The find reveals that our forebears underwent a previously unknown stage of evolution more than a million years before Lucy, the iconic early human ancestor specimen that walked the Earth 3.2 million years ago. Scientists today announced the discovery of the oldest fossil skeleton of a human ancestor.The centerpiece of a treasure trove of new fossils, the skeleton—assigned to a species called Ardipithecus ramidus—belonged to a small-brained, 110-pound (50-kilogram) female nicknamed "Ardi." (See pictures of Ardipithecus ramidus.) The fossil puts to rest the notion, popular since Darwin's time, that a chimpanzee-like missing link—resembling something between humans and today's apes—would eventually be found at the root of the human family tree.Indeed, the new evidence suggests that the study of chimpanzee anatomy and behavior—long used to infer the nature of the earliest human ancestors—is largely irrelevant to understanding our beginnings.One way that helps scientists place fossils into the correct era on the Geologic Time Scale is by using radiometric dating.Also called absolute dating, scientists use the decay of radioactive elements within the fossils or the rocks around the fossils to determine the age of the organism that was preserved.
The fossil record may be incomplete and may never fully completed, but there are still many clues to evolution and how it happens within the fossil record.
Radiometric dating of two layers of volcanic ash that tightly sandwiched the fossil deposits revealed that Ardi lived 4.4 million years ago.
Older hominid fossils have been uncovered, including a skull from Chad at least six million years old and some more fragmentary, slightly younger remains from Kenya and nearby in the Middle Awash.
While important, however, none of those earlier fossils are nearly as revealing as the newly announced remains, which in addition to Ardi's partial skeleton include bones representing at least 36 other individuals.
"All of a sudden you've got fingers and toes and arms and legs and heads and teeth," said Tim White of the University of California, Berkeley, who co-directed the work with Berhane Asfaw, a paleoanthropologist and former director of the National Museum of Ethiopia, and Giday Wolde Gabriel, a geologist at Los Alamos National Laboratory in New Mexico.