Big Brains Arose Separately in Multiple Primate Groups

John Roach
for National Geographic News

July 18, 2008

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.

When the team applied the model to the fossil skull of a 20-million-year-old primate from South America known as Chilecebus carrascoensis, they found it had a body mass of about 1.3 pounds (600 grams) and an EQ of only 1.11.

Living primates have EQs ranging from 1.39 to 2.44, and it's even higher for humans. An EQ value of 1 is the average for modern mammals.

The new measurement of Chilecebus' body mass is nearly half as small as earlier estimates, but the results still confirm the animal's low EQ.

Previously researchers had suspected Chilecebus' small brain was an anomaly, the University of Michigan's Finarelli noted.

"That brain is so small that even drastic lowering of the denominator is not enough to make Chilecebus' brain size look like what we see in modern anthropoids," he said.

The work is described in last month's issue of the American Museum of Natural History publication Novitates.

Bigger is Better?

Robert Martin is the A. Watson III Curator of Biological Anthropology at the Field Museum in Chicago, Illinois, and an expert on mammalian brain expansion.

He said the new paper confirms that primates—like dolphins, whales, and other mammals—developed bigger brains through time.

The one exception, Martin noted, is a controversial paper published in 2005 that found bat brains lack a directional trend. That paper, however, included no comparison with the fossil record.

"When people have done careful work, they always find the same thing: that brain size starts off small and it gets bigger, and in some lineages it gets bigger faster than in others," he said.

(Related: "Human Ancestor Had Lime-Size Brain" [May 14, 2007].)

"This new paper on Chilecebus provides yet another clear example that brain size gets bigger generally in primates," Martin said.