Rumble in the caves over Homo naledi 'burial' theory
Lichen and cave-dwelling snails are at the centre of an academic dispute about whether our earliest relatives mourned and ritually disposed of their dead.
The contenders are academics in the same department at Wits University.
On one side is Francis Thackeray, director of the Wits Institute of Human Evolution, who suggests that the black spots found on the fossil remains of Homo naledi are a residue left by lichen.
He also argues that the damage to bones caused by snails is an indication that the floor of the chamber in which they were found was once covered in leaf litter.
"My hypothesis is that, if there was lichen, there [was a] source of light."
He says snails also need light and the presence of leaf litter suggests that there was once an opening to the Dinaledi chamber, which is 30m underground.
Thackeray has published his argument in the South African Journal of Science.
Now the team led by Lee Berger, which announced the discovery of Homo naledi last year, has responded to Thackeray. The team members are sticking to their theory that the 15 skeletons excavated in the Rising Star cave were carried to the Dinaledi chamber and dropped into it by their contemporaries.
MISHAP THEORY: Professor Francis Thackeray's theory is that the hominids were trapped by a rockfall
BURIAL THEORY: Professor Lee Berger believes the skeletons show that hominids disposed of their dead
Thackeray argues that the black dots on the bones are stains caused by manganese excreted by lichen.
Similar staining, he says, can be found throughout the Cradle of Humankind.
But the lead author of the response to Thackeray, Patrick Randolph-Quinney, writes in the SA Journal of Science that the manganese staining could have other origins.
He points out that manganese is found in ground water and can leach into the bones.
Randolph-Quinney says that there were tidal lines on the bones, strips of manganese staining where the fossil was in contact with soil.
Further evidence of the manganese being attributable to a source other than lichen is that the spots are also found on the underside of the fossils.
Thackeray says he can explain this. His theory is that the 15 individuals found in the chamber died after a rockfall trapped them there.
An opening that later became sealed admitted light.
"In a scenario like this, there will be movement. As the carcass disintegrates and decomposes, bones will change position, allowing lichen to grow on both sides," Thackeray explains.
Randolph-Quinney says there is no evidence of such a rockfall and argues that Thackeray's contention that snails need sunlight is flawed.
He says snails are often found in burial sites, and certain species are omnivorous.
"They are known to go down 100m into caves," says Randolph-Quinney.
When Berger announced his belief last year that the skeletons had been placed in the cave, scientists across the world shunned the idea. Many believed that the hominids would have found it nearly impossible to navigate the passages in total darkness carrying a corpse.
But Randolph-Quinney said they could have made the journey using artificial light, as there is evidence that homo species other than humans were using fire as early as 1.5 million years ago.
He said it was not a big leap to believe that Homo naledi mourned their dead.
Primates such as gorillas and chimpanzee do so, staying with bodies for a time before abandoning them, he said.
"This mourning behaviour didn't arrive out of nowhere. It is part of a continuum.
"With this awareness in other primates, we realise that we are not that special."
UNCOVERING THE EVIDENCE
Over the next decade or so more evidence is likely to emerge that could prove or disprove the theory that Homo naledi was involved in body disposal.
The search now, said Dr Patrick Randolph-Quinney of Wits University, was to find sites that Homo naledi inhabited.
"Excavations will be ongoing at the Rising Star cave. That could help in resolving some of these issues [body disposal]."
He said further excavations in the Dinaledi chamber could produce more skeletons that may present forensic evidence that suggests the bodies were thrown into the cave.
Randolph-Quinney said that none of the 15 skeletons excavated revealed signs of how they died.
The Homo naledi team is also soon expected to release the dates for the fossils, which will establish where naledi is on the evolutionary tree.
Work is being done at another cave in the Cradle of Humankind that could once and for all prove that sunlight entered the Dinaledi chamber.
Professor Francis Thackeray, director of the Institute of Human Evolution at Wits University, is conducting an experiment in which he is growing lichen on bone at various depths in the cave to see how it reacts to light.
"We need to find out what the optimum conditions are for lichen to grow. Give me 10 years and we will know," he said.