A trait shared by East Asians and Denisovans: three rooted molar

As you can see, by checking my most recent posts, I have been reading recent articles and papers about Homo sapiens in Eurasia. Another one caught my eye: Rare dental trait provides morphological evidence of archaic introgression in Asian fossil record, Shara E. Bailey, Jean-Jacques Hublin, and Susan C. Antón, PNAS first published July 8, 2019 https://doi.org/10.1073/pnas.1907557116.

Bailey, Hublin and Antón found a dental trait in modern humans that is quite rare outside of Eastern Asia and the Americas and yet was found in the jawbones of two Denisovans. This suggests that modern Asians inherited this trait from an ancestral population that lived in Asia.

Let me quote the paper's Abstract to summarize its findings:

"The recently described Denisovan hemimandible from Xiahe, China [F. Chen et al., (2019) Nature 569, 409–412], possesses an unusual dental feature: a 3-rooted lower second molar. A survey of the clinical and bioarchaeological literature demonstrates that the 3-rooted lower molar is rare (less than 3.5% occurrence) in non-Asian Homo sapiens. In contrast, its presence in Asian-derived populations can exceed 40% in China and the New World. It has long been thought that the prevalence of 3-rooted lower molars in Asia is a relatively late acquisition occurring well after the origin and dispersal of H. sapiens. However, the presence of a 3-rooted lower second molar in this 160,000-y-old fossil hominin suggests greater antiquity for the trait. Importantly, it also provides morphological evidence of a strong link between archaic and recent Asian H. sapiens populations. This link provides compelling evidence that modern Asian lineages acquired the 3-rooted lower molar via introgression from Denisovans."

So the three-rooted second molar (3RM2) is unusal outside of Asia and it was found in the Xiahe Denisovan fossil which is 160,000 years old.

The paper also tells us that: "he recently described Penghu 1 mandible from Taiwan (190 to 10 ka) also exhibits a 3RM2 ... The Penghu mandible retains 'archaic' features, including a receding symphysis that lacks a chin, a thick mandibular corpus, and large molar crowns similar in size to Denisovans. Like Xiahe, these exceptionally large molars are coupled with agenesis of the third molar. For these reasons, Chen et al.suggest that Penghu 1 may also be closely related to Denisovans. Both mandibles show that the 3RM anomaly existed in archaic Asian hominins before H. sapiens in the region.".

The authors conclude: " the presence of “archaic features” in recent Asians that were once used to suggest continuity from Pleistocene Asian H. erectus may also have been obtained by introgression from Denisovans."

Some interesting facts they report are that " the 3RM has not been reported in the earliest H. sapiens from Asia, nor have we observed the trait in early H. sapiens from Africa or Homo erectus in Asia.* We note, however, that the lack of radiography of many specimens and the absence of the original Zhoukoudian remains make this conclusion preliminary."

The global prevalecy of the 3RM is highest in Asians (this paper cites a 25% prevalence rate for Nepal) and Esquimos. See the table in this other paper for more info on 3RM prevalence data.

The Homo luzonensis, which lived 67 kya in the Philippines (see post), had teeth that shared features of both modern H. sapiens and ancient H. erectus, yet they were far smaller than those of modern humans and... they also had three rooted molars.

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