Peopling the Americas: Not “Out of Japan”

Figure 1 Comparison of Cooper’s Ferry projectile points (A, C, F, G, H) with late Pleistocene age Tachikawa-type stemmed points from the Kamishirataki 2 site on Hokkaido, Japan. (A) Stemmed projectile point haft fragment from LU3. (B) Illustration of Japanese Upper Paleolithic stemmed projectile point from the Kamishiritaki 2 site. (C) Blade fragment of projectile point from LU3. (D) Stemmed projectile point haft fragment from LU3.

(E) Illustration of Japanese Upper Paleolithic stemmed projectile point from the Kamishiritaki 2 site. (F) Stemmed projectile point from PFA2. (G) Stemmed projectile point from PFA2. (H) Stemmed projectile point from PFA2. (I–K) Illustrations of Japanese Upper Paleolithic stemmed projectile points from the Kamishiritaki 2 site (reproduced from Davis et al. 2019, figure 5; copyright AAAS 2019).

A “dual structure model” for the peopling of Japan is hypothesized by anthropologists, where the contemporary populations of Japan – including the mainland Japanese, the northern Ainu, and the southern Ryukyuan – are composed of two historical components: a very old hunter-gatherer population associated with Jomon pottery, spanning 16,000–2300 cal yr BP with even deeper roots in the early Upper Paleolithic of Japan (Adachi, Shinoda, and Izuho 2015), and a much more recent migration of East Asian rice farmers, the Yayoi culture, approximately 2300 cal yr BP (Habu 2004; Hanihara 1991). It is the earlier of the two components that is associated with the production of stemmed points in Japan 16,000–15,000 cal yr BP.

According to the “Out of Japan” model, warming climates following the Last Glacial Maximum (after 18,000–17,000 cal yr BP) invited northward expansion of the people who made the Incipient Jomon points in the Northeast Asian maritime region, who already had developed a successful coastal economy with a marine diet and watercraft.

They are assumed to have spread rapidly along the southern coast of Beringia (which has been characterized as a “kelp highway” (Erlandson et al. 2007)) and into the rich marine environments of the deglaciated Pacific Northwest of North America, bringing their distinctive stemmed points with them (Erlandson 2013).³

In our view, many elements of this model are highly plausible and some of it may be regarded as at least tentatively confirmed by research undertaken during the past two decades. We concur with Erlandson (2013) and others (Davis et al. 2019; Dixon 2001; Erlandson et al. 2007) that Native Americans probably dispersed initially along the Pacific Northwest coast with the aid of watercraft and subsequently spread down the Pacific coast of South America, and that the first occupants of mid-latitude North America probably pursued a coastal economy.

Moreover, whatever the biological relationship between the people who made stemmed points in Japan and California 15,000–13,000 cal yr BP, the similarities in their artifacts are striking (e.g., Davis et al. 2019, figure 5; Erlandson and Braje 2011, figure 4).

Given the explosive growth of genetic data in recent years, the time is right to re-evaluate hypotheses regarding the peopling of the Americas with a synthesis of biological and archaeological evidence (e.g., Pinotti and Santos 2020; Raff and Bolnick 2015). In what follows, we address the “Out of Japan” hypothesis with the data and methods of biological anthropology – both anatomical and genetic – and show that the people who made the Incipient Jomon artifacts are an unlikely source for Indigenous Americans.

We suggest the source populations for the latter must be sought elsewhere.

2. Craniometrics

Human cranial variation has a long and, originally, wrought history within the scientific investigation of the peopling of the Americas. In the nineteenth century, anatomists in the United States measured cranial size across Indigenous peoples in the Western Hemisphere with the intent of identifying differences in intelligence compared to Europeans (e.g., Morton and Combe 1839; see early overview by Hrdlička 1919).

Craniometrics gradually evolved to focus less on racialized questions and more on understanding the ancestral affinities of human populations. Twenty years ago, human cranial variation joined dental variation and molecular genetics as a key dataset for querying the biological affinities of the Indigenous people living in the Western Hemisphere (e.g., González-José et al. 2005; Herrera et al. 2017; Hubbe, Neves, and Harvati 2010; Hubbe et al. 2020; Kuzminsky, Coonerty, and Fehren-Schmitz 2017; Neves and Pucciarelli 1991).

