Scientists estimate the age of the skeleton to be between 3.22 million and 3.6 million years old and said it belongs to a creature who stood about 4ft tall and who walked upright.
Palaeontologists from the University of Witwatersrand in Johannesburg found the skeleton in a cave in Sterkfontein, a former lime quarry that has yielded some of the most important fossil finds in South Africa. The hominid belongs to the group of early human ancestors known as the Australopithecines. It is the oldest and most complete fossil of its kind and affirms Africa as the cradle of mankind.
Philip Tobias, a professor emeritus at the university who has been in charge of the site's excavation for the past 30 years, said yesterday that the details of the hominid's anatomy are quite stunning.
"The heal of the foot is human-like and is suitable for walking on two legs. The big toe is more like that of a chimp, more like a thumb, so that the foot could grasp," he said.
"The canine teeth are small like a human's and not the elongated fangs of chimps and gorillas. It has a delicately constructed face with a protruding upper jaw and a retreating chin."
Ron Clarke, the palaeontologist at Witwatersrand who made the discovery with his two colleagues, Stephen Motsumi and Nkwane Molefe, said the skeleton was older and more complete than the famous "Lucy", an Australopithecene found in 1974 in East Africa.
Professor Tobias said the hominid probably fell down a shaft into the cave.
The skeleton is still embedded in the rock but scientists have revealed enough of the fossil to realise that it is "probably the most momentous find ever made in Africa", Professor Tobias said.
Dr Clarke found four footbones belonging to the skeleton in 1994 and this led him to search the cave for the rest of the skeleton. "It was like searching for a needle in a haystack," Professor Tobias said.
Chris Stringer, head of human origins at the Natural History Museum in London, said: "We need to get it completely out of the ground to assess its value."