Australia: The Land Where Time Began

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Morphological Evidence of Archaic Introgression in Asian Fossil Record Provided by Rare Dental Trait

An unusual feature has been described recently in the hemimandible from Xiahe, China: a 2^nd molar that has 3 roots. It was demonstrated by a survey of the clinical and bioarchaeological literature found that the lower molar with 3 roots is rare, occurring in <3% of non-Asian Homo sapiens. Its presence in populations that are Asian-derived, in contrast, can exceed 40% in China and the New world. It has been thought for a long time that the presence of lower molars with 3 roots in Asia is an acquisition that occurred long after the origin and dispersal of H. sapiens. The presence in this fossil hominin dating to 130,000 BP of a lower 2^nd molar with 3 roots suggests the antiquity of the trait is greater than has been believed. Importantly, it has also provided strong morphological evidence that there was a strong link between archaic and recent H. sapiens populations. Compelling evidence has been provided by this link that modern Asian lineages acquired the 3-rooted lower molar via Denisovan introgression.

The new Denisovan hemimandible from Xiahe, China (Chen et al., 2019), Exhibits a 3-rooted lower molar, which has provided a direct morphological link between archaic and recent Asian Homo sapiens populations, root numbers varies from 1 to 3 (Turner, Nichol & Scott, 1991) or more (Sidow et al., 2000), though mandibular lower molars of genus Homo are commonly 2-rooted (Sidow et al., 2003). Both mesial and distal roots are maintained by 3-rooted lower molars, with a 3^rd accessory root on either side on the distolingual aspect or lingually between the mesial and distal roots. The 3^rd root is not a simple bifurcation of either the mesial or distal root tips. The accessory root can be quite small; it is usually about ⅓ the size of the normal roots (Turner, Nicholl & Scott, 1991). The 3^rd root usually occurs on the mandibular 1^st molar (3RM1) though may also occur on the lower 2^nd and 3^rd molars; referred to collectively as 3RM (Ferraz & Pécora, 1993; Carlsen & Alexandersen, 1990; Erkman & Kaya, 2012). According to Bailey et al. the 3RM may appear either unilaterally or bilaterally; the only twin study which Bailey et al. are aware of shows bilateral development in both twins, which suggests a genetic underpinning (Gabriel, 1948). The 3RM entered the clinical literature in 1844 (Carabelli, 1844), Bolk (Bolk, 1915) calling it radix entomolaris; it was codified into the Arizona Dental Anthropology System in 1991 (Turner, Nicholl & Scott, 1991).

The rarity of 3RM outside of Asia and the New World was confirmed by extensive clinical and bioarchaeological studies. The frequency of 3RM in populations that are Asian derived can exceed 40% (Aleut, Neolithic China), whereas the frequency from 0 to 3.4% in populations that are not Asian derived. In non-Asian H. sapiens the rarity of 3RM is low enough to be explained by mutation alone (Turner, Nichol & Scott, 1991). In northeast Asians and Native Americans the high frequencies of 3RM is a key feature that links the origins of Native Americans to Asia (Turner, 1971).

The 3RM has not been reported in the earliest H. sapiens from Asia (Liao et al., 2019) in spite of its high frequency in recent populations that are Asian-derived, and the trait has not been observed in Africa or Homo erectus in Asia.* It was noted by Bailey et al. that the lack of radiography of many specimens and the absence of the original Zhoukoudian remains make this conclusion preliminary. The earliest example of 3RM prior to the recent discoveries came from an Homo sapiens from the Philippines (Barker, 1978), possibly from the site of Tabon, which has fossil-bearing strata dating to 9 ka, 16.5 ka and as much as 47 ka, and jar burials that are  more recent (Dizon, 2003; Corny, 2008). The mandible that was originally described by Macintosh (Macintosh, Barker & Lamach, 1978) shows a bilateral 3RM that has an accessory root situated lingually between the mesial and distal roots (Barker, 1978). An explanation for its high frequency in Asia, given the lack of early evidence for 3RM, has been the relatively recent acquisition that postdates the origin of Homo sapiens and occurs well after their dispersal into Eurasia,

