A 4.4-million-year-old hominin is shaking up our understanding of human evolution this morning. The first bits of the new species, called Ardipithecus ramidus, were discovered in 1994, and now (it took a while), scientists are publishing an exhaustive analysis of the hominin and the habitat in which it lived. The scientists, working in Ethiopia, found 36 individuals, including one that preserves some of the most important features for studying the evolution of human traits.
In addition to 11 scientific papers, Science is publishing a news account by Ann Gibbons, who visited the Ethiopian field camp and writes about what it took to find these fossils and make sense of them. (One piece of her story is subtitled: "How do you find priceless hominin fossils in a hostile desert? Build a strong team and obsess over the details.")
This remarkably rare skeleton is not the oldest putative hominin, but it is by far the most complete of the earliest specimens. It includes most of the skull and teeth, as well as the pelvis, hands, and feet—parts that the authors say reveal an “intermediate” form of upright walking, considered a hallmark of hominins. “We thought Lucy was the find of the century but, in retrospect, it isn’t,” says paleoanthropologist Andrew Hill of Yale University. “It’s worth the wait.”
Ardipithecus ramidus lived more than a million years before Lucy, an Australopithecus fossil that until now was our best source of information about how humans evolved from a shared ancestor with chimps about 7 million years ago. The new fossil shows that human ancestors--even relatively shortly after this evolutionary split--were much less chimp-like than people thought. The new species walked upright, although its feet had opposable big toes that were.
Good for gripping as it climbed trees. It wasn't a knuckle-dragger. Males and females were about the same size (50 kilograms). They were agile climbers. Perhaps most intriguingly, neither males nor females have the dagger-like teeth that chimps use to fight one another. Their stubby teeth suggest that they were social and cooperative. Many of the characteristics of chimps and gorillas that people thought might have been shared by early hominins instead must have evolved in the great apes after the split with our ancestors.
"What Ardipithecus tells us is that we as humans have been evolving toward what we are today for at least 6 million years," said Owen Lovejoy of Kent State in Ohio during a press conference this morning. "It was one of the most revealing hominid fossils I could ever have imagined."
The scientific analyses of the fossil and news stories about its discovery are available on Science's website.