"This is the world's oldest biped," Bruce Latimer, director of the natural history museum in Cleveland, Ohio, said in the Ethiopian capital Addis Ababa on Saturday.
"It will revolutionise the way we see human evolution."
The bones were found three weeks ago in Ethiopia's Afar region, at a site about 60km from Hadar where Lucy, one of the first hominids, was discovered in 1974.
The Leakey Foundation, which funded the team who found Lucy, dates her 40% intact skeleton back 2.8 million years, but other paleontological sources have said she may be as old as 3.2 million years.
Latimer and his Ethiopian colleague, D Yohannes Haile-Selassie, said the newly discovered skeleton had been determined to be capable of walking upright on two feet because of the nature of the ankle bone.
"I couldn't explain in detail how it walked yet," Latimer said, "but looking at the ankle we know it is a biped."
"Normally, you find one bone or two from an individual and you are happy. Now we have found parts of a skeleton, this is very rare"
Cleveland Natural History Museum
This was the "revolutionary" aspect of the discovery, the scientists said, in that it would help them learn how species like those from which modern mankind, homo sapiens, descended first learned to walk on two feet.
"This skeleton helps us to understand what happened in the joints, how walking upright occurred, what we never had before," Latimer said.
Researchers at the site in northeast Ethiopia have in all unearthed 12 hominid fossils, of which parts of one skeleton were discovered.
"Portions recovered thus far include a complete tibia, parts of a femur, ribs, vertebrae, a clavicle, pelvis, and a complete scapula of an adult," Latimer said.
"Normally, you find one bone or two from an individual and you are happy. Now we have found parts of a skeleton, this is very rare," he explained. "It says a lot more on the individual than isolated bones."
"It is already clear that the individual was larger than Lucy, it has longer legs than Lucy ... but it is older which is strange," he added.
Haile Selassie, a paleontologist from the national museum in Addis Ababa, said: "We have hundreds of pieces that have to be reconstructed and we haven't finished excavating."
The skeleton was the fourth ancient hominid to be found since Lucy, with others discovered in Ethiopia and in South Africa.
The researchers have yet to determine the species and sex of the latest discovery.