Last weekend, I attended a cross-cultural wedding. The guests came from all over the world, spoke languages that were not understood by each other, and had different cultural backgrounds. They all had a similar understanding of the relationship that exists between the bridegroom and the bride. In spite of the fact that pair bonds are rare in other mammal species, they are universal in human society. We don’t know exactly why.
I spoke with a couple before the wedding breakfast who left their children with their grandparents. It’s not uncommon for grandparents to babysit an average of 76 times per year in the UK. We take this as a given. But now, a study offers grandparents the credit that they deserve. Long-term relationships were actually developed because grandmothers helped out with children in prehistoric times.
The importance of grandparents
In evolutionary anthropology, the question of why we form pair bonds (the biological term for a strong affinity between partners – often a male and female pair, but not always) is one of the most puzzling questions. Although humans are apes – our closest living relatives, chimpanzees & bonobos – do not have such long-term male-female relationships.
Hadza women are able to have many children, possibly because of their grandmothers. idobi/wikimedia, CC BY-SA
The ” grandma hypothesis ” was proposed by anthropologists in the late 1990s to explain why female humans stop reproducing around the same age as other great apes but live a significantly longer life. Chimpanzees can live well into their 40s and beyond, but female humans often live for decades after their reproductive years.
The grandmother hypothesis is based on the observations of the Hadza in Tanzania. Hadza people hunt and gather food like our ancestors.
Hadza women who are older dig up tubers for children who can’t do it themselves. This grandmother hypothesis states that daughters are able to have their babies sooner if they receive help from their grandmothers. As time went on, grandmothers who lived longer and took care of their grandchildren more shared their genes with their grandchildren. These genes were spread more widely in the population, and lifespans increased.
The evolution of partnerships
This new study published in the Proceedings of the National Academy of Sciences used computer simulations in order to link this hypothesis with the evolution of human pair bonding. They claim that long-term relationships developed because of a combination between people living longer and men being fertile for longer. This led to an excess of older men who were competing with younger, fertile females.
The study found that humans have a ratio of fertile men to females twice as large as chimpanzees. This makes us a very unique mammal. We are more similar to birds because we have an excess of males. Birds are known for their pair bond.
Grandma’s presence in the family can make it easier to manage. Wavebreakmedia, Shutterstock.
When many males are competing for a small number of females, males who form a strong relationship with one female have more offspring who survive than males who seek out multiple partners. The authors claim that this increased the incentives for males to “guard” their partner against rival males.
The authors claim that while mate-guarding and pair-bonding are not the same, both require a balance between paying attention to your current partner or seeking out a new one. Although the study focuses on male strategies, women are not passive – it takes both to bond.