It is suggested by 2 mandibles that have recently been described that there was a more ancient origin for 3RM and 1 that precedes H. sapiens in the region. The individual from Xiahe, China that is newly described, identified as a Denisovan by palaeoproteomics (Chen et al., 2019), has a 3-rooted lower 2^nd molar (3RM2). ^† This individual has been dated to 160,000 BP. The Penghu 1 mandible from Taiwan, that has recently been described (190 to 10 ka), also exhibits a 3RM2 (Chang et al., 2015). It is clear that the morphology of Penghu 1 falls within the variation that has been described by previous studies (Turner, Nichol & Scott, 1991; Turner, 1971), though the authors suggest the 3RM2 differs from that described by turner et al. (Turner, Nichol & Scott, 1991): The 3^rd root appears lingually as an accessory root between the mesial and distal roots. “Archaic” features are retained by the Penghu mandible, including the receding symphysis lacking a chin, a mandibular corpus that is thick, and large mandibular crowns that are similar in size to Denisovans (Reich et al., 2010). These exceptionally large molars are coupled with agenesis of the 3^rd molar, as in the case of Xiahe (Chang et al., 2015). It is suggested by Chen et al. (Chen et al., 2018) for these reasons that Penghu 1 may also be connected to Denisovans. It is shown by both these mandibles that the 3RM anomaly was present in Asian hominins prior to H. sapiens in the region.

It is suggested by these fossils that have recently been reported that the 3RM

1)    Very likely originated in Asia and

2)    Evolved in a pre-sapiens population.

Moreover, it should be understood as a morphological trait that was transferred to H. sapiens through gene flow from Denisovans, until a 3RM is found in hominins that are more archaic.

It has been documented that gene flow took place between Denisovans and H. sapiens, including a mutation (at EPAS1) that is related to adaptation to high altitudes shared by a Siberian Denisovan and Tibetans of the present (Huerta-Sánchez et al., 2014). Importantly, one of the highest occurrences of 3RM (25%) is shown by Nepal in East Asia. As with the high altitude related mutation, which is retained as a result of positive selection (Huerta-Sánchez et al., 2014), the retention at high frequencies in Asia of 3RM may be related to selection for molar retention in populations with heavy masticatory loading (Turner, 1987). The lower frequencies of 3RM in recent populations with higher Denisovan introgression, e.g., Australia and New Guinea, but with demonstrably less masticatory robusticity (Antón, Carter-Menn & DeLeon, 2011), also explain such selection. Alternatively, frequencies of 3RM may be a reflection of indirect influences that result from selection on another trait under high selection, as has been suggested to be the case of the morphology of the incisor crown and EDAR in North and East Asians and in the New World (Hlusko et al., 2018). Bailey et al. argue that whatever the cause, the 3RM anomaly is an example of a morphological character in recent humans that can be traced clearly to this archaic admixture.

The 3RM is an Asian-derived character that can be traced definitively to Denisovans. There is, therefore, now very clear evidence that gene flow between archaic groups and Home sapiens had the result of transferring identifiable morphological features. A number of H. sapiens fossils that have recently been described from Asia that point to admixture with archaic humans as an explanation for the presence of primitive traits [e.g., Dushan (Wu & Poirier, 1995) and Tianyuan (Shang et al., 2007)]. Bailey et al. suggest that if 3RM was transferred from archaic humans to H. sapiens, other traits may also have been as well. The presence of “archaic features” in recent Asians that were in the past suggested to be continuity from Asian H. erectus (Frayer & Wolpoff, 1993; Lu, 1997; Wolpoff et al., 1994) may have also been obtained by introgression from Denisovans.

Sources & Further reading

Bailey, S. E., et al. (2019). "Rare dental trait provides morphological evidence of archaic introgression in Asian fossil record." Proceedings of the National Academy of Sciences 116(30): 14806-14807.