Raman spectroscopic and elemental analysis of bone from a prehistoric ancestor: Mrs Ples from the Sterkfontein cave

The complementary value of analytical elemental and molecular data for archaeological relics recovered from a burial deposition is illustrated here with the non-destructive analysis of bone specimens from an early human ancestor, known as Mrs Ples. Discovered in 1947 by Robert Broom in the Sterkfontein Cave system, forming part of the Cradle of Humankind in Gauteng Province, South Africa, a skull from Australopithecus africanus dated to 2.1 Mya has been identified as a Hominidae precursor to Homo sapiens. The ability to record novel compositional and structural data from such specimens without any form of chemical or mechanical pretreatment is fundamentally necessary for the preservation of such unique relics while affording conservators the information needed to undertake restoration and the adoption of conservation strategies. Our analyses revealed information about the state of preservation of the samples as well as their depositional context. Raman microspectroscopic mapping produced data on the spatial distribution of phosphate, quartz, carbon, calcite and manganese dioxide signatures on the bone fragments. Extensive deposits of pyrolusite, manganese(II) oxide, on the bone specimens are attributed to hydrogeological processes from ground water irrigation of the skeletal remains in the depositional environment; the identification of distinct quartz- and calcite-rich areas may arise from taphonomic and/or depositional processes. Raman spectroscopic signatures of the organic proteinaceous component of the prehistoric bone are not observed, indicating the degradation of endogenous organics or, if present, were masked by exogeneous materials. The identification of carbon could additionally reflect the remains of biodegradative processes that have operated in the burial site. Elemental mapping using X-ray fluorescence and variable pressure scanning electron microscopy imaging and spatially resolved energy dispersive X-ray spectroscopy provided complementary information about the samples. The result is a comprehensive characterisation of the samples' morphology, elemental and molecular composition from the micro scale to the macroscale, using entirely non-destructive analytical techniques.