Fossils of the bones of a young male and an adult female suggest the newly documented species, called Australopithecus sediba, walked upright and shared many physical traits with the earliest known human Homo species.
The finding of the pre-human, or hominid, fossils - which scientists say are between 1.78 and 1.95 million years old - was published in the journal Science and may answer some key questions about where humans came from.
Lee Berger of the University of the Witwatersrand, Johannesburg, who led the team that found the fossils in August 2008, said the team were hoping to reveal a possible two further skeletons from the same site.
He was reluctant to define the new species as a "missing link" in human evolutionary history, but said it would "contribute enormously to our understanding of what was going on at that moment where the early members of the genus Homo emerged".
"Australopithecus sediba is undoubtedly a highly transitional species with a mosaic of characters that are shared by later hominids... in the line of the genus homo, as well as features that are shared by earlier hominids," he told reporters in a telephone briefing.
Many experts believe the human genus Homo evolved from the Australopithecus genus about two million years ago. One of the best-known pre-humans is "Lucy", the skeleton of a species called Australopithecus afarensis, and this new species is about one million years younger than "Lucy", the scientists said.
The fossils, a juvenile male and an adult female, were found in the Malapa caves in the "Cradle of Humankind" World Heritage Site, 40km outside Johannesburg.
The species had long arms, like an ape, short powerful hands, a very advanced pelvis and long legs capable of striding and possibly running like a human, the researchers said.
The scientists estimate both hominids were about 1.27m, although the child would have grown taller.
The brain size of the younger one was probably between 420 and 450 cubic centimetres, which is small when compared with the human brain of about 1 200 to 1 600 cubic centimetres, they said.
"These fossils give us an extraordinarily detailed look into a new chapter of human evolution... when hominids made the committed change from dependency on life in the trees to life on the ground," said Berger.
Paul Dirks of James Cook University in Australia, who also worked on the study, said he and a team of researchers from around the world identified the fossils of at least 25 other species of animals in the cave, including sabre-toothed cats, a wildcat, a brown hyena, a wild dog, antelopes and a horse.