Earlier this summer, Francis Thackeray published a short paper in the South African Journal of Science suggesting that lichens had deposited manganese upon the surfaces of hominin bone from the Dinaledi Chamber. Knowing that lichens depend upon light for their growth, Thackeray suggested that the Dinaledi bones had once been exposed to light for long enough to explain the lichen growth and manganese deposition.
Thackeray proposed that the geological evidence was somehow wrong, and that the Dinaledi Chamber had once been open to the surface. He elaborated on this view in an essay written for The Conversation:
I strongly believe that there was possibly a temporary entrance into the chamber, in addition to the one used by explorers today. This temporary entrance may been covered up by a rock fall that also trapped the individuals whose bones were found some time later.
Our team, including geologists and geochemists, has now published a response to Thackeray. We review some of the ways that manganese and other trace elements from dolomitic limestone can be deposited on fossil bone, and present additional evidence from the bone surfaces that rules out lichen involvement.
The Dinaledi bones bear traces of manganese and iron oxides. Some of the staining is dendritic, with very tiny features that suggest microbial involvement. The deposition process happens within sediment, in the dark
Several of the hominin fossils have linear tide marks of manganese or iron oxides. These mark an interface of air and sediment in which these bones were embedded.
Looking through a microscope, the bones bear many different patterns of mineral staining and calcite deposition. Some of the fossils have calcium carbonate deposition on top of manganese staining; others have manganese deposition on top of calcium carbonate. Some have successive encrustations of iron oxide, manganese, and iron oxide, as visible in this photo:
Additionally, some fossils have manganese deposits that have been removed by gastropod activity; others have manganese formed on top of old gastropod markings. All of this evidence is consistent with a long history of manganese and iron oxide deposition on the fossils within their current sedimentary environment.
None of this geochemical activity requires light.
One of the things that we have kept at the forefront of our papers describing the Rising Star cave system is that we must not set aside or ignore any of the geological evidence.
The geological evidence from the Dinaledi Chamber shows that it was an isolated depositional environment during and after the bones of Homo naledi entered the chamber. The sediments with the bones of H. naledi are very different in their chemical composition and particle characteristics from the neighboring Dragon’s Back Chamber. Dragon’s Back sediments have some input of surface material, the Dinaledi Chamber doesn’t. With articulated skeletal parts, complete representation of skeletons, and abundant fragile juvenile material, the bones of Homo naledi reject the hypothesis that the Dinaledi deposit could be a secondary accumulation from some other source. This is all compatible with the ceiling of the chamber, which is a chert layer with at most small thin fissures and no substantial openings, except for the vertical crack called the “Chute”.
Although the bone has been within the Dinaledi Chamber, that doesn’t mean it has been static. There has been erosion of the deposit since it began to form, with sediment and other material exiting the chamber through floor drains and some of the hominin bone being reworked along with the chamber’s surface. Bone within the Dinaledi Chamber was modified by gastropods and other cave-dwelling organisms, and minerals were deposited on its surfaces. The surfaces of many bones have been etched by the slightly acidic groundwater. Within parts of the deposit that are mere centimeters apart, the chemical circumstances differ.
It’s a very complicated picture, and whenever we make a new observation we must integrate it into the entire body of evidence. That work continues. This summer, members of our team published a review of the team’s 3D data collection strategies, including some mapping data from the cave. And our original research article on the context of the fossil assemblage remains open access in eLife, with detailed maps and analyses.
From earlier this year: “Deliberate deposition and Homo naledi”
Dirks, P. H., Berger, L. R., Roberts, E. M., Kramers, J. D., Hawks, J., Randolph-Quinney, P. S., ... & Schmid, P. (2015). Geological and taphonomic context for the new hominin species Homo naledi from the Dinaledi Chamber, South Africa. eLife, 4, e09561.
Dirks, P. H. G. M., Berger, L. R., Hawks, J., Randolph-Quinney, P. S., Backwell, L. R., & Roberts, E. M. (2016). Deliberate body disposal by hominins in the Dinaledi Chamber, Cradle of Humankind, South Africa?. Journal of human evolution 96: 149-153. doi:10.1016/j.jhevol.2016.04.007
Kruger, A., Randolph-Quinney, P. S., & Elliott, M. (2016). Multimodal spatial mapping and visualisation of Dinaledi Chamber and Rising Star Cave. South African Journal of Science, 112(5/6) doi:10.17159/sajs.2016/20160032
Randolph-Quinney, P. S., L. R. Backwell, L. R. Berger, J. Hawks, P. H. G. M Dirks, E. M. Roberts, G. Nhauro, J. Kramers. 2016. Response to Thackeray (2016) – The possibility of lichen growth on bones of Homo naledi: Were they exposed to light? South African Journal of Science doi:10.17159/sajs.2016/a0177
Thackeray, J. F. (2016). The possibility of lichen growth on bones of Homo naledi: Were they exposed to light?: scientific correspondence. South African Journal of Science, 112(7-8), 1-5. doi:10.17159/sajs.2016/a0167
Val, A. (2016). Deliberate body disposal by hominins in the Dinaledi Chamber, Cradle of Humankind, South Africa? Journal of Human Evolution 96: 145-148. doi:10.1016/j.jhevol.2016.02.004