for National Geographic News
The ancestors of modern-day primates in the Americas had tiny brains just like their counterparts in Africa and Eurasia, according to a new study.
Because modern anthropoid, or humanlike, primates in both regions have large brains relative to their body sizes, the finding suggests that one of the hallmarks of primate biology—increasing brain size—happened independently in isolated groups.
"Both sides today have large brains, so that transformation has to have occurred subsequent to the evolutionary split," between New World primates and Old World monkeys, said John Finarelli, a study co-author and evolutionary biologist at the University of Michigan in Ann Arbor.
John Flynn, Frick Curator of Fossil Mammals at the American Museum of Natural History in New York, also co-authored the paper.
He added that "we have to now change our thinking about the pathways and processes that led to getting a big brain."
For example, knowing that the brain expanded in multiple groups raises questions about the genetic controls for brain size and how the expansions affect skull growth and shape, Flynn noted.
More Grey Matter
An increase in brain size relative to body size is called encephalization.
Animals with large encephalization quotients, or EQs, are those with bigger brains relative to their body sizes compared to the group average.
Some scientists had suspected big brains arose several times during primate evolution, but it had been difficult to determine precisely when separate events occurred, Flynn noted.
The new research is based on 80 different measurements taken of the skulls, jaws, and teeth of 17 living New World monkeys, such as capuchins and marmosets, to determine which features are the best predictors of body size.
The results were then used to create a model that researchers could use to plug in available data from a particular fossil skull, jaw, or tooth and obtain a body-size estimate.
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