In the power of two - the evolutionary origin of the relationship / Blake Edgar

Bonding in pairs may have been the wisest move our ancestors ever made

Mammals do not excel at monogamy. In less than 10% of the species the pairs mate exclusively only with each other. Even in the primate wing monogamy is not very common. Although 15% to 29% of primate species prefer to live in pairs, only a few single species are committed to monogamy as humans know it: exclusive sexual partnership between two individuals.

Of course, humans don't follow the rules fanatically either. They conduct affairs "on the side", get divorced, and in some cultures even marry several partners. In fact, polygamy is found in most societies in the world. However, even in places where polygamy is allowed, this is a rare condition. Almost always, society is organized according to the premise that the majority of the population will bond in permanent couples that will maintain sexual exclusivity. And monogamy seems to have benefited humanity. "Pair bonds", as scientists call monogamous relationships, are a crucial evolutionary adaptation that appeared in some ancestor and became central to human society and our evolutionary success. "Pair bonds give us a great advantage over many other species," says anthropologist Bernard Spey of the University of Montreal.

The monogamous couple also creates the basis for something unique to the human species: the extensive and complex social systems in which we live. Young apes establish their family ties only through the mother. Humans, on the other hand, bond with the families of both parents and expand the number of bonds in each generation. Human social networks also include other families and even separate groups, in ever-expanding circles of connection. In Shafey's view, such group bonds and monogamy are "two of the most influential features of human society."

For decades, scientists have tried to understand the origins of human monogamy and its consequences. Basic questions, such as when we started forming pairs for life, what is the advantage of them and how mating has benefited our success as a species, remain in dispute and without an unequivocal answer, but new studies bring us closer to solving the mystery.

Origin of pairing

It is quite possible that our earliest ancestors were already monogamous. According to anthropologist K. Owen Lovejoy of Kent State University, the fossil evidence suggests that monogamy existed even before Ardipithecus ramidus (Ardipithecus ramidus), the species that gained publicity due to the 4.4 million-year-old incomplete female skeleton, discovered in the Middle Awash region of Ethiopia and known as "Ardi" for short. According to Owen's hypothesis, shortly after the split from the common ancestor of the two evolutionary branches of man and the great apes, more than seven million years ago, our ancestors adopted three revolutionary behaviors: carrying food with the hands freed up by walking upright, creating mating bonds, and hiding the external signs of ovulation . The combined evolution of these innovations gave hominins, the tribe that formed when early humans split from chimpanzees, a reproductive advantage over great apes.

According to this hypothesis, the ancient polygamous mating system was replaced by a pair arrangement when low-status hominin males diverted the energy they spent fighting each other to find food to serve to females as an incentive to mate. The females began to prefer reliable food providers over aggressive competitors, and became associated with the more successful harvesters. Eventually, the females lost their skin swelling and other signs of sexual availability, which would have attracted other males while their mates were busy foraging.

As evidence, Lovejoy cites the dentition of A. ramidus. The difference in the size of the fangs between males and females in the species A. Ramidos considerably smaller than the differences in the fangs of contemporary apes and fossil apes. In many species of great apes, evolution has refined the dagger-like fangs of the males into threatening weapons, used in the struggle for females. With the ancient hominins, things were different. Remember the fangs in the gaping mouth of a male gorilla, then look at our fangs. Male and female humans have small, short fangs, a non-threatening trait unique to hominins, including the earliest Ardipithecus remains.

There is also a certain correlation between the mating behavior of primates and sexual dimorphism, i.e. the differences in weight and body size between males and females of the same species. The greater the dimorphism of a particular primate species, the more likely the males will fight each other over females. At one end of the range, polygamous gorilla males are heavier and more than twice as large as females. At the other extreme, male and female gibbons, which are often monogamous, are almost identical in size. In this arc, humans are closer to gibbons: men may be 20% heavier and larger than women.

However, there is a limit to what can be inferred from fossils. Paleoanthropologist J. Michael Fluken of the University of Arkansas cautions against jumping to conclusions about hominin social behavior based on fossilized bones. For example, the species to which "Lucy" belonged, Australopithecus afarensis, which lived 3.9 to 3 million years ago, had small fangs like Ardipithecus. However, according to his skeleton the sexual dimorphism of A. afarensis was between that of a chimpanzee and that of a gorilla. “We find considerable dimorphism in body size suggesting that the males [of A. Afransis] competed for the females, but the loss of dimorphism of the tusks, which indicates that they did not [compete for females], is a mystery,” says Flucken.

Many anthropologists also disagree with Lovejoy's conclusion that monogamy, fostered by males gathering food for their mates and offspring, was a hominin strategy for millions of years. In an article in the Monthly Evolutionary Anthropology in 2013, Shafey claimed that the special features of the family and the human social structure (monogamy, family ties through both parents and expanding social circles) appeared in stages. Before the first stage, according to Shafey, male and female hominins were free to choose sex partners. Then there was a transition to polygamy, as it exists in the gorillas. But it takes hard work to maintain a large number of females: fighting with other males and protecting the females. Monogamy may have emerged as the best way to reduce the effort involved in polygamy.

