From: Elizabeth Preston, Science AAAS
Published June 25, 2012 09:23 AM

Inner Ears Reveal Speed of Early Primates

It's 20 million years ago in the forests of Argentina, and Homunculus patagonicus is on the move. The monkey travels quickly, swinging between tree branches as it goes. Scientists have a good idea of how Homunculus got around thanks to a new fossil analysis of its ear canals and those of 15 other ancient primates. These previously hidden passages reveal some surprises about the locomotion of extinct primates—including hints that our own ancestors spent their lives moving at a higher velocity than today's apes.

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Wherever skeletons of ancient primates exist, anthropologists have minutely analyzed arm, leg, and foot bones to learn about the animals' locomotion. Some of these primates seem to have bodies built for leaping. Others look like they moved more deliberately. But in species such as H. patagonicus, there's hardly anything to go on aside from skulls.

That's where the inner ear canals come in. "The semicircular canals function essentially as angular accelerometers for the head," helping an animal keep its balance while its head jerks around, says Timothy Ryan, an anthropologist at Pennsylvania State University, University Park. In the new study, he and colleagues used computed tomography scans to peer inside the skulls of 16 extinct primates, spanning 35 million years of evolution, and reconstruct the architecture of their inner ears.

Also called the bony labyrinth, the area in question is a set of three twisting cavities, one oriented along each axis of the body. The sloshing of fluid inside the canals provides information for an animal's system of balance. An earlier study of living and recently extinct mammals showed that more agile or acrobatic animals have bigger semicircular canals relative to their body size. A sedentary sloth, for example, has small and insensitive canals. A gibbon needs larger, more sensitive canals to keep its head and gaze stabilized while it trapezes through the tree branches.

Article continues at Science AAAS

Modern chimpanzee image via Shutterstock

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