Saturday 10 September 2016

Evidence of Lichen growth on the bones of Homo naledi.

In 2013 scientists in South Africa described the discovery of a remarkable new Hominin species in the Dinaledi Cave System in Gauteng State, South Africa (part of the Maropeng Cradle of Humankind World Heritage Site). Homo naledi was similar to small Modern Humans in size, but had a cranial capacity more typical of an Australopithicene and a post-cranial skeleton similar to that of early members of the genus Homo. The exact age of the remains is unclear, but based upon the level of development they are thought to be about two million years old. Remarkably, all of the remains attributed to the species, over 1550 pieces of bone belonging to at least fifteen individuals, were found within a single chamber, the Rising Star Chamber (Dinaledi  means 'Rising Star' in Sotho, so sometimes the terms Dinaledi Chamber and Rising Star Cave System are encountered), which could only be reached via a long, twisting and narrow entrance tunnel (so much so that small bodied female potholing enthusiasts had to be recruited and trained to carry out much of the excavation work), and which in the absence of modern torches would have been completely dark. The presence of a large number of bodies in such an inaccessible chamber has been seen as an indicator of advanced funerary behaviour in Homo naledi, though such behaviour is such an apparently primitive Hominid has been hotly contested.

In a paper published in the South African Journal of Science on 27 July 2016, Francis Thackeray of the Evolutionary Studies Institute at the University of the Witwatersrand presents evidence that Lichens may have formerly grown on the bones of Homo naledi, which if true would mean the bones have been exposed to sunlight since they were deposited (Lichens are photosynthetic organisms that cannot live in complete darkness), implying that the chamber was not as inaccessible when the remains were deposited.

Lichens are symbiotic organisms, each consisting of a Fungus and an Alga, the Fungus obtaining nutrients from the substrate (surface upon which the Lichen sits), while the Alga produces carbohydrates through photosynthesis. They the Fungi and Algae involved can often form relationships with more than one member of the other group, and sometimes more complex communities, with more than one Fungus or Algae.

Lichens are common around the Maropeng Cradle of Humanity site, where they grown on a variety of rock types. However they are not found on exposed surfaces outside the caves, in an area that is arid and gets extremely hot, nor in the inner, dark parts of the cave system, where there is no light and photosynthesis is impossible, but favour areas around the entrances to the cave system, where low light levels are combined with limited but available moisture.

 A chert rock at Kromdraai, showing the distribution of actively growing green Lichen which is dispersing in small thalli from larger, denser central mats. The size of the thalli generally decreases outwards from then central mats. The small Lichen spots to the right are relatively young growths. Lichen is a fungal–algal–bacterial symbiont, and dispersal from central thalli is facilitated by fungal spores. Note the presence of adjacent black spots of manganese dioxide (upper margin of photograph). Scale: centimetre squares are shown in black and white. Benjamin Lans in Thakeray (2016).

Lichens absorb a variety of chemicals from the rock they grown on, most of which are essential to their life proceses, but some are considerably less so, and have to be excreted by the Lichen to prevent damage to their metabolism. One of these is manganese, a metal element essential to photosynthesis and some enzyme reactions, but highly toxic in excess, and abundant in many rock systems, including those around Maropeng. Lichens deal with excess manganese by depositing it as manganese dioxide at fixed spots on the surface of the rock. Over time these manganese dioxide spots react with water to form manganese oxy-hydroxide, which since it is black in colour this creates a distinctive mottled pattern on the rock surface, indicative of the former presence of Lichens even when these are no longer present.

 A chert rock at Kromdraai, showing the distribution of spots of manganese oxy-hydroxide, which may relate to former growth of lichen which dispersed in spots. Note the presence of actively growing green lichen, which in this instance may be very young compared with earlier phases of Lichen growth and deposition of manganese oxy-hydroxide. Jean-Baptiste Fourvel  in Thakeray (2016).
Many of the bones from the Dinaledi Chamber also have a dark mottled pattern of manganese oxy-hydroxide blotches on their surface, which Thakeray interprets as evidence of Lichen growth on the surface of the bones. If this is true, then at some point in the past light must have been able to directly enter the chamber, which can only be explained by the former presence of a second entrance to the cave system, which has presumably subsequently been closed of by geological activity in the area. The former presence of such an entrance directly from the surface to the Dinaledi Chamber would imply that the site was accessible to Homo nedali without recourse to the tortuous entrance used by modern explorers. Such an open cave, with some light available within, may have been used as a living space, rather than a burial chamber, removing the need to explain how such an early Hominid was able to indulge in elaborate funerary behaviour.

Tibia shaft specimen UW 101-1070, Homo naledi from the Dinaledi Chamber, with dotted coatings of manganese oxy-hydroxide. It is suggested that the black dots result, at least in part, from the growth of Lichen as a Bacterial–Algal–Fungal symbiont that includes a photobiont. The growth of Lichen on such bone surfaces, even for a limited time, may have occurred in subdued but essential lighting. Note the distribution of manganese oxy-hydroxide, extending from a continuous mat to more dotted occurrences; this pattern is potentially analogous to the dispersal of lichen from a central thallus. Francis Thakeray in Thakeray (2016).

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