As part of the Ernest Everett (E.E.) Just Symposium, University of Utah anthropology proessor Kristen Hawkes presented her research on the evolution of human behavior. At Dartmouth, Hawkes discussed findings that support the grandmother hypothesis: the theory that the trait for human females having long post-reproductive lives has been selected because it allows grandmothers to help their offspring survive and assist in raising their grandchildren.
Hawkes started this discussion by explaining the differences in life history among humans and other primates, with most of her data coming from studies of chimpanzees. She and her team found that the fraction of female adult chimpanzees that make it to adulthood and live it past their childbearing years, which ends at age forty-five, is markedly lower than that of humans; only two percent of female chimpanzees survive past forty-five, while about seventy-seven percent of female humans live past the same age. Hawkes and her team set out to discover the reason female humans have a long post-reproductive span and the evolutionary advantage that this trait conveys.
Hawkes and other anthropologists postulate that this extended post-childbearing period has been selected because these grandmothers live on to help their offspring reproduce and survive; these two evolutionary benefits would justify long post-fertility periods. While chimpanzee mothers wait until their current child is totally independent, which can take about five or six years, human mothers reproduce before their current child can fend for itself and, on average, wait three years or less before they have another child.
While chimpanzees and other primates raise their children alone, human mothers also receive outside help, allowing them to have children more frequently. This group effort creates a rearing environment that is drastically different from those of other apes and helps to distinguish humans from other primates.
Because human mothers have to distribute their attention to several children at the same time, humans are additionally much more social than their evolutionary peers. Hawkes believes that this unique human rearing environment creates shared intentionality, an idea set forth by Michael Tomasello, which states that humans want to be connected and work together in society to achieve common goals.
In the future, Hawkes and her team will continue to evaluate human behavior and how this behavior helps humans evolve. Specifically, she will assess how shared intentionality adds to the grandmother hypothesis. She also plans to investigate some of the differences between chimpanzees and humans at the molecular level to aid in the understanding of the differences in aging between these two species.