The Human Family’s Earliest Ancestors

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

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)
Smithsonian Magazine | Subscribe

(Continued from page 1)

In 1959, archaeologist Louis Leakey and his wife Mary, working in Olduvai Gorge in Tanzania, discovered a bit of hominid jawbone that would later become known as Paranthropus boisei. The 1.75-million-year-old fossil was the first of many hominids the Leakeys, their son Richard and their associates would find in East Africa, strengthening the case that hominids indeed originated in Africa. Their work inspired American and European researchers to sweep through the Great Rift Valley, a geologic fault that runs through Kenya, Tanzania and Ethiopia and exposes rock layers that are millions of years old.

In 1974, paleoanthropologists Donald Johanson and Tom Gray, digging in Hadar, Ethiopia, found the partial skeleton of the earliest known hominid at the time—a female they called Lucy, after the Beatles’ song “Lucy in the Sky with Diamonds,” which was playing in camp as they celebrated. At 3.2 million years old, Lucy was remarkably primitive, with a brain and body about the size of a chimpanzee’s. But her ankle, knee and pelvis showed that she walked upright like us.

This meant Lucy was a hominid—only humans and our close relatives in the human family habitually walk upright on the ground. A member of the species Australopithecus afarensis, which lived from 3.9 million to 2.9 million years ago, Lucy helped answer some key questions. She confirmed that upright walking evolved long before hominids began using stone tools—about 2.6 million years ago—and before their brains began to expand dramatically. But her upright posture and gait raised new questions. How long had it taken to evolve the anatomy to balance on two feet? What prompted some ancient ape to stand up and begin walking down the path toward humanness? And what kind of ape was it?

Lucy, of course, couldn’t answer those questions. But what came before her? For 20 years after her discovery, it was as if the earliest chapter of the human story were missing.

One of the first teams to search for lucy’s ancestor was the Middle Awash project, which formed in 1981 when White and Asfaw joined Berkeley archaeologist J. Desmond Clark to search for fossils and stone tools in Ethiopia. They got off to a promising start—finding 3.9-million-year-old fragments of a skull and a slightly younger thighbone—but they were unable to return to the Middle Awash until 1990, because Ethiopian officials imposed a moratorium on searching for fossils while they rewrote their antiquities laws. Finally, in 1992, White’s graduate student, Gen Suwa, saw a glint in the desert near Aramis. It was the root of a tooth, a molar, and its size and shape indicated that it belonged to a hominid. Suwa and other members of the Middle Awash project soon collected other fossils, including a child’s lower jaw with a milk molar still attached. State-of-the-art dating methods indicated that they were 4.4 million years old.

The team proposed in the journal Nature in 1994 that the fossils—now known as Ardipithecus ramidus—represented the “long-sought potential root species for the Hominidae,” meaning that the fossils belonged to a new species of hominid that could have given rise to all later hominids. The idea that it was a member of the human family was based primarily on its teeth—in particular, the absence of large, dagger-like canines sharpened by the lower teeth. Living and extinct apes have such teeth, while hominids don’t. But the gold standard for being a hominid was upright walking. So was A. ramidus really a hominid or an extinct ape?

White joked at the time that he would be delighted with more fossils—in particular, a skull and thighbone. It was as if he had placed an order. Within two months, another graduate student of White’s, Ethiopian paleoanthropologist Yohannes Haile-Selassie, spotted two pieces of a bone from the palm of a hand—their first sign of Ardi. The team members eventually found 125 pieces of Ardi’s skeleton. She had been a muscular female who stood almost four feet tall but could have weighed as much as 110 pounds, with a body and brain roughly the same size as a chimpanzee’s. As they got a good look at Ardi’s body plan, they soon realized that they were looking at an entirely new type of hominid.

It was the find of a lifetime. But they were daunted by Ardi’s condition. Her bones were so brittle that they crumbled when touched. White called them “road kill.”

The researchers spent three field seasons digging out entire blocks of sedimentary rock surrounding the fossils, encasing the blocks in plaster and driving them to the National Museum of Ethiopia in Addis Ababa. In the museum lab, White painstakingly injected glue from syringes into each fragment and then used dental tools and brushes, often under a microscope, to remove the silty clay from the glue-hardened fossils. Meanwhile, Suwa, today a paleoanthropologist at the University of Tokyo, analyzed key fossils with modified CT scanners to see what was inside them and used computer imaging to digitally restore the crushed skull. Finally, he and anatomist C. Owen Lovejoy worked from the fossils and the computer images to make physical models of the skull and pelvis.

It’s a measure of the particularity, complexity and thoroughness of the researchers’ efforts to understand Ardi in depth that they took 15 years to publish their detailed findings, which appeared this past October in a series of 11 papers in the journal Science. In short, they wrote that Ardi and fossils from 35 other members of her species, all found in the Middle Awash, represented a new type of early hominid that wasn’t much like a chimpanzee, gorilla or a human. “We have seen the ancestor and it’s not a chimpanzee,” says White.

This came as a surprise to researchers who had proposed that the earliest hominids would look and act a lot like chimpanzees. They are our closest living relatives, sharing 96 percent of our DNA, and they are capable of tool use and complex social behavior. But Ardi’s discoverers proposed that chimpanzees have changed so dramatically as they have evolved over the past six million years or so, that today’s chimpanzees make poor models for the last common ancestor we shared.


Comment on this Story

comments powered by Disqus