The research team, working in the north-east of Ethiopia, believe the remains of the hominid, or primitive human, date back four million years.
They say initial study of the bones indicates the creature was bipedal - it walked around on two legs.
The fossils were found just 60km (40 miles) from the site where the famous hominid Lucy was discovered.
Lucy (Australopithecus afarensis), whose remains were unearthed in 1974, lived 3.2 million years ago and is thought to have given rise to the Homo line that ended in modern humans.
The as yet unnamed fossil creature, found in February  at a new site called Mille in the Afar region of Ethiopia, looks to be even older than Lucy.
The remains include a complete tibia from the lower part of the leg, parts of the thighbone or femur, ribs, vertebrae, a collarbone, pelvis and a complete shoulder blade.
"The discovery of 12 early hominid fossil specimens estimated to be between 3.8 and 4 million years old will be important in terms of understanding the early phases of human evolution before Lucy," Yohannes Haile Selassie told a news conference.
Researchers are often happy to find isolated bones belonging to human ancestors of this age, so to find a partial skeleton is exceptional.
The team that found it says the discovery is also significant because, due to the structure of the ankle bone, the individual almost certainly walked upright like modern people.
The find, one of a series of hominid fossils which are still being unearthed, held many mysteries, said Bruce Latimer, of the Cleveland Museum of Natural History, US, who made the discovery with his Ethiopian colleague.
"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 said.
It is currently too early to say what sex or species the creature was, say the researchers.
"We have a pelvis which can tell us whether it was male or female. But the whole pelvis is embedded in a rock matrix. That's going to take a lot of time to clean up," Dr Haile Selassie told the BBC News website.
The team had to return home because they were nearing the end of their field season, leaving the excavation unfinished. But there are plans to return to the discovery site in December.
"If you want to look at the sex, stature and what species it is, you have to have all the elements that can be retrieved from the excavation," the Ethiopian researcher added.
The discovery of the remains of at least nine primitive hominids of similar age to the latest find was announced in January.
Those fossils, which were uncovered at As Duma in the north of Ethiopia, were mostly teeth and jaw fragments, but also include parts of hands and feet.
Bipedalism is a crucial aspect of the human form, palaeoanthropologists believe - but there is a great deal of debate about when exactly this ability first arose in our lineage.
There is some evidence that much older hominids could walk upright.
Foot bones from the species Ardipithecus ramidus and Ardipithecus kadabba have features diagnostic of bipedalism.
And recent computed tomography (CT) scans of the thighbone of a six-million-year-old Kenyan creature known as Orrorin tugenensis suggest it might have had quite a human gait.
And a seven-million-year-old hominid from Chad, known as Sahelanthropus tchadensis and nicknamed Toumaï, may also have been bipedal. The assessment is based on an analysis of where the animal's spine would have entered the skull and the position of muscle attachments on its head.
May 27, 2005
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