An international team of scientists reported on Thursday that the skeleton of an early human who lived 4.4 million years ago shows humans did not evolve from chimpanzee-like ancestors.
The 17-year investigation into the discovery of the extremely fragile remains of the small “ground ape” found in the Afar region of Ethiopia will be described Friday in a special issue of the journal Science. The journal will also contain 11 papers about the discovery.
The fossil, nicknamed “Ardi,” is the earliest skeleton known from the human branch of the primate family tree. The branch includes Homosapiens as well as species closer to humans than to chimpanzees and bonobos. The discoveries provide new insights about how hominids - the family of “great apes” comprising humans, chimpanzees, gorillas and orangutans - may have emerged from an ancestral ape.
Until the discovery of Ardi, the earliest well-known stage of human evolution was Australopithecus, the small-brained, fully bipedal “ape man” that lived between four million and one million years ago. The most famous Australopithecus fossil is the 3.2-million-year-old “Lucy,” which was named after the Beatles song “Lucy in the Sky with Diamonds.” Lucy was found in 1974 about 45 miles north of where Ardi would later be discovered. Ardi’s skeleton and associated Ardipithecus ramidus remains are older and more primitive than Australopithecus.
After Lucy’s discovery, there was some expectation that when earlier hominid remains were found, they would converge to a chimpanzee-like anatomy, based on the genetic similarity of humans and chimps. The Ardipithecus ramidus fossils do not, however, corroborate this expectation.
Ardi’s skeleton contains enough of the skull, teeth, pelvis, legs, feet, arms, and hands to estimate her body weight and height; that she walked on two legs on the ground, but climbed trees and spent time in them as well; and that she probably was omnivorous. What may come as a surprise is that Ardi and her companions did not have limb proportions like chimps or gorillas, but rather like those of extinct apes or even monkeys, and that her hands are also not chimpanzee- or gorilla-like, but more closely related to earlier extinct apes.
Scientists said the findings suggest that hominids and African apes have each followed different evolutionary paths, and that we can no longer consider chimps as “proxies” for our last common ancestor.
“Darwin was very wise on this matter,” Tim White from the University of California Berkeley, who helped lead the research team, observed.
“Darwin said we have to be really careful. The only way we’re really going to know what this last common ancestor looked like is to go and find it. Well, at 4.4 million years ago we found something pretty close to it. And, just like Darwin appreciated, evolution of the ape lineages and the human lineage has been going on independently since the time those lines split, since that last common ancestor we shared,” Mr. White said.