By Beatrice Debut 

Maropeng (South Africa) (AFP) - The fossilised bones of 15 members of a previously unknown branch of the human family tree have been discovered in a cave in South Africa, scientists said Thursday, hailing the find as a breakthrough in evolution research.

About 1,500 fossils were found deep in a cave system outside Johannesburg, hidden in a chamber only accessible via several steep climbs and narrow rock crevasses.

The hominid -- described as a "new species" of human -- has been named Homo naledi after the "Rising Star" cave where the bones were found. Naledi means "star" in Sesotho, a local South African language.

Experts are uncertain how old the bones are, but say they were probably placed there after death -- a discovery that shines light on ancient human rituals.

"We have just met a new species of human relative that deliberately disposed of its dead," Lee Berger, project leader and palaeoanthropologist at Johannesburg's University of the Witwatersrand, announced as the fossils were unveiled.

"Until this moment in history we thought the idea of ritualised behaviours directed towards the dead... was actually unique to Homo sapiens.

"We saw ourselves as different. We have now seen, we believe, a species that had that same capability -- and it is an extraordinary thing."

The bones were first discovered in 2013 by Witwatersrand University scientists and volunteer cavers in the Cradle of Humankind, a UNESCO World Heritage Site, 50 kilometres (30 miles) northwest of Johannesburg.

- 'Tool-using capabilities' -

Ancient human remains have been found in the area since excavations begun in the 1920s.

"The discovery of so many fossils belonging to at least 15 individuals is remarkable," said Professor Chris Stringer, from the Natural History Museum in London, one of the lead analysts on the discovery.

The find highlighted "the complexity of the human family tree and the need for further research to understand the history and ultimate origins of our species," he added.

Scientists say the hands, wrists and feet of the bodies were similar to modern humans, but the brain size and upper body were much more like the earliest humans.

"Homo naledi had a tiny brain, about the size of an average orange, perched atop a very slender body," said John Hawks, of the University of Wisconsin-Madison, a senior author on the academic paper detailing the new species.

The hominid stood approximately 1.5 metres (about 5 feet) tall and weighed about 45 kilos (almost 100 pounds).

"The hands suggest tool-using capabilities," said Tracy Kivell of the University of Kent, in Britain, who was part of the team that studied Homo naledi's anatomy.

"Surprisingly, Homo naledi has extremely curved fingers, more curved than almost any other species of early hominin, which clearly demonstrates climbing capabilities," she said.

The first expedition to the cave chamber in 2013 lasted 21 days and involved more than 60 specialist cavers and scientists working in dangerous conditions, squeezing through tiny gaps in the rock.

Since then scientists have been studying the bones, which are from infants, children, adults and elderly individuals, before revealing their conclusions.

"This chamber has not given up all of its secrets," Berger, an American from Kansas, added.

"There are potentially hundreds if not thousands of remains of Homo naledi still down there."

Tracing the human odyssey is one of the thorniest areas of anthropology, and experts can disagree fiercely over whether a find merits to be called a new species.

Many of the fossils will be exhibited at the Cradle of Humankind until October 11 when they will return to the University of the Witwatersrand before a decision is made on a proposed world tour.

"Today will be written into the history books as one of those moments in which the world learnt something new and remarkable," South African deputy president Cyril Ramaphosa told the audience at the unveiling.

"We are delighted that discoveries that we would never have imagined have been found here at the southern tip of the African continent.

"Despite our individual differences in appearance, language, beliefs and cultural practices, we are bound together by a common ancestry."