Increased brain size arose independently in isolated groups of primates

July 10th, 2008 - 1:37 pm ICT by ANI  

Washington, July 10 (ANI): A new research has determined that one of the hallmarks of primate biology, increased brain size, arose independently in isolated groupsthe platyrrhines of the Americas and the catarrhines of Africa and Eurasia.

This research is based on a fresh study of an old fossil by John Flynn, Frick Curator of Paleontology at the American Museum of Natural History, who along with his colleagues, determined that the brains of the ancestors of modern neotropical primates were as small as those of their early fossil simian counterparts in the Old World.

Primatologists have long suspected that increased encephalization may have arisen at different points in the primate evolutionary tree, but this is the first clear demonstration of independent brain size increase in New and Old World anthropoids, said Flynn.

Encephalization is the increase in brain size relative to body size.

Animals with large encephalization quotients (E.Q.s) are those with bigger brains relative to their body size in comparison to the average for an entire group.

Most primates and dolphins have high E.Q.s relative to other mammals, although some primates (especially apes and humans) have higher E.Q.s than others.

At the heart of the new research is the development of more accurate equations for estimating body size in platyrrhines, or New World monkeys.

Most fossils are fragments of skulls or teeth so, to help in estimating their body size (and then E.Q.), Flynn and colleagues collected 80 measurements of the skulls, jaws, and teeth of 17 different species of living New World monkeys that ranged across the full spectrum of body sizes.

This study is one of the first to estimate body size with platyrrhines instead of their better-studied counterparts from the Old World, and this detailed analysis uses new statistical approaches to tease out which characteristics correlate best with body size.

The goal is to apply this equation to fossilized specimens.

Chilecebus, found high in the Andes and described by Flynn and collaborators in 1995, is one such fossil.

The skull dates to 20 million years ago and is the oldest and most complete well-dated primate skull from the New World.

Flynn and colleagues more accurately estimate that Chilecebus weighed about 583 grams and had an E.Q. of only 1.11a much smaller relative brain size than any living New or Old World anthropoid, which have E.Q.s ranging from 1.39-2.44 (and even higher for humans).

The result is clear: early fossil members of both the New World and Old World anthropoid lineages had small brain sizes, thus the larger brain sizes seen in both groups today must have arisen independently, said Flynn. (ANI)

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