Acting on a tip from spelunkers two years ago, scientists in South Africa discovered what the cavers had only dimly glimpsed through a crack in a limestone wall deep in the Rising Star Cave: lots and lots of old bones.
The remains covered the earthen floor beyond the narrow opening. This was, the scientists concluded, a large, dark chamber for the dead of a previously unidentified species of the early human lineage — Homo naledi.
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The new hominin species was announced on Thursday by an international team of more than 60 scientists led by Lee R. Berger, an American paleoanthropologist who is a professor of human evolution studies at the University of the Witwatersrand in Johannesburg. The species name, H. naledi, refers to the cave where the bones lay undisturbed for so long; “naledi” means “star” in the local Sesotho language.
In two papers published this week in the open-access journal eLife, the researchers said that the more than 1,550 fossil elements documenting the discovery constituted the largest sample for any hominin species in a single African site, and one of the largest anywhere in the world. Further, the scientists said, that sample is probably a small fraction of the fossils yet to be recovered from the chamber. So far the team has recovered parts of at least 15 individuals.
“With almost every bone in the body represented multiple times, Homo naledi is already practically the best-known fossil member of our lineage,” Dr. Berger said.
The finding, like so many others in science, was the result of pure luck followed by considerable effort.
Two local cavers, Rick Hunter and Steven Tucker, found the narrow entrance to the chamber, measuring no more than seven and a half inches wide. They were skinny enough to squeeze through, and in the light of their headlamps they saw the bones all around them. When they showed the fossil pictures to Pedro Boshoff, a caver who is also a geologist, he alerted Dr. Berger, who organized an investigation.
Just getting into the chamber and bringing out samples proved to be a huge challenge. The narrow opening was the only way in.
Paul Dirks, a geologist at James Cook University in Australia, who was lead author of the journal paper describing the chamber, said the investigators first had a steep climb up a stone block called the Dragon’s Back and then a drop down to the entrance passage — all of this in the total absence of natural light.
For the two extended investigations of the chamber in 2013 and 2014, Dr. Berger rounded up the international team of scientists and then recruited six excavating scientists through notices on social media. One special requirement: They had to be slender enough to crawl through that crack in the wall.
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One of the six, who were all women and were called “underground astronauts,” was Marina Elliott of Simon Fraser University in British Columbia. She said the collection and removal of the fossils involved “some of the most difficult and dangerous conditions ever encountered in the search for human origins.”
Besides introducing a new member of the prehuman family, the discovery suggests that some early hominins intentionally deposited bodies of their dead in a remote and largely inaccessible cave chamber, a behavior previously considered limited to modern humans. Some of the scientists referred to the practice as a ritualized treatment of their dead, but by “ritual” they said they meant a deliberate and repeated practice, not necessarily a kind of religious rite.
“It’s very, very fascinating,” said Ian Tattersall, an authority on human evolution at the American Museum of Natural History in New York, who was not involved in the research.
“No question there’s at least one new species here,” he added, “but there may be debate over the Homo designation, though the species is quite different from anything else we have seen.”
A colleague of Dr. Tattersall’s at the museum, Eric Delson, who is a professor at Lehman College of the City University of New York, was also impressed, saying, “Berger does it again!”
Dr. Delson was referring to Dr. Berger’s previous headline discovery, published in 2010, also involving cave deposits near Johannesburg. He found many fewer fossils that time, but enough to conclude that he was looking at a new species, which he named Australopithecus sediba. Geologists said the individuals lived 1.78 million to 1.95 million years ago, when australopithecines and early species of Homo were contemporaries.
Researchers analyzing the H. naledi fossils have not yet nailed down their age, which is difficult to measure because of the muddled chamber sediments and the absence of other fauna remains nearby. Some of its primitive anatomy, like a brain no larger than an average orange, Dr. Berger said, indicated that the species evolved near or at the root of the Homo genus, meaning it must be in excess of 2.5 million to 2.8 million years old. Geologists think the cave is no older than three million years.
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The field work and two years of analysis for Dr. Berger’s latest discovery were supported by the University of the Witwatersrand, the National Geographic Society and the South African Department of Science and Technology/National Research Foundation. In addition to the journal articles, the findings will be featured in the October issue of National Geographic Magazine and in a two-hour NOVA/National Geographic documentary to air Wednesday on PBS.
Scientists on the discovery team and those not involved in the research noted the mosaic of contrasting anatomical features, including more modern-looking jaws and teeth and feet, that warrant the hominin’s placement as a species in the genus Homo, not Australopithecus, the genus that includes the famous Lucy species that lived 3.2 million years ago. The hands of the newly discovered specimens reminded some scientists of the earliest previously identified specimens of Homo habilis, who were apparently among the first toolmakers.
At a news conference on Wednesday, John Hawks of the University of Wisconsin, Madison, a senior author of the paper describing the new species, said it was “unlike any other species seen before,” noting that a small skull with a brain one-third the size of modern human braincases was perched atop a very slender body. An average H. naledi was about five feet tall and weighed almost 100 pounds, he said.