Fossils of a newly discovered extinct human species have emerged from deep in a South African cave, paleontologists reported on Thursday, describing a prehistoric pit intentionally filled with thousands of bones.
Discovered inside South Africa's Rising Star cave, these newly discovered human cousins have been dubbed Homo naledi (nuh-LEE-dee), meaning "Star Man" in the local Sotho language, by the discovery team led by paleontologist Lee Berger of South Africa's University of the Witwatersrand.
Berger formally announced the discovery on Thursday morning in a South African government ceremony.
"This is the first time we've found human fossils alone in a chamber like this in Africa," Berger said on a press call Wednesday. Caving expeditions in 2013 and 2014 recovered some 1,550 fossils from a small stone chamber roughly 100 feet underground, he said.
The chamber pit — located at the bottom of a long, skinny cave chute only 7.5 inches wide in places — yielded the bony remains of perhaps 15 individuals, from infants to adults, described in two papers in the journal eLife.
"In a word, they are weird," paleoanthropologist Bernard Wood of George Washington University, who was not involved in the discovery, told BuzzFeed News.
Wood noted the extinct species sported a mixture of features that look like modern human ones, such as small teeth and flattish feet, and more ancient characteristics including an apelike pelvis and brow ridge.
This leaves researchers puzzled over where the new species fits in the human family tree, Wood said. "It's kind of a Sherlock Holmes case where they belong in human evolution."
Based on human-looking feet, Homo naledi likely walked like modern people, said John Hawks, a study team co-author at the University of Wisconsin, Madison. But they also likely climbed trees like apes, judging their from curved fingers, stout chests, and apelike shoulders, he said. The team estimates they weighed about 90 to 120 pounds as adults.
The mystery is deepened by the scientists' inability to say exactly when the newly discovered species lived. The clay coating the bottom of the discovery pit defies standard dating techniques. Based on their looks, Berger said, they could have lived as long as 2.8 million years ago to only a few tens of thousands of years ago.
Homo naledi stood less than 5 feet tall, and appears most closely related to Homo erectus ("Upright Man"), a human predecessor species that lived in Africa and Asia within the last 2 million years.
But the new species had smaller brains, roughly one-third the size of modern human ones. That's surprisingly small for creatures that negotiated a deep, dark cave and carefully deposited bodies in a pit.
Berger and his colleagues argue that the species' humanlike hands and feet raise the possibility of Homo naledi being a direct ancestor of modern humans.
But many researchers outside the discovery team told BuzzFeed News that the fossils more likely came later and add to an emerging picture of the human family "tree" as more of a "bush," filled with tangled loose ends — extinct cousins such as the Neanderthals and Homo naledi. "I would be surprised if these bones are more than a million years old," Wood said.
What's more likely, according to paleontologist Richard Potts of the Smithsonian's Natural History Museum in Washington, D.C., is that the species was an isolated population of early human cousins, who shrank in stature and brain size similar to the "hobbit" early human species found in 2003 on the Indonesian island of Flores, who lived perhaps 88,000 years ago.
Another paleontologist, Tim White of the University of California, Berkeley, told BuzzFeed News that he doubts that Homo naledi even counts as a new human species, and is more likely a small-brained population of Homo erectus, either of ancient or more recent vintage. He called the description of the chamber and fossils very preliminary, a view echoed by other researchers.
"Really the work is only beginning — we have a lot of bones here to look at and more questions than answers," paleontologist William Kimbel of Arizona State University in Tempe told BuzzFeed News. "The exact place of this species in human evolution is always going to be hard to answer until we have an age for them."
In a commentary accompanying the discovery studies in eLife, paleontologist Chris Stringer of the Natural History Museum in London expressed frustration that Berger's team hadn't tried radiocarbon dating, which would have been able to tell if the fossils were at least 50,000 years old.
In response, Berger said that radiocarbon dating would require destruction of some of the bones and was "unethical and perhaps illegal" until the description study was published. The team is now starting radiocarbon dating, said the discovery team's Paul Dirks of James Cook University in Australia, and studying new techniques of radioactive dating that might yield a better, older age.
The unearthing of the new species was accompanied by a great deal of publicity over the last two years, raising hackles among other paleontologists.
After the discovery of bones in the the pit by cavers in 2013, Berger recruited a team of six women with paleontology and caving experience on social media. They did much of the excavation and mapping of the burial chamber in two month-long excavation campaigns. Retrieval of the fossils from tight spaces by the team was widely reported by news outlets and even broadcast online.
Instead of taking years to assess the fossils and publish reports on their features in a long series of studies, White complained that the discovery team had tried to publish a dozen papers on the find. But this effort led to their rejection by the prestigious journal Nature, a diss widely discussed in paleontology.
"We simply were unable to make this large description of an assemblage of fossils 'fit' the Nature format," Berger told BuzzFeed News by email. The team and the journal editors, he said, "were both dissatisfied with the attempt to describe the largest assemblage of fossil hominins ever discovered in Africa in a 2,500-word article."
Instead, Berger and his colleagues published the paper in the journal eLife, not a typical paleontology outlet but one that published a 35-page detailing of the fossils and 37-page description of the cave.
"We have not 'rushed' these papers into print," Berger added. "They represent two years of analysis and tens of thousands of person hours of effort by our large team, something smaller teams working only on summer vacations simply cannot replicate."
The publication comes just ahead of a NOVA and National Geographic documentary on the find, Dawn of Humanity, premiering on PBS on Sept. 16 and streaming online Sept. 10. The discovery also stars on the cover of the October National Geographic magazine. (Berger received funding from the National Geographic Society as well as the South African government for his work.)
"Showmanship seems to have gotten ahead of the science," White wrote by email, criticizing the apparent timing of the studies coinciding with the documentary. "So at this point, even though the entertainment value is undeniable, and the potential scientific value is apparent, the scientific SIGNIFICANCE remains undetermined."
Berger, however, gave no ground. "We find National Geographic and PBS excellent partners in communicating to an audience outside of the narrow scientific field of palaeoanthropology," he said. He called his team, "quite frankly unapologetic about communicating the science of human origins to the public through the media of television, print and the internet and will continue to do so."
The most intriguing discovery of the excavations is the find that the bodies in the cave seem to have been lowered one by one into the pit over great length of time.
"Wow, what a mystery!" Potts said. "Who dunnit?"
Because the fossils appear to have accumulated in several layers of clay at the bottom of the pit, the team argued that they were deposited there over time, perhaps as a deliberate mass grave, rather than as a result of dying in some simultaneous catastrophe long ago.
The discovery team suggests these extinct human cousins relied on torches to repeatedly travel some 260 feet from the opening of Rising Star cave to the fossil pit and leave their dead there. Mastery of fire might also explain their small teeth, enabling them to eat softer food.
Stopping short of calling it a cultural burial practice, Berger's team concludes that Homo naledi deposited its dead in a deep pit in order to protect them from scavengers.
The burials include no grave goods, no tools, and no other signs of symbolic burials, Potts noted, unlike those seen with prehistoric modern humans and even our vanished Neanderthal cousins.
He suspects that more recent, but still prehistoric, foul play at the hands of our own ancestors might better explain how the South African cave was filled with the bones of a now-extinct human species.
If the individuals in the cave pit had lived relatively recently, Potts said, then members of our own species, Homo sapiens, "might have unceremoniously disposed of them."