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Ardi (right) lived in a forest in Africa. Her fossil skeleton shows that she walked upright and yet had an opposable toe, good for climbing trees. (Tim White 2009, From Science Oct. 2 issue; Illustration by J.H. Matternes)

The Human Family's Earliest Ancestors

Studies of hominid fossils, like 4.4-million-year-old "Ardi," are changing ideas about human origins

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In his lab at Kent State University, Lovejoy recently demonstrated why Ardi is so unusual. He gently lined up four bones from Ardi’s hand on his lab bench, and he showed how they fit together in a way that allowed Ardi’s hand to bend far backward at the wrist. By comparison, a chimpanzee’s wrist is stiff, which allows the animal to put its weight on its knuckles as it moves on the ground—knuckle walking. “If you wanted to evolve Ardi’s hand, you couldn’t do it from this,” he said, waving a set of bones from a chimpanzee hand in the air. If Lovejoy is right, this means Ardi—and our upright-walking ancestors—never went through a knuckle-walking stage after they came down from the trees to live on the ground, as some experts have long believed.

As evidence that Ardi walked upright on the ground, Lovejoy pointed to a cast of her upper pelvic blades, which are shorter and broader than an ape’s. They would have let her balance on one leg at a time while walking upright. “This is a monstrous change—this thing has been a biped for a very long time,” Lovejoy said.

But Ardi didn’t walk like us or, for that matter, like Lucy either. Ardi’s lower pelvis, like a chimpanzee’s, had powerful hip and thigh muscles that would have made it difficult to run as fast or as far as modern humans can without injuring her hamstrings. And she had an opposable big toe, so her foot was able to grasp branches, suggesting she still spent a lot of time in the trees—to escape predators, pick fruit or even sleep, presumably in nests made of branches and leaves. This unexpected combination of traits was a “shocker,” says Lovejoy.

He and his colleagues have proposed that Ardi represents an early stage of human evolution when an ancient ape body plan was being remodeled to live in two worlds—in the trees and on the ground, where hominids increasingly foraged for plants, eggs and small critters.

The Ardi research also challenged the long-held views that hominids evolved in a grassy savanna, says Middle Awash project geologist Giday WoldeGabriel of Los Alamos National Laboratory. The Ardi researchers’ thorough canvassing—“You crawl on your hands and knees, collecting every piece of bone, every piece of wood, every seed, every snail, every scrap,” White says—indicates that Ardi lived in woodland with a closed canopy, so little light reached grass and plants on the forest floor. Analyzing thousands of specimens of fossilized plants and animals, as well as hundreds of samples of chemicals in sediments and tooth enamel, the researchers found evidence of such forest species as hackberry, fig and palm trees in her environment. Ardi lived alongside monkeys, kudu antelopes and peafowl—animals that prefer woodlands, not open grasslands.

Ardi is also providing insights into ancient hominid behavior. Moving from the trees to the ground meant that hominids became easier prey. Those that were better at cooperating could live in larger social groups and were less likely to become a big cat’s next meal. At the same time, A. ramidus males were not much larger than females and they had evolved small, unsharpened canine teeth. That’s similar to modern humans, who are largely cooperative, and in contrast to modern chimpanzees, whose males use their size to dominate females and brandish their dagger-like canines to intimidate other males.

As hominids began increasingly to work together, Lovejoy says, they also adopted other previously unseen behaviors—to regularly carry food in their hands, which allowed them to provision mates or their young more effectively. This behavior, in turn, may have allowed males to form tighter bonds with female mates and to invest in the upbringing of their offspring in a way not seen in African apes. All this reinforced the shift to life on the ground, upright walking and social cooperation, says Lovejoy.

Not everyone is convinced that Ardi walked upright, in part because the critical evidence comes from her pelvis, which was crushed. While most researchers agree that she is a hominid, based on features in her teeth and skull, they say she could be a type of hominid that was a distant cousin of our direct ancestor—a newfound offshoot on the human family tree. “I think it’s solid” that Ardi is a hominid, if you define hominids by their skull and teeth, says Rick Potts, a paleoanthropologist at the Smithsonian’s National Museum of Natural History. But, like many others who have not seen the fossils, he has yet to be convinced that the crushed but reconstructed pelvis proves upright walking, which could mean that Ardi might have been an extinct ape that was “experimenting” with some degree of upright walking. “The period between four million to seven million years is when we know the least,” says Potts. “Understanding what is a great ape and what is a hominid is tough.”

As researchers sort out where Ardi sits in the human family tree, they agree that she is advancing fundamental questions about human evolution: How can we identify the earliest members of the human family? How do we recognize the first stages of upright walking? What did our common ancestor with chimpanzees look like? “We didn’t have much at all before,” says Bill Kimbel, an Arizona State University paleoanthropologist. “Ardipithecus gives us a prism to look through to test alternatives.”

After Ardi’s discovery, researchers naturally began to wonder what came before her. They didn’t have long to wait.

Starting in 1997, Haile-Selassie, now at the Cleveland Museum of Natural History, found fossils between 5.2 million and 5.8 million years old in the Middle Awash. A toe bone suggested its owner had walked upright. The bones looked so much like a primitive version of A. ramidus he proposed these fossils belonged to her direct ancestor—a new species he eventually named Ardipithecus kadabba.

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