Shefey does not speculate on when the change occurred and what species were involved, but other researchers speculate on the period from two million years ago to a million and a half years ago. This is a period after the appearance of the biological type Homo, in which the physical changes of Homo erectus appeared, apparently the first hominin species to successfully migrate out of Africa. Homo erectus had a much larger body than its predecessors, about twice that of Lucy's species, and the proportions of its organs were more similar to those of modern humans. It also appears that its conjugal dimorphism was less than that of the Australopithecines and Mini Homo more ancient. Limited fossil evidence suggests that Homo erectus females were more similar in physical size to males, reaching a level of dimorphism similar to that of modern humans. All this perhaps indicates that the way of life of Homo erectus was less competitive than that of his ancestors. Since primates of similar body size tend to be monogamous, this change could indicate a shift to more exclusive reproductive behavior.

Strategic partnership

If scientists disagree on the issue of when humans became monogamous, it is difficult to expect them to agree on why. Two separate research teams published two studies in 2013 in which they conducted a statistical analysis of the scientific literature in order to determine which behaviors were likely to lead to monogamy. Both studies tried to choose the best explanation for monogamy out of three hypotheses known as "female spacing", "avoidance of female perpetrators" and "male parental care".

The female spacing hypothesis posits that monogamy evolved after females began to expand their territories to improve their access to the limited food resources, a process that resulted in them moving away from each other. In this state of affairs, the males had difficulty finding many mates and keeping them. Being content with one mate made life easier, reduced the risk of the male being injured while guarding his territory and helped him ensure that his mate's offspring were indeed his.

Zoologists Dieter Lucas and Tim Clutton-Brock of the University of Cambridge found evidence for this idea in a statistical analysis of 2,545 mammal species. They described their findings in an article published in the journal Science. According to them, the data shows that mammals started out as individuals, but different species switched to monogamy 61 times during their evolutionary history. Monogamy appeared more often in carnivores and primates, which could indicate that a species tends to mate when the females need rich but rare food (such as high-protein carrion of animals or ripe fruits), which can usually only be found by searching over a large area. These findings provide the strongest statistical support for the conclusion that single and increasingly distant females motivated males to select single mates.

Lucas admits that even if the hypothesis works for non-human mammals, one can understand why it is less suitable for humans: it is difficult to combine the inherent sociality of humans with a hypothesis that depends on the spacing of available females. Our ancestors may have been too social for females to be spread across the savannah like other mammals. However, the theory might apply to humans if monogamy appeared in hominins before our tendency to form groups emerged.

The second leading hypothesis claims that the origin of monogamy is in the threat of lethal violence against offspring. If a rival male challenges or defeats a dominant male in the community, he may kill offspring that are not his offspring. When this happens, the mothers will stop producing milk and go back to ovulating, thus giving the attacking male a chance to spread his genes. To prevent the murder, the female chooses a male partner who can protect her and her children.

Anthropologist Keith Opie from University College London provides evidence for the theory of avoiding culpable homicide in a study published in the Proceedings of the American Academy of Sciences (PNAS). Opie and his colleagues ran computer simulations of the evolutionary history of 230 primate species, then performed a Bayesian statistical analysis to determine which of the three main hypotheses for the origin of monogamy was more likely to be true. They identified a high correlation between monogamy in primates and between each of the influencing factors in the various hypotheses, but only increasing the risk of infanticide consistently resulted in the appearance of monogamy in many branches of the primate evolutionary tree.

The biology of modern primates and their behavior increases the likelihood that infanticide is the factor that spurred monogamy. Primate babies are at a particularly high risk of such murder: it takes a long time for their large brains to develop, so the babies are dependent on adults and vulnerable for a long period after birth. Homicide has been observed in more than 50 primate species. Usually it is a male that comes from outside the group and attacks a young that has not yet been weaned in an attempt to control the females or gain access to them. However, this evidence is limited: almost all of these species have polygamous or free-breeding systems, so the distribution of oligicide in primates today is inconsistent with the prediction that monogamy would evolve when such oligicide is a major threat.

The third hypothesis for the development of monogamy focuses on the male's participation in parental duties. When a baby becomes too expensive, in terms of food and energy, for a single female, the father who stays with his family and provides food and other help increases his offspring's chances of survival and encourages closer bonds with the mother. A related idea, put forward by University of Notre Dame anthropologist Lee Gettler, suggests that even carrying the offspring by the father encourages monogamy. The mothers have to meet the heavy nutritional requirements of breastfeeding the babies, and in the society of primates or human hunter-gatherers, carrying a baby, especially in the absence of a carrier or other means of carrying, requires an investment of energy similar to that of breastfeeding. If the male carries the baby, the female can fulfill her own energy needs by gathering food.

