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The bond between mother and child may offer clues to the mystery of why humans live longer than expected for their size – and shed new light on what it means to be human.

“That’s one of the really mysterious things about humans, the fact that we live these super-long lives compared to so many other mammals,” said Matthew Zipple, a Klarman postdoctoral fellow in neurobiology and behavior in the College of Arts and Sciences. “What we’re putting forward is that part of the explanation for our longevity is this other major aspect of our lives, which is the relationship between a mother and her child.”

The article “Maternal Care Drives the Evolution of Long, Slow Lives” was published in Proceedings of the National Academy of Sciences on June 14.

In their models, Zipple and co-authors consistently found that in species where offspring survival depends on longer-term maternal presence, the species tends to develop longer lifespans and slower lifespans, which is characterized by how long it lives the animal and how often it reproduces.

“As we see these connections between maternal survival and offspring fitness strengthen, we see the evolution of animals that have longer lives and reproduce less frequently — the same pattern we see in humans,” Zipple said. “And the nice thing about this pattern is that it’s general to mammals in general, because we know that these relationships exist in other species outside of primates, like hyenas, whales and elephants.”

Zipple and co-authors provide a universal mathematical model that demonstrates the relationship between maternal survival and offspring fitness, on the one hand, and life span, on the other. Two additional empirical models include the types of data on maternal survival and offspring fitness collected by field ecologists. Zipple said the hope is that these models can be further tested and used by field ecologists to predict how maternal care and survival affect the evolution of a species’ lifespan.

“We hope that we’ve made the model clear enough that field ecologists can take their existing long-term demographic data that they’ve been collecting for decades and apply it to this model and come up with this estimate of how much maternal care expect mother have shaped the evolution of their learning system,” Zipple said.

The work is based on the mother-grandmother hypothesis, based on observations of human populations from the 18th and 19th centuries, that offspring are more likely to survive if their mothers and grandmothers are in their lives. This theory has been used primarily as an explanation for menopause in humans, Zipple said, because ceasing reproduction reduces the risk of death and allows older women to focus on caring for offspring.

Zipple’s models are both broader and more specific, encompassing more of the ways in which a mother’s presence or absence in her offspring’s life affects its fitness. The team makes predictions based on results from Zipple’s doctoral research on baboons and other primates about how offspring fare if the mother dies after weaning but before the offspring reach puberty, which Zipple found leads to short- and long-term, even between generations, negative effects on primate offspring and grandchildren.

“We wanted to expand the mother-grandmother hypothesis to look at these specific ways that we know in primates that the mother’s survival benefits her offspring,” Zipple said. “And ask what are the broader and perhaps more subtle ways in which the benefits of a mother’s presence in a person’s life might lead to the evolution of longevity.” We are also trying to explain this phenomenon in a much wider range of animals.”

For Zipple, who spent six months during his PhD observing mother baboons with their babies in the field, the link between motherhood and longevity reinforces his observations and underscores the importance and power of maternal care.

“When you watch mothers and babies in nonhuman primates interact, you can just see in the babies’ faces that there is nothing more important in the world than their mother’s presence,” Zipple said. “So for me, the behavioral work, combined with the demographic studies, really reinforced this common evolutionary thread that we share with our closest primate relatives — which is that there is a period of time where the whole world is our mother, and while it gets weaker over time, it never goes away. Part of the long-term aspiration of this line of research is to relate this to longevity, to relate these two mysterious and central aspects of what it is to be human.”

Co-authors include H. Kern Reeve, Professor of Neurobiology and Behavior (A&S), and Orca Jimmy Peniston, Kenai Peninsula College at the University of Alaska Anchorage.