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.
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