U.S.-led scientists say they've discovered a 3.6-million-year-old partial skeleton in Ethiopia, making it 400,000 years older than the famous "Lucy" skeleton.

Scientists from The Cleveland Museum of Natural History, Kent State University, Case Western Reserve University, Ethiopia's Addis Ababa University and the Berkeley Geochronology Center said the find suggests advanced human-like, upright walking occurred much earlier than previously thought.

The researchers, led by Yohannes Haile-Selassie, curator of physical anthropology at the Cleveland museum, said the partial skeleton belongs to "Lucy's" species -- Australopithecus afarensis. It was excavated during a five-year period following the discovery of a fragment of its lower arm bone in 2005.

The specimen was nicknamed "Kadanuumuu" (kah-dah-nuu-muu) by the researchers. "Kadanuumuu" means "big man" in the Afar language and reflects its large size, the scientists said, noting the male hominid stood between 5 and 5 1/2 feet tall, while "Lucy" stood at about 3 1/2 feet.

"This individual was fully bipedal and had the ability to walk almost like modern humans," said Haile-Selassie. "As a result of this discovery, we can now confidently say 'Lucy' and her relatives were almost as proficient as we are walking on two legs, and the elongation of our legs came earlier in our evolution than previously thought," Haile-Selassie said.

The study appears in the early online edition of the Proceedings of the National Academy of Sciences.