Paleontologists working in Ethiopia have discovered the remains of at least nine primitive human ancestors that are up to 4.5 million years old.
The specimens belong to a hominid species called Ardipithicus ramidus, a transitional creature with significant ape characteristics. The fossils are mostly teeth and jaw fragments, with some hands and feet bones, according to nine researchers from universities in the United States and Spain.
The discoveries were made over a four-year span beginning in 1999 in digs at the As Duma site in Ethiopia's Afar region, which has yielded many important fossils. The details appear in the latest issue of the journal Nature.
Among the specimens, the recovered canine teeth are smaller and blunt, similar to those of other human ancestors. But most of its teeth, including molars, are like those of great apes. The size and wear of the teeth suggest A. ramidus ate a plant-based diet, the researchers reported.
Geological and radiocarbon tests show the specimens are between 4.3 million and 4.5 million years old.
Scientists know little about A. ramidus. A few skeletal fragments suggest it was even smaller than Australopithecus afarensis, the 3.6 million-year-old species widely known by the nearly complete "Lucy" fossil that measures about four feet tall.
Evidence from other A. ramidus specimens shows its skull rested directly atop its spinal column, rather than in front like apes. This suggests it could walk upright, or had bipedal abilities.
Other fossils found at the As Duma site show that A. ramidus lived alongside monkeys, mole rats and cow-like grazing animals. But details of the environment are sketchy.
Originally, scientists theorized that the earliest human ancestors lived on the savannah and began walking upright to see across the open landscape. But pollen and other evidence from As Duma suggest the diverse habitat had swamps, grass and even some woods.
The first A. ramidus fossils were reported in 1994. With the nine additional specimens, labs now have fragments from as many as 60 individuals