While appreciable amounts of Denisovan DNA are found in various ethnic populations in southeast Asia, their physical remains have eluded archaeologists. That was until December 2018 when a hominin permanent lower molar was recovered from a breccia block at Tam Ngu Hao 2 (Cobra Cave), Huà Pan province, Laos.
The inconspicuous Tam Ngu Hao 2 (Cobra Cave) with archaeologists examining finds: ref 2.
The story of the discovery was recounted in a National Geographic article (1): “On December 3, 2018, geologist and caver Eric Suzzoni headed to the hollow on a reconnaissance trip ahead of Shackelford’s first visit inside, and he collected bits of rock and bone to show the team. He clambered down from the cave just before lunch to pass around his many fossil finds. “At some point Eric said, Oh, but I got something here,” Demeter says. From his front shirt pocket, Suzzoni pulled out the unusual molar.”
“Almost immediately, we knew that it was hominin of some sort,”
Shackelford says. “But it wasn't modern human.”
Personally, I think, that from the moment they saw it, Shackelford and Demeter had a very strong suspicion that the tooth belonged to a Denisovan. As they passed it between them, I believe they must’ve had the image of Denisova 4 in their minds.
Despite the toils the team had endured in the rainforest clad karst hills day after day, and in Laura Shackelford’s case 10 years searching for hominin fossils in Laos, the real work of analysing their find had just begun. It would be another 4 years before they published their journal article, on the tooth morphology and species designation in Nature Communications (3). Below is the image of the fossil (TNH2-1) they presented in their paper:
Demeter et al. (3), Fig 2: The Denisovan tooth. Original caption reads: Views of the TNH2-1 specimen. Pictures of TNH2-1 in occlusal (a), inferior (b), mesial (c), distal (d), buccal (e) and lingual (f) views.
The authors tried various methods to identify the tooth. However, they took a strategic approach carrying out microCT analysis of the entire tooth, before invasive/destructive sampling for DNA, thus ensuring full morphological data were saved.
They conclude: “Morphometric analyses of the external and internal crown structural organisation allow us to reject a number of hypotheses regarding species assignment. TNH2-1 has large crown dimensions and a complex occlusal surface that differentiates it from the smaller and morphologically simpler teeth of H. floresiensis49, H. luzonensis50 and H. sapiens. The EDJ shape shows a mixture of Neanderthal-like and H. erectus-like features, closely resembling the M1 morphology of the Denisovan specimen from Xiahe. The similarities between TNH2-1 and H. erectus are mostly related to the proportionally lower crown, although H. erectus molars display even lower molar crowns and a narrower occlusal basin. The Lao fossil shows clear Neanderthal-like features such as a well developed mid-trigonid crest and internally-positioned mesial dentine horns, but differs with its much lower EDJ topography and occlusal basin shape.
The differences from Neanderthals that we observe do not preclude TNH2-1 from belonging to this taxon and would make it the south-eastern-most Neanderthal fossil ever discovered. However, considering the morphological particularities of TNH2- 1 in unison, as well as the high-degree of morphodimensional similarities with the molars of the Denisovan specimen from Xiahe, the most parsimonious hypothesis is that TNH2-1 belongs to this sister group of Neanderthals.”
The paper also looks at the age of the layers in which the tooth was found and the accompanying fauna. Adapted from the paper: Three bovid teeth recovered from the upper fossil-bearing breccia (LU2) were directly dated using coupled uranium series and electron spin resonance (US-ESR). This provided a weighted mean age estimate of 151 ± 37 thousand years ago (kyr) and an age range of 188–117 kyr. Two large blocks of breccia (LCC1 and LCC2) from LU2 (upper) and one block of the silty clay unit (LCC3) from LU1 (lower) were removed for luminescence dating. These samples produced coeval age estimates of 143 ± 24 kyr (LCC1) and 133 ± 19 kyr (LCC2) for the deposition of the LU2 breccia and 248 ± 31 kyr (LCC3) for the underlying LU1 silty clay deposit, which lies unconformably below the layer containing the tooth. These ages are in stratigraphic agreement with the age of the overlying flowstone (CCF1), which was precipitated earlier than 104 ± 27 kyr based on the weighted mean of U-series age estimates on four separate sub-samples of flowstone carbonate. Bayesian modelling was performed on all independent age estimates to determine an overall geochronological framework for the site and tooth. The fossiliferous breccia including the tooth was deposited between 164 and 131 kyr (at 68% confidence limit).
The faunal assemblage (mainly teeth) was assessed to see whether the fauna of the LU2 layer were in agreement with the age determination: “The fauna bears close affinities to those known from the late Middle Pleistocene of southern China and northern Indochina and, to a lesser extent, Java, which is consistent with the sedimentary chronology of the site. It can be assigned to the “Stegodon-Ailuropoda faunal complex”. We note the absence of Neogene taxa that persist in the Early Pleistocene and that of two key-species, Pachycrocuta brevirostris and Gigantopithecus blacki, which are good indicators of pre-300 kyr faunas in the region.”
Faunal remains from Tam Ngu Hao 2 (Cobra Cave), Laos. Photo credit: ref 3.
In terms of molecular analysis, for example ancient DNA and ancient proteins, the results were somewhat disappointing. No ancient DNA could be extracted as far as I am aware – the paper does not make this explicit – and while endogenous proteins were found in the sampled enamel, these only showed that the source of the tooth was from the genus Homo and female. As the authors say: “No high-confidence peptides overlapped diagnostic amino acid positions with sequence differences between H. sapiens, Denisovans, or Neanderthals, making further taxonomic assignment based on palaeoproteomics impossible.”
I can only say, that while the paper makes a very strong case for the unerupted molar being that of a female Denisovan of 5-8 years of age it is a real shame that the conditions under which it was found prevent a completely undeniable species determination being made.
Below I show images of TNH1 and Denisova 4, as a visual comparison:
Occusal views of Denisovan molars. Left: Denisova 4 ref 4, Right TNH1 ref 3.
References
(1). Wei-Haas, M. and M. Greshko (2022). Tooth from mysterious human relative adds new wrinkles to their story. National Geographic, at: https://www.nationalgeographic.com/science/article/denisovan-tooth-found-in-laos-adds-new-wrinkles-to-their-story accessed 25/07/2022
(2). The Past (2022). Denisovan tooth found in Laos? At: https://the-past.com/news/denisovan-tooth-found-in-laos/ accessed 25/07/2022
(3). Demeter, F., Zanolli, C., Westaway, K.E., Joannes-Boyau, R., Duringer, P., Morley, M.W., Welker, F., Rüther, P.L., Skinner, M.M., McColl, H. and Gaunitz, C., 2022. A Middle Pleistocene Denisovan molar from the Annamite Chain of northern Laos. Nature Communications, 13(1), pp.1-17.
(4). Zubova AV, Chikisheva TA, Shunkov MV. The morphology of permanent molars from the Paleolithic layers of Denisova Cave. Archaeology, Ethnology & Anthropology of Eurasia. 2017 Apr 3;45(1):121-34.
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