January 24, 2015
In the book Mothers and Others, renowned primatologist Sarah Hrdy argues that cooperative breeding played a crucial role in the evolution of human social intelligence . In her opening chapter, Apes on a Plane, Hrdy introduces a humorous yet informative comparison. When faced with air travel, humans manage, one by one, to file themselves in a semi-organized fashion onto a plane, and, arm rests and leg room notwithstanding, manage to share space in an equitable fashion such that they are able to take off and land without ripping one another limb from limb. Were fifty or more unrelated chimpanzees to board an aircraft for a similar journey, flight attendants would have quite a different experience as their passengers screamed and attacked one another until all but one bloodied winner remained. The point that Hrdy means to illustrate is that human beings, when compared to one of their closest living relatives, have a profound capacity for social tolerance, and an unprecedented ability to not only endure but enjoy complete strangers in a confined space for hours on end. While there is substantial variation in the human temperament during travel (I, for one, tend to exit planes in a borderline murderous rampage) an overwhelming theme emerges: our species demonstrates a propensity for cooperation and altruism that is greatly exaggerated when compared to non-human animals.
And cooperation is essential to our species, in ways that some would argue it is not for others. Consider, for example, the female chimpanzee. Females are less gregarious than their male counterparts, foraging alone or with close maternal kin . While grooming, mating and other social behaviors are important components to a chimpanzee mother’s daily life, she is well-equipped to nurse, carry and raise her offspring in a manner that is shockingly independent when contrasted to that of the modern human female. Sitting aboard a KLM flight to Kilimanjaro, it dawned on me just how cooperative the experience of human child maintenance could be. The fates have mandated that all of my travel experiences be encumbered by screaming youths, and as such I was seated behind a young couple as they struggled to soothe a cranky two-year-old. Both parents played with and nurtured the child, invested not only in appeasing their own offspring but the passengers around them. Those nearby, when cries transitioned into muted sniffles, made silly faces at the child and engaged him with the famous “peek-a-boo” gesture. When pacifiers were thrown to the floor, they were returned dutifully by the nearest neighbor. When seats were jarred by petite yet thrashing legs, those in them remained calm. For the sake of propriety, I will refrain from describing how a chimpanzee might respond to an unruly infant belonging to another mother (but see section 3c of Pusey & Schroepfer-Walker 2013  for a recent review of infanticide directed by female chimpanzees). And while my personal temperament tends to range closer to “disgruntled chimpanzee” than “charming human female”, I, too, cooed and awed at the child when it ceased to screech in my general direction.
Airplanes are one of the more extreme examples of human cooperation, as in addition to calming screaming children we all work together to load luggage, pass beverages, and engage in friendly conversation. Hrdy’s comparison is important as it represents a general overview of the comparative method that primatologists use to understand the evolution of human behavior and psychology. By examining a single topic, e.g., food sharing or child care, within a cluster of closely-related species, we can identify dissimilarities, allowing us to infer potential adaptations. Comparing the present cooperative tendencies of chimpanzees and those of modern humans, we can surmise that since the last common ancestor of humans (Genus: Homo) and chimpanzees (Genus: Pan) at 4.8  – 8 million years ago (MYA), humans have undergone considerable changes in their neuro-anatomy, behavior, locomotion and cognition that have all may have contributed to our unique place in the animal kingdom.
And while I have evolved to the point that I can refrain from thrashing a noisy child whose loss will not impair my own fitness, I find myself drawing comparisons between my life and that of a chimpanzee on a daily basis. After deeming the airplane child cute and no longer a menace, and making a taxi ride from the airport to Oyster Bay in Dar es Salaam, I managed to dive into a buffet of Indian food like a tactless animal for one last meat-filled binge. Like a female chimp, foraging will always be my number one priority.
 Hrdy, S.B. (2000). Mothers and Others: The evolutionary origins of mutual understanding. Cambridge, Massachusetts: Harvard University Press.
 Wrangham, R.W., & Smuts, B. B. (1980). Sex differences in the behavioral ecology of chimpanzees in the Gombe National Park, Tanzania. Journal of Reproduction and Fertility, 28, 13-31.
 Pusey, A.E., & Schroepfer-Walker, K. (2013). Female competition in chimpanzees. Philosophical Transactions of the Royal Society B: Biological Sciences, 368(1631), 20130077.
 Chen, F. C., & Li, W. H. (2001). Genomic divergences between humans and other hominoids and the effective population size of the common ancestor of humans and chimpanzees. The American Journal of Human Genetics, 68(2), 444-456.
 Langergraber, K.E. et al. (2012). Generation times in wild chimpanzees and gorillas suggest earlier divergence times in great ape and human evolution. Proceedings of the National Academy of Sciences USA. 109, 15716–15721.