The Homo naledi fossils found in the Cradle of Humankind are far younger than scientists initially thought, suggesting the species may have lived alongside the first humans in Africa, researchers announced on Tuesday.
Using six different dating techniques, an international team of scientists led by University of the Witwatersrand (Wits) paleoanthropologist Lee Berger, have pegged the age of the fossils at between 335,000 and 236,000 years old, a fraction of their initial estimate of about 2-million years.
The startlingly young age of these primitive-brained hominins raises a host of questions, including how best to interpret archaeological finds that hold evidence of early modern human activities such as tool-making, said Berger. "We have to step back and say are they [H naledi] responsible for these activities, are they interacting with modern humans, is gene exchange possible? We just don’t know," he said in a telephone interview.
The fact that they had identified one species — H naledi — that shared the world with modern humans suggested there might others that had yet to be found, said Berger, who co-authored three new papers on H naledi published on Tuesday in the open access journal eLife. The work was carried out by researchers from Wits, James Cook University in Australia, the University of Wisconsin, Madison and more than 30 international institutions.
H naledi emerged from the biggest single African fossil site discovered to date, located in the Dinaledi chamber of the Rising Star cave system in the Cradle of Humankind. No other species bar birds or mice were identified among the thousands of fossils in the chamber, and when Berger made the announcement in September 2015 they hypothesised that this previously unknown species may have deliberately placed its dead in the cavern. However, many scientists were sceptical of this idea, and others questioned whether H naledi was even a new species.
On Tuesday Berger’s team announced the discovery of a second chamber in the Rising Star cave system containing more fossil remains of the same species. Dubbed the Lesedi chamber, it is about 100m from the Dinaledi chamber. So far 130 hominin fossils have been excavated from the Lesedi chamber, including those from at least three individuals — two adults and a young child.
H naledi was previously described by the team as having a unique combination of primitive and modern features, with a brain the size of an orange and broad ape-like shoulders and ribs. Its teeth are similar to those of humans, and the shape of its hands and its broad thumbs indicate it could make tools. Its curved fingers suggest it climbed trees, but it had long slender legs and humanlike feet that suggest it could walk long distances too.
One of the adult hominins found in the Lesedi chamber has an almost complete skeleton and skull, and has been dubbed "Neo", which means gift in Sesotho.
The scientists said the discovery of the Lesedi chamber fossils added weight to their hypothesis that H naledi deliberately disposed of its dead in these hard-to-reach chambers. The fossils have yet to be dated, but the team hypothesised they were within the same age range as those from the Dinaledi chamber.
Chris Stringer from the Natural History Museum in London said he remained sceptical of the idea that H naledi had deliberately placed its dead in the chamber. "Although no other satisfactory explanation for the deposition of the naledi remains has yet been proposed, many experts (including me) consider such complex behaviour unlikely for a creature with a brain size close to that of a gorilla, particularly when a requirement for the controlled use of fire (for lighting) probably has to be added in. Perhaps further exploration will reveal other, closer, entrances or sink holes which were temporarily open, through which the remains could have been introduced by accidental or natural processes?" he said.
Stringer said H naledi’s discovery and dating were a reminder that 95% of Africa had yet to be explored for its fossil human record. "Its history even within the last 500,000 years may well be as complex as that of Eurasia with its five known kinds of humans — Homo erectus, heidelbergensis, neanderthalensis, Denisovans and floresiensis. At around 300,000 years ago, there were probably at least three kinds of humans across the African continent: heidelbergensis/rhodesiensis, early Homo sapiens, and naledi — and who knows what else might be out there?," he said.
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