They believe the two-million-year-old fossilised skeleton of a child, found in South Africa, is that of an entirely new species and an intermediate stage between our ape-like ancestors and modern man.
And they claim it could help us crack one of the great mysteries of our evolutionary tree - exactly when humans began to walk on two feet.
While most finds are little more than scattered fragments of bone or teeth, the latest discovery is an almost complete skeleton.
The bones were found in the Malapa Cave in the Sterkfontein region of South Africa by Professor Lee Berger, of Witwatersrand University in Johannesburg.
The caves were made a Unesco world heritage site in 1994 after an almost complete fossil of a 3.3million-year-old Australopithecus was found there.
Professor Phillip Tobias, an anthropologist at the university, said that the discovery was a 'wonderful and exciting' find. 'To find a skeleton as opposed to a couple of teeth or an arm bone is a rarity,' he said.
'It is one thing to find a lower jaw with a couple of teeth, but it is another thing to find the jaw joined on to the skull, and those in turn uniting further down with the spinal column, pelvis and the limb bones.'
The discovery, along with a number of other partially complete fossils, could now help scientists understand how our ancestors evolved from the apelike Australopithecus, which emerged in Africa around 3.9million years ago, to Homo habilis, the first human-type species which appeared around 2.5million years ago.
The skeleton, which will be unveiled on Thursday and has already been visited by the country's president Jacob Zuma, is believed to be an evolutionary intermediate between these two species.
It includes a pelvis and whole limbs which could reveal whether the new species walked upright or on all fours.
Hand bones could also say how dextrous the species was and give researchers the first clue as to when humans' ability to hold stone tools first emerged.
Dr Simon Underdown, an expert in evolution from Oxford Brookes University, said: 'A find like this could really increase our understanding of our early ancestors.'
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