In the animal kingdom, most species keep reproducing until they die. In humans, however, females live for decades even if they are no longer fertile. This has baffled scientists, but some studies suggest this ability might have evolutionary benefits.
Women's Role in Species Survival
When a woman experiences menopause, she can continue living in her 70s or 80s. The only female species living past reproduction are aphids (Quadrartus yoshinomiyai) and four types of whales (beluga, killer, narwhal, and short-finned pilot).
At around age 50, a woman's ovaries stop producing egg cells. By this time, a woman is officially liberated from her reproductive duties and biological functions of menstruation, pregnancy, and lactation.
Many primates die out after their reproductive years, but humans live on decades later. From an evolutionary perspective, this suggests women's importance to our species' survival beyond their direct ability to bear offspring.
In the 1960s, experts developed the "grandmother theory," which states that grandmothers enable mothers to bear more children. It suggests that ceasing reproduction allows older women to devote their time and resources to take care of their children's children. This way, grandmothers can still ensure the survival of their genes, which are just two generations down the family tree. This is considered a better strategy than for older women to attempt childbirth since it becomes riskier with age.
READ ALSO: Menopause Explained: Why Women Stop Ovulating While Men Keeps on Producing Sperm
Scientific Evidence of Grandmother Effect
The postmenopausal longevity of human females was once considered an evolutionary conundrum because a shorter reproductive lifespan would impair fitness and would not be expected to evolve. However, mathematical simulations have shown that the grandmother effect is a vital reason human life spans moved from an apelike pattern to our modern one.
Two research studies strengthened this notion. In the first study entitled "Limits to Fitness Benefits of Prolonged Post-reproductive Lifespan in Women," experts analyzed church birth and death records in Finland for individuals born from 1731 to 1890. It was discovered that having a maternal grandmother aged 50-75 years old increases the survival of a child who is five years old or younger.
This effect disappears when the grandmother reaches 75 years, likely because they can no longer help with child-rearing. Meanwhile, the impact on longevity was mainly seen with maternal grandmothers, not paternal grandmothers.
The second study, "Using Geographic Distance as a Potential Proxy for Help in the Assessment of the Grandmother Hypothesis," drills into this effect. It was shown that the benefit is observed not only with the existence of a grandmother but her proximity also matters. This means that the shorter the distance between grandmother and grandchild, the more involved the grandmother can be.
For centuries, grandmothers have played a significant role in helping their family units grow. In our modern society, however, more families live alone without their grandparents. Over the 20th century, the nuclear family has changed, mainly due to geographic dispersion.
At the beginning of the 20th century, only 7% of older women lived alone, which peaked at 38% in the 1990s. If this dispersion continues, our modern society could reverse the evolutionary role played by the grandmothers. As a result, we may also lose the benefits we have achieved from this effect.
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