After taking a fresh look at an old fossil, John Flynn, Frick Curator
of Paleontology at the American Museum of Natural History, and
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. This means one of the hallmarks
of primate biology, increased brain size, arose independently in
isolated groups-the platyrrhines of the Americas and the catarrhines of
Africa and Eurasia.
"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," says
Flynn of the paper that appeared in the Museum's publication Novitates
this June. 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 paper 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 in Nature, 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. In the Novitates paper,
Flynn and colleagues more accurately estimate that Chilecebus weighed
about 583 grams and had an E.Q. of only 1.11-a 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," says
Flynn. "Documenting that large brains evolved separately several times
within Primates will enhance understanding of the timing and pathways
of brain expansion and its effects on skull growth and shape, and may
lead to new insights into the genetic controls on encephalization."
Eric
Delson, the Chair of Anthropology at Lehman College, City University of
New York and a Research Associate at the Museum, concurs. "This work
confirms that brain size increase may be one of the common
characteristics of all primates," he says. "The relatively small brain
of Chilecebus contrasts with that of the slightly younger (16.5 million
years ago), larger brained fossil Killikaike found in Argentina and
described two years ago. It is probable that brain size also increased
independently in the lemurs of Madagascar, as well as in the apes (of
which humans are the extreme case) and the cercopithecid monkeys of
Africa and Asia."
American Museum of Natural History. July 2008.
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