In 2017, a team of “underground astronauts” was exploring a deep, dark, remote part of the Rising Star cave system in the Cradle of Humankind World Heritage Site when they stumbled on small yellowed fragments in a narrow crevice.
“These were on a tiny ledge, sitting 80cm above the cave floor in a passageway that’s about 15cm wide, on a small shelf of forming limestone,” said Lee Berger, project leader and director of the Centre for Exploration of the Human Journey at the University of the Witwatersrand and an explorer at large for the National Geographic Society. “Initially, they didn’t know what they were.”
The space was so tight and cramped, Berger said, that only a few people could see the area. Painstakingly, the team marked the fragments, preparing them for excavation.
“Imagine working for hours recovering these [fragments] in these spaces, but they eventually did and after reconstruction and study, we realised that we had made a beautiful and wonderful discovery — one of the rarest types of discoveries there are,” Berger said.
“What it clearly was, was a skull, a part of it, and some teeth that had deteriorated on to the top of this small ledge and then begun to dribble over the front of it. In this deep, dark remote passage, we discovered a beautiful skull of a child.”
The discovery of the remains of the Homo naledi child, which is an ancient human relative, was revealed to the world on Thursday from work led by Berger and a team of 21 researchers from Wits University and 13 other universities.
They have named the child, who was four to six years old, “Leti”, from the Setswana word “letimela”, meaning “the lost one”. The child is thought to have died about 250 000 years ago and the discovery of its remains is “the rarest of the type of early hominid fossils that we find — the fossil of a child”, Berger said.
This is because the bones of children are fragile and don’t preserve well.
Leti was found some 12m beyond the Dinaledi Chamber, the original site of discovery of the first Homo naledi remains that were revealed in 2015. Leti’s remains were found in a passage that measures only 15cm wide and 80cm long, just beyond an area the team calls the “Chaos Chamber”.
“The area where Leti was found is part of a spider web of cramped passages,” said Maropeng Ramalepa, a member of the exploration team who was responsible for bringing the remains to the surface.
Marina Elliott, one of the original underground astronauts in the first Rising Star expedition that discovered Homo naledi and the leader of the excavation team that recovered Leti, said excavating the child’s remains was “very difficult”.
“The spaces in the Dinaledi sub-system are really interesting and we’re learning more about them as the team explores,” she said. “In 2017, one of our primary goals was to really start looking in all those spaces around the Dinaledi area from the first excavations just to better understand those spaces, how they might have formed, and how the Homo naledi material might have got into those areas.”
It was in the course of those explorations the team identified the material where Leti was found.
The skull and its context are described in two separate papers in the open access journal, PaleoAnthropology.
“This is the first partial skull of a child of Homo naledi yet recovered and this begins to give us insight into all stages of life of this remarkable species,” said Juliet Brophy, who led the study on Leti’s skull and dentition.
“We know that this individual was a child because two of those teeth are deciduous or baby teeth, the other four are permanent or adult teeth,” said Brophy. “The two decidious teeth that we have are worn while the four adult teeth are unworn. From this, we can estimate the age of the specimen.”
Leti’s skull was found alone, and no other parts of its body have been recovered. The skull consists of 28 skull fragments and six teeth and when reconstructed shows the frontal orbits and top of the skull with some dentition.
The teeth are consistent in size, shape and morphology with the other Homo naledi specimens from Dinaledi, Brophy said. “That consistency and lack of variation within the whole Rising Star cave system suggests that we’re likely looking at a population of related individuals that used the cave to deposit their dead.”
Almost 2 000 individual fragments of more than two dozen individuals at all life stages of Homo naledi have been recovered since the Rising Star cave system was discovered in 2013.
“This makes this the richest site for fossil hominins on the continent of Africa and makes Homo naledi one of the best-known ancient hominin species ever discovered,” according to John Hawks, a biological anthropologist and lead author of a previous study on the fossil skeleton of a male naledi nicknamed Neo that was also found at the Rising Star cave.
Leti’s brain size is estimated at about 480 to 610 cubic centimetres. “This would have been around 90% to 95% of its adult brain capacity,” said Debra Bolter, co-author on the paper and a specialist in growth and development. “The size of Leti’s brain makes it very comparable to adult members of the species found so far.”
It has yet to be established how old Leti’s remains are. But, since other fossils of Homo naledi were found in the nearby Dinaledi Chamber and dated to between 335 000 and 241 000 years ago, it’s likely Leti is from a similar period, according to Tebogo Makhubela, who was part of the geological team investigating the discovery.
Since it was found, the Rising Star cave system has become one of the most prolific sites of discovery for hominin fossils in the world. Work is continuing throughout the cave system and new discoveries are likely to shed further light on whether these chambers and passages are a burial ground of Homo naledi, as the team originally controversially hypothesised.
“Why is this child’s skull sitting alone on this little ledge? There is no mandible, there’s no parts of the body. If the body had been there it’s almost certain that the hard preserved bones of the limbs would be there, which leaves it likely that this child’s skull came in here only as a skull,” Berger said. “And yet there are no signs of scavenging, no signs of predation on this. We can tell from the sediments that they have not been moved by any forces of strong water.
“That leaves us with perhaps one of the greatest enigmas. How does a small skull of a small child end up there? Is this a continuation of the behaviour that we’ve seen and hypothesised for Homo naledi … Hopefully as we continue to explore and continue to make discoveries we may reach a point where we can confidently say that we have found a non-human species of ancient human relative that practices ritualised mortuary practices in the deep aspects of the Rising Star cave.”