds at an early stage of fetal development but these disappear in a later stage of pregnancy leaving the clitoris exposed in all but humans. In the human fetus, however, these folds increase so that by the time the baby is born the clitoris is fully concealed by the labia. It has been suggeted that these flaps of skin have the primary role of protecting the interior of the body from microbes and infection which can more readily be a problem in water. [2]

The hymen

The hymen is a membrane-like tissue which is considered part of the external genitalia, whereas the internal vaginal orifice is partly covered by the labia majora. Only two non-human primate species are known to have hymens: lemurs and chimpanzees (Cold & McGrath 1999). Although it has yet to be determined, given the change to an upright posture and a reduction into the size of the birth canal, it is not likely the function of the human hymen is necessarily the same as the one in lemurs and chimpanzees (Hobday et al. 1997). Currently, it is unclear when this trait may have evolved, as it is unlikely to be a trait shared with chimpanzees given the morphological changes from chimpanzees to humans. In this context, this suggests that the hymen evolved three times. The hymen in Hobday’s exaptation theory serves as a homoplasy, and it is postulated this evolutionary adaptation may have benefitted humans for more hygienic reasons, which may not have been necessary in primate ancestors.

Although there is as yet no concensus on what the purpose of the hymen might be in humans, Elaine Morgan proposed it might be an adaptation to an aquatic environment where hominids were more at risk to microbes rarely encountered on land and that the hymen evolved as an independent adaptation to avoid vaginal infections caused by microbes within the aquatic environment (Morgan 1972). As hominids walked upright more frequently within the aquatic habitat, the likelihood of contracting vaginal infections increased, she claims. These vaginal infections were considered to cause infertility and as a result, reduce reproductive success (Hobday et al. 1997). Therefore, females that possessed these tissues to ward off infections were more likely to be reproductively successful and pass on the adaptation to offspring.[3]

The penis

During prenatal development of the male genitalia a fully extendable foreskin is formed. In humans it completely covers the penis and is non-retractable in 96% of boys when they are born. By the time they have reached five years old this has been reduced to only 5%. As with the labia and hymen in females, it has been hypothesised that the purpose of a non-retractable foreskin in male children could be to protect the urethra from infection by microbes at an early age, before they are able to properly clean themselves. In most mammal species, the mother will groom her offspring's genitals and anal areas herself, but in humans, perhaps because of a reduced olfactory sense and a tendency to rely on water for hygiene, we have lost this urge.

Apes and monkeys all have a bone in their penis whereas human males do not, allowing the human penis to be far more flexible than in other apes. But the most obvious difference between the other primates and human males is the size of the penis. In chimps it is usually no bigger than the size of a human finger: