Humans are cooperative breeders like many other primates. However, monogamy was a necessary key step before cooperative breeding evolved in humans.
“It takes a village to raise a child” – the proverb is not unheard of and often put forward to support community living and emphasize the importance of joint families. But we have to admit that some very wise person said it for it is truest to the core.
When a woman is pregnant help of all sorts come from all directions, from kins, next to kins, friends, associates and even strangers. When the baby is born, the little one is welcomed with a grand celebration. The child doesn’t even remember the number of laps he or she has traversed before developing a certain level of consciousness. Older siblings change their diapers, aunts and uncles take them on small outdoor trips, grandmothers bathe them, grandfathers play with them and keep an watchful eye always looking out for their safety, fathers rock their babies to sleep while mothers rest after the demanding ordeal of childbirth and the parental duties that come after it. Such is the current state of affairs in the modern human society.
Why? Because, humans are cooperative breeders. In cooperative breeding, parental duties are performed by non-parents besides the parents themselves. The helpers may be older siblings or grandparents who have direct genetic relations to the child, or kins who are distantly related and non-kins who are unrelated genetically.
And it was cooperative breeding that helped humans emerge as a successful species among all hominins that lived alongside our own kind even until some thousands of years ago. But something very significant may have preceded the evolution of cooperative breeding in ancient human societies. It is monogamy. Males are more inclined to invest on a child when paternity is confirmed than when paternity is uncertain. The same is true for paternal grandparents, aunts and uncles. Since in most societies (71%) the woman relocates to the husband’s home after marriage, help from the father and paternal grandparents are essential for the survival and well-being of the newborn. High relatedness between siblings is also necessary if help is to be expected in child rearing from older siblings.
There is a lot to explore on this topic. Let’s get to them one by one.
Coefficient of Relatedness and benefits of Kinship
The Kin Selection Theory proposed by W. D. Hamilton states that altruistic behaviour evolves when the genetic benefit to relatives (rb) is greater than the cost incurred by the giver (c), or rb > c.
Individuals tend to show proportionately more altruistic behaviour towards kins to whom they are genetically more related than to those they share less genes with. Every act of altruism involves a cost for the giver and benefits for the receiver. When there is genetic relation between the giver and the receiver there is an indirect benefit for the giver as well. The indirect benefit to the giver increases with the degree of genetic relatedness between the giver and the receiver. In other words when help is bestowed on near-relatives benefit to cost ratio (rb/c) is higher compared to when help is bestowed on far-relatives and non-relatives.
When related individuals live in groups they help each other out. Even in present day societies we observe this behaviour. It is more prominent in some surviving hunter gatherer human communities. Genetic relatedness between members is high in these communities and relatives live in close-knit groups. Altruism develops in these groups as help given to family members involves higher benefit to cost ratio because of high genetic relatedness. Cooperative breeding also come under altruistic acts.
Cooperative Breeding and Genetic Relatedness
In cooperative breeding, non-parents help the parents with parental duties offsetting some of the child rearing costs incurred by the parents. But, what benefit is there for the helpers? Simple logic says that helpers help the couple or the breeding pair at a cost of their own reproduction. They could have found a mate and reproduced themselves instead of helping others with child rearing. Well, there is a benefit when they have a genetic relation with the couple and eventually with the child they are helping rear.
Let’s look at the typical construct of a modern family. It involves just the couple and their children when it is a nuclear family. Sometimes a modern family also involves the paternal grandparents or maternal grandparents depending upon whether the society has patrilineal or matrilineal residence. In larger joint families uncles, aunts, cousins and their grandparents also reside under the same roof. The type of marriage prevalent decides the genetic relation family members share with one another.
While evidence of polygyny was found in 85% of human societies most of the population practiced monogamy.
Indirect fitness benefits are the main driving force behind the evolution of cooperative breeding.
