New research suggests that evolutionary changes in cognitive development underlie the extensive social and behavioral differences that exist between two closely related species of great apes. The study, published online on January 28th in Current Biology, a Cell Press publication, enhances our understanding of our two closest living relatives, chimpanzees and the lesser-known bonobos, and may provide key insight into human evolution.

Although chimpanzees and bonobos have a very close genetic relationship with each other, the two species display major differences in their physical appearance, behavior, and cognition. For example, when compared to chimpanzees, bonobos seem to be much more peaceful and easygoing, retaining juvenile levels of play as adults, exhibiting low levels of aggression towards one another, and being much more likely than adult chimpanzees to share resources. It has been suggested that these differences might be a result of species-specific shifts in the developmental pathways that link infancy with adulthood.

"Thus far, there has been no direct test of the hypothesis that certain aspects of behavior or cognition in adult bonobos represent developmentally delayed forms of the traits found in chimpanzees," explains the lead study author, Victoria Wobber of the Department of Human Evolutionary Biology at Harvard University. "We tested this hypothesis by comparing skills of semi-free-ranging infant, juvenile, and adult bonobos and chimpanzees in three feeding competition tasks, given the prediction that this area in particular differs between the two species."

Wobber and colleagues observed that as chimpanzees reached adulthood, they became more and more intolerant of sharing food, whereas bonobos retained juvenile levels of food-related tolerance. Furthermore, chimpanzees consistently outperformed bonobos of the same age in tests where the subjects had to out which experimenters held a food reward. "Bonobos took longer to develop the same skill level shown even among the youngest of the chimpanzees that were tested," says Wobber. "It seemed as if adult chimpanzees were able to exhibit more social restraint than adult bonobos."

The findings support the hypothesis that developmental delays play a role in shaping differences in the social psychology and behavior of chimpanzees and bonobos. "Taken together, our results indicate that these social and cognitive differences between these two closely related species result from evolutionary changes in brain development," says Wobber. "Intriguingly, it has been suggested that the crucial cognitive adaptation of humans relative to other apes may be the accelerated development of social skills in infants. If we can understand the evolutionary processes by which developmental changes occurred in bonobos, perhaps inferences can be made about our own species' evolution."