How Older Generations Contribute to a Long Lifespan

Humans possess many unique traits which separate them from other species. Perhaps most impressive is the species ability to live into old age. For most other animals, natural selection heavily favors reproduction, and animals that live past their useful reproductive period quickly move on to mortality. Natural selection generally produces traits that favor producing new members of the species, and an animal’s survival decreases in direct correlation with its ability to procreate. 

The fact that natural selection seems to work differently for humans is a common puzzle for evolutionary biologists. The likelihood of a rapid onset of mortality is present even in one of humanity’s closest animal cousins, the chimpanzee. If other primates age rapidly and become more susceptible to disease after losing their reproduction ability, why doesn’t it happen for humans? Human women don’t just get a few extra years after their fertile period closes; they typically live for decades. 

Despite what may be immediately apparent, scientists believe that humans evolved a long life span through natural selection. Humans were longer-lived than other species well before the advent of modern health care and safety practices, and the laws of natural selection, while diminished, are still in effect past the age when a human can reproduce. 

One popular theory, the Grandmother Hypothesis, suggests that maternal grandmothers can increase the likelihood that their genes will continue to be passed on by ensuring the fitness of their grandchildren. A long-lived woman who can help care for her grandchildren grants them a higher chance of successfully reproducing, ensuring that the grandmother’s genes survive to the next generation. The chances of surviving to reproduce are reduced in families without long-lived grandmothers. This creates a “force” that pushes evolution to select long-lived genes regardless of reproductive ability. 

The implications of natural selection producing longer-lived humans are deeply rooted in cooperation. For example, the concept of grandmotherly support is not present in chimpanzee populations, and there is no motive for natural selection to reward long-lived chimpanzees. Humans, on the other hand, have evolved the ability to cooperate and integrate. This cooperation allows grandparents to significantly increase the likelihood that their grandchildren will survive to reproduce, directly motivating natural selection.