The one thing everyone agrees is that the fossils themselves are spectacular. In 2015, researchers unveiled 1,500 hominin fossil fragments found deep in a South African cave, excavated by six cavers who were all skinny, short, and female. The hominin, a new species the team christened Homo naledi, was an unusual mix of the old and modern. Their heads were small, suggesting an early hominin perhaps more than a million years old. But their feet were stiff for walking upright and their hands adept like modern humans.
So in the media frenzy that followed—a National Geographic cover, a documentary, numerous articles—the question kept coming up: How old are these Homo naledi fossils, really? What do they tell us, if anything, about the origin of Homo sapiens?
To that first question, the researchers now have an answer: 236,000 to 335,000 years old. As for the second question, well, it’s complicated. “You can’t tell simple stories anymore,” says Lee Berger, a paleoanthropologist at the University of the Witwatersrand who led the research. “This is the gigantic message out of Homo naledi.” The age of these fossils puts these strange, small-brained yet human-like hominins in South Africa just before the emergence of the first anatomically modern Homo sapiens.
The Homo naledi fossils were hidden in a pitch-black cave nearly impossible to access. What else would researchers find if they looked harder? “We don’t have many fossils from this time period at all in Africa,” says Carol Ward, an anthropologist at the University of Missouri, who was not involved in the Homo naledi research. But Homo naledi suggests more diversity in Africa in this period than previously thought.
In fact, fossils found over the past several decades have increasingly complicated our understanding of human evolution. Our early ancestors did not simply become bigger brained and more upright over time. They also begat other lineages—Homo naledi is an example; Homo neanderthalensis in Europe is another—of which modern humans are the only extant branch. The dating of Homo naledi to a minimum of 236,000 years old provokes more questions, and Berger is no stranger to provocations.
All of this is addressed in three new papers from Berger and his collaborators—totaling over 100 pages—published today in the journal eLife. The first paper provides the age of the original fossils. The second announces the discovery of a second chamber in the same South African cave system, containing over 130 Homo naledi fossils including a nearly complete skull. The third and most speculative paper takes up the question of how Homo naledi evolved and revisits a controversial hypothesis for the presence of these fossils in the cave.
The dating paper has solid evidence—at least as solid as you can get with ancient bones. “They have thrown basically every available dating method in the book at the remains,” says Justin Adams, a paleontologist at Monash University who has also worked in South African caves. The study uses a total of six methods, including measuring the levels of uranium and thorium in layers of stone that have formed over the fossils. This gave a minimum age of 236,000 years. An even more informative method looked at the fossils themselves, specifically three teeth. Radiation from radioactive elements like uranium and cosmic rays are fairly predictable over time and they leave a signature when they bombard tooth enamel; the more evidence of this radiation, the older the tooth. This method is called electron spin resonance dating.
“It’s taken a long time to do this properly,” says Paul Dirks, a geologist at James Cook University who led the dating work. Other paleontologists had criticized the team for rushing to announce Homo naledi in 2015 before the dating work was complete. Even though the team didn’t give a possible age for the fossils, the discovery announcement fueled speculation that the fossils were more than a million years old based on the ancient-looking morphology. “Not publishing a date actually exposed the frailty of many of these morphological interpretations,” Dirks says. It’s happened before in the history of paleoanthropology. Susan Antón, a anthropologist at NYU who was not involved in this work, notes that researchers had previously misdated hominin fossils found on the island of Java in Indonesia when they looked at morphology alone.
The team is now working on dating fossils in Lesedi, the second chamber, which look morphologically identical to the Homo naledi in the first chamber. Lesedi was a little easier to access. Berger, who couldn’t fit in the first chamber, did get to see the inside of Lesedi—once. “I got stuck coming out and decided I’m never coming in again,” says Berger. His team named that section of the chamber the Berger Box.
This second chamber of Homo naledi bones further complicates Berger’s most controversial hypothesis—that the bodies were intentionally buried in the cave, which suggests some kind of complex culture. They based this hypothesis in part on the difficulty of accessing the first chamber and the lack of animal marks or bones to suggest animals moving the bones. Well, this second chamber is slightly easier to access, and it contains plenty of animal bones. This neither proves the the hypothesis or conclusively disproves it—but that’s the challenge of working with scant evidence in paleoanthropology.
To some experts in the field, this leap to a burial hypothesis based on scant evidence was ludicrous. After the initial discovery was announced, a researcher told reporters, “Intentional corpse disposal is a nice sound bite.” Adams, who also works in South African caves, said that the team had excavated only a small portion of the original chamber and did not look hard enough for evidence that the cave may have had other, more accessible entrances in the past. “To reconstruct the life history of the chamber from one pit is speculative,” he says. “I just don’t understand the rush to put out one hypothesis.”
Berger says they did not excavate everything to leave material for future paleoanthropologists with more advanced technologies. It’s still early days for the excavation of this cave system. (He was coy about whether the cave system had more chambers with fossils.) Efforts to extract ancient DNA from the bones have so far failed, but he’s confident that they’ll get there eventually. That could provide a more conclusive answer to how Homo naledi and humans are related.
The unique completeness of these fossils also allows paleoanthropologists to answer some questions that have often eluded them with less complete sets in the past. The set includes over a dozen individuals, both adults and children. Antón is excited to see if the bones can be used to reconstruct how the Homo naledi moved and walked and lived. How efficient was their gait? How much food did they need to survive?
The discovery that another hominin, so different from us, lived as recently as 236,000 years ago adds more mystery to the question of why humans are the only surviving members of this once diverse family. More than one paleoanthropologist I talked to likened humans to invasive species—like rats or pigeons, flexible in diet and behavior. It’s still too soon to know exactly how we’re related to Homo naledi and why we survived but they didn’t. Whatever the answer, it will force us to consider what it means to be human.