Tuesday, July 7, 2015

A look inside an ancient primate's brain yields surprises

Fifteen million years ago the earliest known member of the subfamily cercopithecine roamed the planet. Victoriapithecus is the most ancient old world monkey scientists have discovered to date. This frugivorous primate was found on an island in Lake Victoria (Benefit, 1999). What we know about this species is a result of one fossil, a skull. A recent study by Gonzales and colleagues studied the inside of this monkey's skull (also called an endocast) to learn more about the brain structure of Victoriapithecus.

Endocast of A. sediba, Photo credit: Lee Berger
Using high-resolution X-ray imaging, researchers were able to create a 3D model of what Victoriapithecus's brain looked like. The results were surprising. At only 35.6cm3, this monkey had a brain about the size of a plum: small when compared to the animal's body size.  Most primates this size have a brain that's about the size of an orange.

The olfactory bulb, the part of the brain in which sense of smell is processed, was three times larger than expected. It was along the lower end of what we see for strepsirrhines, a group of primates with a greater reliance on sense of smell when compared to haplorhines (new and old world monkeys, tarsiers, and apes).

Despite having a small brain and a greater reliance on smell, characteristics typically seen in the so-called lower primates, there is more to Victoriapithecus than meets the eye. Micro CT scans were able to show the wrinkles and folds in the brain.  Given that this animal had quite a small brain, the numerous ridges and folds (sulci and gyri) show that this species was complex.  Folds on the brain are generally linked to greater intelligence. The organization of Victoriapithecus's brain and the sulci and gyri are what we see in present-day cercopithecines. Thus, it looks like a having a large brain is not a prerequisite for having a complex brain.

Links of interest:
Brain size and evolution
Developmental pattern of primate brains
How snakes may have influenced primate brain evolution

Literature cited:

Benefit, B. R. Victoriapithecus: the key to Old World monkey and catarrhine origins. Evol. Anthropol. 7, 155174 (1999). 

Gonzales, L., Benefit, B., McCrossin, M., Spoor, F. Cerebral complexity preceded enlarged brain size and reduced olfactory bulbs in Old World monkeys. Nature Communications, 2015; 6: 7580 DOI: 10.1038/ncomms8580

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