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MARGINALIA

You’ll Never Guess Who Walked In!

Ardi redefines the branch between apes and hominins

Pat Shipman

When a Chimp Is Not a Chimp

If Ardi was living in woodlands, weighed about as much as a chimp, was about as tall as a chimp, and had arms and legs and hands and feet like a chimp’s, why was she not a chimpanzee? Let me count the ways.

First, she doesn’t have apelike teeth. She has humanlike teeth, with small canines that barely project beyond the level of the tooth row.

Second, her dental proportions are wrong. In chimps, the front teeth—incisors and canines—dominate the dentition; in humans, the opposite is true. Ardi’s teeth lie somewhere in the middle of the two.

Third, the Ardipithecus remains known so far indicate that there is little sexual dimorphism in the species in canine or body size. This is one of the oddest features about Ardipithecus. Strong sexual dimorphism is common among living Old World monkeys and apes, among fossil primates older than Ardipithecus (such as Proconsul), and among hominins in the genus Australopithecus that are more recent than Ardi. If something Proconsul-like evolved into Ardi, and later into Australopithecus, sexual dimorphism was first lost and then regained.

Fourth, although her arms and fingers are long like a chimp’s, Ardi’s wrists and hands have no adaptations for brachiating branch to branch like a young chimpanzee or for knuckle-walking on the ground like an adult chimp. Ardi walked quadrupedally in the trees, placing her hands palm-down on branches and grasping with her big toes. Because she was relatively large, Ardi was probably a slow and careful climber on large branches much of the time.

Then there is the most surprising and unchimplike feature of all. According to team member C. Owen Lovejoy of Kent State University, Ardi had a pelvis like no other primate known to science. The upper part of the pelvis in humans forms a large bowl—consisting of the short, curving iliac blades—that cradles the viscera. In a chimp, the flat, long and narrow iliac blades lie behind (or dorsal to) the viscera. In humans, muscles attached to the broad iliac blades balance the body weight over a single pivot leg during bipedal walking. These muscles are differently aligned in apes and cannot serve this function, which is why chimps stagger from side to side when they walk bipedally. Ardi’s upper pelvis was a primary adaptation to bipedal walking on the ground, much closer in shape to Lucy’s pelvis than to a chimp’s. Paradoxically, the lower part of Ardi’s pelvis is very apelike, which is important in climbing vertical trunks with long arms wrapped around the trunk, like an ape or a telephone lineman.

“Ardi isn’t a transitional biped,” says Lovejoy. “She’s a biped from a species that’s going to keep its grasping toe and climbing thigh musculature, thank you very much.” He views Ardi’s locomotion and anatomy as mosaics of those in living species.

Ardi was clearly not chimp, nor Lucy nor human. Paleoartist Jay Matternes reconstructed her as having fur everywhere but on the center of her face, which protruded more than expected for a hominin. The fur, the long arms and the divergent big toe give Ardi the gestalt of an odd chimp, but that bipedal gait transforms Ardi back into a hominin.

The lasting messages from this tour-de-force analysis are complex, but the most important lesson of all is simple. Studying the behavior and anatomy of living animals provides us with an understanding of how anatomy links form and function. Genetic comparisons reveal the branching patterns that occurred in evolution and how one living species is related to another. Only the fossils really show us the past.




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