A species of monkey from South America called Azara's owl monkey may shed light on how parental care can strengthen monogamy. These monkeys live in small family groups: a pair of parents, an adult male and female, a baby and one or two young. The mother carries the newborn on her hip immediately after birth, but the father takes on most of the carrying and care from the age of two weeks onwards. The adult couple "keeps in touch" with frequent tail touches, and it is possible that the physical proximity of the male to the female and his offspring deepens the emotional bonds.

Indeed, a study published in March 2014 in Proceedings of the Royal Society B presents genetic evidence that pairs of owl monkeys remain monogamous, the first genetic confirmation in a non-human primate. DNA collected from several groups showed that all the females, and all but one of the males, in 17 couples were very likely the parents of their 35 offspring. "They are completely committed to a monogamous relationship in a genetic sense," says anthropologist Eduardo Fernandez-Doc, one of the authors of the study, who now works at Yale University. The reproductive relationships between owl monkeys last an average of nine years, and the monkeys that stay together longer reproduce more - the evolutionary definition of success for any reproductive system.

What do the two most recent statistical studies say about the parental care hypothesis? Both concluded that of all the competing theories, parental care seems the least likely reason for the formation of a monogamous pairing - but, as Lucas says, "Parental care may explain why sex remaining monogamous.”

It takes a village

To raise an ape as smart and social as a human requires more than a pair of monogamous parents, says anthropologist Sarah Hardy of the University of California, Davis. A human baby consumes 13 million calories on its long journey from birth to adulthood - a heavy burden for the mother, even if she has a supportive partner. This need may explain why, in many societies, human mothers rely on "additional parents" (such as relatives of the couple, or other members of the group) to provide food and care for the child. "Human mothers agree to have others hold their babies immediately after birth," says Hardy, "which is amazing and completely different from apes." No ape behaves in a way that is close to "additional parenting".

According to Hardy, joint rearing of children, i.e. a social system in which "additional parents" help take care of the offspring, developed in our ancient ancestors starting with Homo erectus almost two million years ago. This species had a larger body and brain than its predecessors. According to the estimate, the body action of Homo erectus required a 40% increase in metabolic energy compared to previous hominins. If Homo erectus was indeed the creature that paved the human path of late development and prolonged dependence on parents, extended parenting may have been essential to support the energetic demands of raising larger-brained infants.

Karin Eisler and Karl van Scheik, both of the University of Zurich, conclude that without co-rearing children, the early homosexual species could not have broken the "gray ceiling", the presumed barrier that limits the brain volume of great apes to a maximum of about 700 cubic centimeters. To pay the energy bill for larger brains, the animal must reduce its birth rate, its growth rate, or both. But humans have achieved shorter suction periods and greater success in breeding than is expected in a creature with a brain volume of 1,100-1,700 cc. Eisler and Van Scheik attribute this success to the "additional parents", which allowed Homo erectus to produce offspring more often, and also to provide them with enough energy for the growth of a large brain.

If so, it was cooperation, whether it was done through monogamous couples, whether through nuclear families or through tribes, that allowed humans to succeed where all our fossil ancestors and their relatives failed. In fact, cooperation may be the greatest skill we've acquired in the past two million years. This skill allowed our young species (in evolutionary terms) to survive periods of environmental change and stress, and it may also determine its future.

About the author

Blake Edgar is one of the authors of the book "From Lucy to Language" and other books, and an editor in the journal Archeology.

in brief

Even in societies where polygamy is permitted, monogamy is the most common human mating arrangement. In this sense, we are unusual animals: less than 10% of mammals form exclusive sexual relationships.

The question of how we became like this has been at the center of scientific discussions for decades, and has not yet received a definitive answer. However, new studies clarify the issue.

We now know that the first hominins, who appeared more than seven million years ago, were probably monogamous. Humans remained monogamous (for the most part) for a good reason: it helped them evolve into their current form of big-brained world conquerors.

More on the subject

Reexamining Human Origins in Light of Ardipithecus ramidus. C. Owen Lovejoy in Science, Vol. 326, pages 74, 74e1-74e8; October 2, 2009.

Monogamy, Strongly Bonded Groups, and the Evolution of Human Social Structure. Bernard Chapais in Evolutionary Anthropology, Vol. 22, no. 2, pages 52-65; March/April 2013.

The Evolution of Social Monogamy in Mammals. D. Lukas and TH Clutton-Brock in Science, Vol. 341, pages 526-530; August 2, 2013.

Male Infanticide Leads to Social Monogamy in Primates. Christopher Opie et al. in Proceedings of the National Academy of Sciences USA, Vol. 110, no. 33, pages 13,328-13,332; August 13, 2013.

The article was published with the permission of Scientific American Israel