Since, in humans birth interval is much less compared to other primates it allows offspring to be produced in quick succession. Interval between two pregnancies may be as less as one year. This shortened birth interval is an outcome of cooperative breeding rather than the cause.
Children of different ages may live with their parents before they reach a certain age and cease to be dependent on their parents. Full siblings, born out of monogamy, have as much genetic similarity between themselves (r = 0.5) as they would to their own children (r = 0.5). Thus, older siblings gain the same indirect benefits when they help their mothers with parenting responsibilities as they would with bringing up their own children when they are ready to reproduce. Being juveniles yet to reach reproductive age they do not have anything to lose when they offer their time to take care of their younger siblings.
Under polygyny, which is still prevalent in many societies, women are monogamous (monoandrous) as they are mating with only one male. Thus, the genetic relatedness between siblings remains the same. Even maternal and paternal grandparents share the same genetic relatedness to their grandchildren as under a monogamous arrangement. As grandparents are past their reproductive age, helping with parental duties grants them some extra points of indirect benefits in both monogamous and polygynous set up.
Monogamy and Cooperative Breeding
Cooperative breeding increases survival chances of the new born and also shortens birth intervals for the mother. As we have seen before, in both monogamy and polygyny where the mother is monogamous in the sense that she mates with only one male, the genetic relatedness between siblings and between the newborn and its grandparents and other kin is preserved in a similar manner. Genetic relatedness doesn’t change helping behaviour in the either cases as proportion of genes shared remains optimum.
In humans, the father cares for the newborn along with the mother, unlike many other primate species where males do not participate in sharing parental duties. The real problem arises when paternity in uncertain. In such scenarios parental duties rest entirely on the mother, older siblings and maternal grandparents. In a promiscuous or polyandrous setup, it is very easy for the male to have doubts about the paternity of a child. The need for male help with parenting is said to have pushed for monogamy in many species, which is argued to hold true for humans as well.
Modern human societies project a non-paternity rate of 1.7% – 3.3%, which is much lower than that found in monogamous birds (20%) and mammals (5%). Paternal care is only found in those mammalian species where paternity is confirmed and the pair is monogamous.
Both promiscuity and polyandry exist in human societies. In polygamous and promiscuous mating systems siblings have lesser genetic relatedness to step sisters and brothers. So delaying their own reproduction (r = 0.5) or own well being (r = 1) and dedicating that time towards raising a step sister or brother (r < 0.5 or r = 0.25) seems futile.
When we consider other species that show cooperative breeding, we find that they are three times less promiscuous than species that do not show cooperative breeding. A study of mammalian species reveal that female helpers are more common in those mammalian species where the female mates exclusively with just one male. Studies across all eusocial species groups show that monogamy always evolved before the evolution of eusociality which is a form of cooperative breeding itself.
Under a monogamous setting older siblings have 50% genetic relation with the newborn (r = 0.5). The value of genetic relatedness is equal to that they would have with their own children when they reach reproductive age. Even paternity is confirmed to elicit paternal investment in offspring. It is assumed that the “Monogamy Bottleneck Hypothesis” is applicable to human evolutionary history as well. When humans developed larger brains, higher calorie intake and their newborns demanded greater care paternal help in child rearing was solicited. Monogamy was the best arrangement under which the optimal care could be provided to ensure higher survival of the human offspring.
However, the modal mating system among humans at present is serial monogamy. Since serial monogamy in essence of genetic relatedness between siblings and grandparents is similar to that of polygamy or promiscuity, it is not considered conducive to the development of cooperative breeding. Serial monogamy came into existence due to higher mortality of mates, separation of mating partners and a long human reproductive career. But still, a short association between couples for some years was probably enough to raise offspring to self-dependency. As modern divorce laws, monetary settlement and child custody laws evolved in human societies serial monogamy ceased to be a hindrance to child upbringing in modern times.
Many evidences point towards the evolution of monogamy before the development of cooperative breeding in humans. So, it might be said with some confidence that monogamy evolved before cooperative breeding in humans.