Two skeletons uncovered which may be previously undiscovered species of pre-human

• A skull from Australopithecus sediba, which lived nearly two million years ago. Picture: Getty Images

Two partial skeletons unearthed in a South African cave may belong to a previously unclassified species of pre-human, scientists revealed yesterday.

In a discovery that could rewrite the history of human evolution, the remains of a male aged eight or nine and a female in her late 20s or early 30s suggest the species walked upright and shared many physical traits with the earliest documented Homo species.

Details of the pre-human, or hominid, fossils have been published in the latest edition of the journal Science and may answer some key questions about where humans came from.

Professor Lee Berger, a paleoanthropologist at the University of the Witwatersrand, Johannesburg, who led the team that found the fossils in August 2008, said the they were hoping to reveal a possible two further skeletons from the same site.

The remarkable discovery was made by Prof Berger's nine-year-old son, Matthew, who found the fossil after tripping over a log while chasing his dog, Tau.

His 44-year-old father at first thought he had stumbled across the remains of an ancient antelope but recalled yesterday how he noticed a hominid lower jaw jutting out the fossil: "I almost died right there. I mean, what are the odds?"

Dated to between 1.78 and 1.95 million years old, the skeletons make Scotland's oldest human settlement – a camp used by hunters near Biggar, Lanarkshire, about 14,000 years ago – seem positively modern.

While reluctant to define the new species as a "missing link" in human evolutionary history, Prof Berger 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." He added: "Australopithecus sediba (which means 'southern ape, wellspring'] is undoubtedly a highly transitional species with a mosaic of characteristics that are shared by later hominids … in the line of the genus homo, as well as features that are shared by earlier hominids."

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. The new species is about one million years younger than Lucy, scientists believe.

• The species has been named after the Sediba area near Johannesburg where the remains were found

The oldest known hominid is "Ardi" – a female member of the Ardipithecus ramidus species that lived 4.4 million years ago.

The new skeletons, which may be those of a mother and son, were found in the Malapa caves in the Cradle of Humankind World Heritage Site, 25 miles 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 scientists estimate both hominids were about 4ft 2in tall, although the child would not have reached its full height.

Prof Berger added: "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."

Some scientists have cast doubt on the importance of the discoveries. Dr Darren Curnoe, a specialist in human evolution at the University of New South Wales, said the significance of the discovery had been "surrounded by hype and overinterpretation". He said: "It is way too primitive to be the ancestor of the human genus Homo."