THE two amateur cavers had to feel their way along the cave's winding passages, crawl on their stomachs through an opening less than 10 inches high, ascend a jagged wall, cross a narrow ledge dubbed the "Dragon's Back," and make a 400-foot descent, sideways, through a vertical crack before finally arriving at the prize: a 30-foot-long chamber probably between 2 million and 3 million years old.

American paleoanthropologist Lee Berger had asked the men to keep their eyes open for fossils, though the well-explored cave at the Cradle of Humankind World Heritage Site had given up most of its treasures decades ago.

What they found in September 2013 nearly took their breath away: fossil fragments of a relative of the human species, and a cache of bones and teeth buried in ancient clay that would eventually number more than 1,500 — the largest hominin fossil discovery of its kind in Africa.

After a month of excavation under some of the most difficult and dangerous of conditions, then two years of analysis by more than 50 international experts, Berger and the leaders of the expedition announced Thursday that those fossil fragments do indeed belong to a new species of human relative they are calling Homo naledi.

"It was soon apparent that what I thought was an individual skeleton was dozens of individuals," Berger, a researcher in human evolution at the University of the Witwatersrand in Johannesburg, said during a Wednesday teleconference for the media hosted by the National Geographic Society, which helped support the expedition. "With every bone in the body represented multiple times, it is already practically the best-known fossil member of our lineage."

The announcement by the university, National Geographic and the South African Department of Science and Technology/National Research Foundation, was made in Johannesburg, about 25 miles northwest of the cave complex, called the Rising Star, where the fossils were found. ("Naledi" means "star" in Sesotho, a local South African language, and the chamber where the fossils were unearthed was called Dinaledi, or "many stars.")

Although the fragments have not yet been dated, the scientists said they could well represent one of the most primitive members of the genus Homo, which includes today's humans.

Perhaps more remarkable, they added, was that the pieces belonged to at least 15 individuals of the same species — men, women, children and infants — and all of them appeared to have been deliberately placed there after death.

The implication was as astonishing as the initial discovery: It suggested the ritualized disposal of bodies.

"It's enormously surprising to see a very primitive member of the genus, not very humanlike overall, to do something unique to humans," Berger said.

"To see it in a small-brained hominid is completely surprising. None of us expected it."

H. naledi is an unusual combination of the primitive and the modern, the scientists said. Its brain was no larger than a baseball; its shoulders and torso primitive; its fingers long and curved, allowing H. naledi to climb and swing from the trees.

At the same time, H. naledi's wrist bones indicated that it used tools. Its long legs and feet, nearly indistinguishable from those of modern man, allowed it not only to walk upright but also to travel for many miles at a time.

"One of the most exciting things for us is we discovered something new in biology. We had never seen a creature like this before," said John Hawks, an anthropologist at the University of Wisconsin and the lead scientist in the analysis, which included experts in primitive feet, teeth, hands and skulls.

"H. naledi is unlike anything in our genus. . . . When you look at the anatomical elements across the body, it's an enormous assemblage of fossils. The task was to interpret these fossils and put them in the context of evolution and where they fit on the human tree."

The unprecedented number of fragments found made the scientists feel confident that they had found a new species of human relative.

"We have 190 teeth, and they are represented multiple times," Hawks said. "We have more than a dozen molars, and the differences are typically less variable than in small populations of humans.

Every member of every team . . . agreed we were looking at the same species. The hand may be telling us a different story than the shoulder, but it would beg belief that we mixed things" belonging to different species.

The fossils were recovered over 21 days in November 2013 and seven days in March 2014 by a team assembled by Berger and his colleagues through social media.

The archaeologists thought the only way to mount a major excavation was to have a team of people slender enough, but also experienced enough, to handle the hazardous work.

The six who were eventually chosen were all women, including American University PhD candidate Becca Peixotto, who took part in the Wednesday teleconference from Washington.

"I saw an ad on Facebook," Peixotto said, and she immediately signed on.

"We had a lot of spectacular moments, and a few stand out: a fragment of a skull from the center of chamber.

"It took days to excavate, and the removal of this fossil was complicated by overlying fossils. Late one evening, it was finally free from the soil and packaged in a box big enough to hold the fragment and small enough to fit [through the narrow cracks of the cave].

"Then it was all hands on deck. . . . We formed a bucket brigade to pass the skull box up the chute, out the slot, down the Dragon's Back . . . through the crawl, up another ladder, out to the surface. . . . There was huge cheering as it reached the light of day."

The National Geographic Society, which has funded many of Berger's expeditions, is featuring the discovery of H. naledi on the cover of its magazine next month and in Dawn of Humanity," a special by "Nova" and National Geographic that premieres Sept. 16 and begins streaming online Thursday.

Two papers about the discovery also will be published in the open-access scientific journal eLife.

The age of the fossils will be difficult to determine, Berger says, because they were not fused into rock, which can be dated, and they wanted to wait to do radiocarbon dating until they knew more about what they had.

What he did say, with confidence, was that H. naledi "comes near or at the root of the genus Homo," in excess of 2.5 million years ago.

"This is an anatomical mosaic that evolutionary history gave us," Hawks said. And at the very least, until more can be determined, "it gives us a different model for how things could fit in our own origins."

In the meantime, Berger said, the team plans on further expeditions and excavations.

"This chamber has not given up all its secrets."