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.
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.
When the researchers scanned the extinct animals' bony labyrinths, some unexpected results emerged. One came from the species Apidium phiomense. Found fossilized in Egypt, this is one of the earliest anthropoids (a group that includes monkeys, apes, and humans). Apidium's skeleton suggests a creature adapted for leaping. Inside its skull, though, were the smaller canals of a less agile animal. "That was definitely a surprise," Ryan says. Given the previous research in living species, mismatches between an animal's locomotive style and its canal size should be uncommon. Apidium may have been slower than we thought, Ryan notes, or its inner ear may have lagged behind while its skeleton evolved rapidly for agility.
Another twist came from a species of Proconsul, "the best-known early ape," Ryan says. From its extensively studied skeletal fossils, "It was considered to be kind of a slow, cautious quadruped in the trees," Ryan says. The ear canals of Proconsul heseloni were larger than expected, suggesting a more agile animal. "Now we believe that it's probably more like a macaque," Ryan says, a primate that moves at a modest pace but is able to leap and clamber at times.
The findings, published this month in the Proceedings of the Royal Society B, "suggest that the basal ape, that first common ancestor of apes and humans, was faster than we would have thought," Ryan says. The slower locomotion of today's gorillas and humans, rather than being inherent to apes, may have evolved later on.
"This is really valuable because it gives us another source of data to say what an extinct organism might have been doing," says Laura MacLatchy, an anthropologist at the University of Michigan, Ann Arbor, who was not involved in the research. She points out, however, that P. heseloni is on the smaller side of the four or five species of Proconsul. The larger species may have moved more slowly. Rather than representing how the original apes moved, P. heseloni might simply be a more agile member of a diverse genus.
Researchers will need to delve deeper into the fossil evidence to resolve the apparent mismatches between the inner ear and skeleton, as in Apidium. Ryan says that further studies in living primates, too, will help clarify the relationship between an animal's semicircular canals and its style of movement. Eventually, we may be able to put more of our long-fossilized relatives back into motion.