Thursday, December 15, 2011
Endangered orangutans offer a new evolutionary model for early humans
Starving orangutans in Borneo are yielding new ideas about human evolution. The way in which the orangutans cope with food-limited environments may give information about what early human ancestors were facing.
The study is being led by Nathaniel Dominy, an associate professor of anthropology at Dartmouth College. The apes that gave rise to the earliest human ancestors had teeth that are much like orangutan teeth, particularly those living in Borneo. Dominy suggests that the orangutans' diet may have exerted a selective pressure on their molar teeth. If we understand the physical properties of their food, then we may have some idea of why humans evolved the teeth that we have.
The five-year study described in the Biology Letters paper documents the adaptive metabolism of these apes in the protein-deficient times that characterize the environment for the majority of a four- to five-year cycle. Their urine was collected and analyzed for dietary markers, such as ketones, which increase when the body breaks down fat for energy. When fruit abundance was lowest, the ketones surged, demonstrating that the animals were burning their fat reserves—using more energy than they were taking in. When the fat is depleted, however, muscle tissue is cannibalized.
Professor Dominy considers the lean years for orangutans on Borneo to be a selective pressure that led to evolutionary adaptations since the population became isolated 400,000 years ago, and the larger molars and more robust jaws developed in response to the hard, tough foods they consumed during the periods between fruit availability. Recent studies of wear patterns on the huge molars of early hominids suggest that they only ate a more physically challenging diet some of the time; they may be displaying an adaptation that helped them to get through evolutionary pinch points, similar to what the Borneo orangutans encounter.