The discovery of a new human ancestor in 2015 stunned palaeontologists across the globe. Headlines lauded the work for rewriting our history; for filling gaps in the evolutionary record, while others claimed it had the potential to upend everything we know about our cultures and behaviours. This ancestor was dubbed Homo naledi.
Now, following the discovery of a second remote cave chamber on the site where the original remains were found, the story has taken a twist. Advanced dating techniques suggest Homo naledi was much younger than thought and may have lived alongside Homo sapiens – the first time it has been demonstrated that another species of hominin survived in Africa. Furthermore, the discovery of remains of an ancient child, and a partial skeleton of an adult male with a remarkably well-preserved skull, adds evidence to support the idea Homo naledi discarded its dead in mass graves.
Read more: New ancestor challenges view of human evolution
To put this latest discovery into context, we need to journey back to Friday, September 13, 2013. Deep in the Rising Cave system of Malmani dolomites, in Bloubank River valley, South Africa, lies a remote alcove known as the Dinaledi Chamber. While exploring the system, cavers Rick Hunter and Steven Tucker found a narrow "chimney" measuring 39ft long leading to an underground room, the surface of which was littered with fossils. Photos were sent to geologist Pedro Boshoff and paleoanthropologist Lee Berger and excavation began.
Almost two years to the date from the initial discovery, researchers from the University of Witwatersrand (Wits) in South Africa, the National Geographic Society and the South African National Research Foundation, including Berger, unveiled Homo naledi in the eLife journal alongside an exclusive feature in National Geographic Magazine.
Comprising of more than 1,550 numbered fossils belonging to 15 distinct individuals of the same species, the discovery presented the largest such find in Africa and the best-known fossil member of our lineage. Homo naledi has a combination of features that scientists never expected to find in the same species, from a very small brain to complex, human-like feet.
The fossils show a mix of human and neanderthal features, combined with traits similar to those found on Lucy, one of the oldest known human ancestors. When alive, Homo naledi would have stood about 1.5 metres tall, with the average adult weighing about 45kg.
"They're a little bit taller and more slender than some other fossil hominins that we've found," Dr Tracy Kivell from the University of Kent, part of a team who studied aspects of Homo naledi's anatomy, told WIRED. "They've got longer legs and human-like feet. Perhaps the face is a little more recognisable as a human shape. But we would look at them and see them as being something quite distinct from modern humans."
At the time, the study said the fossils could be more than two million years old, given their similarity to Lucy, or less than a few hundred thousand years old due to the more recently evolved features. We now have a more exact date.
Using a combination of so-called optically stimulated luminescence dating – measuring radiation in ancient sediments – with Uranium-Thorium dating and palaeomagnetic analyses, teams from the University of the Witwatersrand, James Cook University, Australia, the University of Wisconsin, Madison, and more than 30 international institutions, dated the remains to between 335 and 236 thousand years old. This places this population of primitive small-brained hominins at a time and place that it is likely they lived alongside Homo sapiens, said to be "surprisingly recent."
Like the original discovery, the latest fossil remains have primitive features shared with some of the earliest known fossil members of our genus, such as Homo rudolfensis and Homo habilis. Read more: Brain scans suggest our unique, human-like thought evolved 1.8 million years ago
After finalising the description of the new species in 2015, experts had predicted that the fossils should be around the age of these other primitive species. Instead, the fossils from the Dinaledi Chamber are barely more than one-tenth that age. The dating of Homo naledi is the conclusion of the paper, The age of Homo naledi and associated sediments in the Rising Star Cave, South Africa, led by Professor Paul Dirks from James Cook University and the University of the Witwatersrand and published in eLife.
At such a young age, in a period known as the late Middle Pleistocene, it was thought only Homo sapiens existed in Africa. More critically, it is at precisely this time that the fossil record shows the rise of what has been called "modern human behaviour" in southern Africa – behaviour attributed, until now, to the rise of modern humans and thought to represent the origins of complex modern human activities such as self-adornment and complex tools.
In an accompanying paper led by Berger, now professor at the University of the Witwatersrand in Johannesburg, entitled Homo naledi and Pleistocene hominin evolution in subequatorial Africa, he describes the discovery of the second chamber as having a "significant impact on our interpretation" of archaeological understanding.
"We can no longer assume 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 behavioural breakthroughs in the archaeological record of Africa," says Berger. "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."
He added that the "abundant" Homo naledi fossils found in the second chamber, including one of the most complete skeletons of a hominin ever discovered, gives weight to the argument that Homo naledi deliberately disposed of its dead in these remote, hard to reach caverns. This second chamber has been named the Lesedi Chamber and sits around a hundred metres from Dinaledi. "Lesedi" means "light" in the Setswana language. It is almost as difficult to access and contains what lead author of the third paper in the eLife series, John Hawks from the University of Wisconsin-Madison and Wits University, described as "spectacular" fossils of naledi.
Hawks points out that, while the Lesedi Chamber is "easier" to get into than the Dinaledi Chamber, the term is relative. "I have never been inside either of the chambers and never will be. In fact, I watched [Lee] Berger being stuck for almost an hour, trying to get out of the narrow underground squeeze of Lesedi." Berger eventually had to be extracted using ropes tied to his wrists. Hawks also explained that the very presence of the second chamber, distant from the first and almost as difficult to reach, highlights the extraordinary effort it took for Homo naledi to reach these places.
"This likely adds weight to the hypothesis that Homo naledi was using dark, remote places to cache its dead," says Hawks. "What are the odds of a second, almost identical occurrence happening by chance?"
So far, the scientists have uncovered more than 130 hominin specimens from the Lesedi Chamber. They are "nearly identical in every way" to those from the Dinaledi Chamber, a remarkable finding in and of itself; they belong to at least three individuals, and the team believes there are more fossils yet to be discovered.
"There is no doubt they belong to the same species," said Hawks. "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."
"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," Berger concluded.
The next step is to date the Lesedi Chamber fossils. Those not being studied will now be put on public display at the Maropeng, the Official Visitors Centre for the Cradle of Humankind World Heritage Site from May 25 to join the original 2015 specimens. This exhibit of the largest display of original fossil hominin material in history forms part of an exhibition called Almost Human.
This article was originally published by WIRED UK
