Dating of Homo naledi fossils from the Dinaledi Chamber of the Rising Star cave system, South Africa, shows that they were deposited between about 335,000 and 236,000 years ago.
A reconstruction of Homo naledi’s head by paleoartist John Gurche, who spent some 700 hours recreating the head from bone scans. Image credit: John Gurche / Mark Thiessen / National Geographic.
Species of ancient humans and the extinct relatives of our ancestors are typically described from a limited number of fossils.
However, this was not the case with Homo naledi. More than 1,500 fossils representing at least 15 individuals were unearthed from the Dinaledi Chamber of the Rising Star cave system in 2013-14.
After the discovery was reported, a number of questions still remained. Not least among these questions was: ‘how old were the fossils?’
The material was undated, and predictions ranged from anywhere between 2 million years old and 100,000 years old.
Homo naledi shared several traits with some of the earliest known fossil members of the genus Homo, such as Homo rudolfensis and Homo habilis. As a result, many paleoanthropologists guessed that Homo naledi was an old species in our family tree, and possibly one of the earliest species to evolve in the genus.
Now, Professor Paul Dirks of James Cook University and the University of the Witwatersrand and co-authors report in the journal eLife that the Homo naledi fossils are most likely between 236,000 and 335,000 years old.
“The dating of Homo naledi was extremely challenging. Eventually, six independent dating methods allowed us to constrain the age of this population of Homo naledi to a period known as the late Middle Pleistocene,” Prof. Dirks said.
“Homo naledi may have survived for as long as two million years alongside other species of hominins in Africa. At such a young age, in a period known as the late Middle Pleistocene, it was previously thought that only Homo sapiens existed in Africa.”
Prof. Dirks and co-authors used a combination of optically stimulated luminescence dating of sediments with Uranium-Thorium dating and paleomagnetic analyses of flowstones to establish how the sediments relate to the geological timescale in the Dinaledi Chamber.
Direct dating of the teeth of Homo naledi, using Uranium series dating (U-series) and electron spin resonance dating (ESR), provided the final age range.
“We used double blinds wherever possible,” said co-author Professor Jan Kramers, of the University of Johannesburg.
“It was crucial to figure out how the sediments within the Dinaledi Chamber are layered, in order to build a framework for understanding all of the dates obtained,” added co-author Dr. Hannah Hilbert-Wolf, a geologist from James Cook University.
“Of course we were surprised at the young age, but as we realized that all the geological formations in the chamber were young, the U-series and ESR results were perhaps less of a surprise in the end,” said co-author Professor Eric Roberts, from James Cook University and University of the Witwatersrand.
Chronostratigraphic summary of radio-isotopic dating results, and interpretation of the depositional ranges of stratigraphic units, flowstones and Homo naledi fossils in the Dinaledi Chamber. Image credit: Dirks et al, doi: 10.7554/eLife.24231.
In an accompanying paper, also published today in the journal eLife, the researchers discuss the importance of finding such a primitive species at such a time and place.
They noted that the discovery will have a significant impact on our interpretation of archaeological assemblages and understanding which species made them.
“We can no longer assume that we know which species made which tools, or even assume that it was modern humans that were the innovators of some of these critical technological and behavioral breakthroughs in the archaeological record of Africa,” said senior Prof. Lee Berger, a paleoanthropologist at the University of Witwatersrand.
“If there is one other species out there that shared the world with ‘modern humans’ in Africa, it is very likely there are others. We just need to find them.”
“I think some scientists assumed they knew how human evolution happened, but these new fossil discoveries, plus what we know from genetics, tell us that the southern half of Africa was home to a diversity that we’ve never seen anywhere else,” said co-author John Hawks, the Vilas-Borghesi distinguished achievement professor of anthropology at the University of Wisconsin-Madison.
“Recently, the fossil hominin record has been full of surprises, and the age of Homo naledi is not going to be the last surprise that comes out of these caves I suspect,” Prof. Berger said.
In a third paper published at the same time in the journal eLife, Prof. Hawks and co-authors announce the discovery of a second chamber in the Rising Star cave system, which contains more fossils of Homo naledi.
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Paul HGM Dirks et al. 2017. The age of Homo naledi and associated sediments in the Rising Star Cave, South Africa. eLife 6: e24231; doi: 10.7554/eLife.24231
Lee R. Berger et al. 2017. Homo naledi and Pleistocene hominin evolution in subequatorial Africa. eLife 6: e24234; doi: 10.7554/eLife.24234
