Teeth are the most likely parts of skeletons to survive for long periods because of their armour by a layer of enamel made of hydroxyapatite (Ca₅(PO₄)₃(OH)). Dental enamel is the hardest material in the bodies of vertebrate animals and lies midway between fluorite and feldspar on Moh’s scale of hardness (value 5). Like the mineral apatite, teeth survive abrasion, comminution and dissolution for long periods in the surface environment. Subdivision of fossil hominin species and even among different groups of living humans relies to a marked extent on the morphology of their teeth’s biting and chewing surface. Although there are intriguing examples in Neolithic jawbones of dental cavities having been filled it is rather lack of attention to teeth that characterises hominin fossils. As well as horrifying signs of mandibular erosion due to massive root abscesses, a great many hominin remains carry large accumulations of dental plaque or calculus made of mineralised biofilm laid down by oral bacteria. Even assiduous brushing only delays the build up. Grisly as this inevitability might seem, plaque is an excellent means of preserving not only the bacteria but traces of what an individual ate. As fossil DNA is a guide to ancestry and relatedness among fossil hominins, so far going back to about 430 ka in the case of a Spanish Homo heidelburgensis, plaque potentially may reveal details of diet and to some extent social behaviour elaborating beyond the possibilities presented by carbon isotopes and dental wear patterns.

Plaque deposits have already shown that Neanderthals had a very varied vegetable diet and that they cooked their food, the sugars thereby released encouraging bacterial biofilms. There have even been hints that they used medicinal herbs, such as yarrow and chamomile. Now a large multinational team of scientists has taken this fascinating line of study a step further using short DNA fragments to identify the actual oral microbes and even plant and animal species that dominated the diets of 8 cave-dwelling Neanderthals found in Spain, Belgium and Italy (Weyrich, L.S. and 30 others 2017. Neanderthal behaviour, diet, and disease inferred from ancient DNA in dental calculus. Nature, v. 543; online doi:10.1038/nature21674). The Spanish individuals found in El Sidrón cave seem to have been mainly vegetarian (mushrooms, pine nuts and edible moss) whereas two from the Spy cave in Belgium feasted on wooly rhinoceros and mouflon sheep. One of the El Sidrón Neanderthals had a dental abscess, and was probably in great pain, and whose calculus contained evidence of ingestion of tissue from poplar trees, known to contain salicylic acid (the active ingredient in aspirin): an example of self-medication. The unfortunate individual was also suffering from acute diarrhoea brought on by a eukaryote parasite (microsporidium). Astonishingly, DNA from several plant fungi, including Penicillium rubens (penicillin) also occurred in this individual’s calculus, from eating mouldy plant material: predating modern antibiotics by more than forty-five thousand years!

More predictable findings from the unfortunate El Sidrón individual was a spectrum of common plaque colonising bacteria. But another surprise was Methanobrevibacter oralis, an archaea common in the human mouth ecosystem, for which a complete genome was reconstructed. It is different from that in the Methanobrevibacter oralis found in living humans and the team were able to use a molecular clock approach to date the divergence between the two sub-species. This seems to have occurred between 112-143 ka ago, long after the divergence of Neanerthals and anatomically modern humans, judged to be around 450 to 750 ka ago. The authors suggest that ‘commensal microbial species were transferred between the two hosts during subsequent interactions, potentially in the Near East’. Two alternative ‘interactions’ occurred to one commentator: kissing or exchange of chewed food (Callaway, E, 2017. Plaque DNA hints at Neanderthal lifestyle. Nature, v. 543, p. 163). Intriguingly the date, albeit imprecise, overlaps with estimates for the timing of Neanderthal – modern human interbreeding as the latter began to leave Africa: not only do living non-Africans share genes with Neanderthals, the may also share saliva and oral bacteria.

For more information on recent human evolution see here.