Initially, craniometric evidence suggested that the earliest people occupying the Americas (< 8000 years ago) differed from later people, with the former having crania that were more anteroposteriorly elongated (e.g., González-José et al. 2005; Neves and Hubbe 2005; Neves and Pucciarelli 1991).

In contrast to other lines of evidence, these cranial shape data suggested that the earliest people in the Americas had a stronger biological affinity with people living in the South Pacific rather than East Asia, and perhaps evidenced two waves of migration (e.g., González-José et al. 2005; Hubbe, Neves, and Harvati 2010; Neves and Hubbe 2005; Neves and Pucciarelli 1991).

As larger sample sizes and more nuanced analyses became available, the morphological distinction between early and later Indigenous Americans was increasingly realized to be less obvious or abrupt than originally thought (Kuzminsky, Coonerty, and Fehren-Schmitz 2017, 2018). However, the craniofacial shapes of Indigenous populations living in the Americas did change over time, providing a rich dataset for probing the biological affinities and models of dispersion among populations within the Americas (e.g., Herrera et al. 2017; Menéndez et al. 2015).

In terms of understanding the deeper ancestry of Indigenous Americans, four skulls from Quintana Roo, Mexico, add an interesting twist (Hubbe, Neves, and Harvati 2010). These skeletal remains are from people who lived 13,000–12,000 cal yr BP on the Yucatan peninsula (Chatters et al. 2014). Two of the skulls show morphological affinities with North American arctic populations; another has strong similarities to Paleoamericans in South America; and the fourth has similarities to Europeans (Hubbe et al. 2020).

As the authors of that study note, “At the very least, it provokes researchers to reevaluate the validity of extrapolations made in the past” (Hubbe et al. 2020, 16).

To infer ancestry from anatomical variation, that variation would ideally be influenced by many different genomic loci and result from only neutral evolution. This is a high hurdle for human cranial variation, given the complex interplay of genetic, non-genetic, and developmental forces that influence its shape, and how intertwined cranial variation is with mastication, thermoregulation, mate choice, etc. (Lieberman 2011). Quantitative genetic-driven studies of human cranial variation enable us to unravel some of this intricacy (reviewed in von Cramon-Taubadel 2014).

Two recent studies are of particular relevance to the use of Indigenous American craniofacial variation to infer ancestry. De Azevedo et al.’s (2017) study of Indigenous Americans found that the observed craniofacial variation accords with a single ancestral source, supporting the “Beringian standstill” model (Tamm et al. 2007). They also found that the variation accrued since that diverse ancestral population deviates from a neutral evolutionary model, indicating that microevolutionary forces have been at play (de Azevedo et al. 2017).

Katz, Grote, and Weaver (2017) explored how the shift from foraging to farming may have influenced human craniofacial form globally. Results showed a modest but significant directional effect between populations that rely on the much softer versus tougher diets (Katz, Grote, and Weaver 2017).

Given the Katz, Grote, and Weaver (2017) results, it is quite likely that the diversity of subsistence strategies that Indigenous Americans adopted as they migrated across the Western Hemisphere contributed to the microevolutionary forces inferred by de Azevedo et al. (2017). Considering the vast latitudinal range of the Americas, climate likely played a role, too (Menendez 2018; von Cramon-Taubadel 2014), further obscuring evidence of ancestry.

In our view, craniometrics does not provide an adequate basis for identifying the source population(s) of Native Americans.

3. Dental anthropology

One hundred years ago, Aleš Hrdlička (1920) wrote a seminal paper on shovel-shaped incisors and demonstrated how a morphological crown trait could provide clues on long-term human population history. Specifically, he noted the widespread occurrence of the trait in Native Americans, which differs significantly from the lower frequencies and trait expressions typical of Euro- and African-Americans.⁴

Half a century later, Christy G. Turner II (1971) used another trait, a three-rooted lower first molar (3RM1), to develop a model for the peopling of the Americas,⁵ which subsequently became a component of the “three-wave model,” based on linguistic, dental, and genetic data (Greenberg, Turner, and Zegura 1986).

Aware of the limitations of single traits (or genes) for reconstructing population history, Turner and his students later identified and classified over three dozen crown and root traits that culminated in the Arizona State University Dental Anthropology System (ASUDAS) (Edgar 2017; Turner, Nichol, and Scott 1991; Scott and Irish 2017). This standardization ushered in a new era of global dental morphology research.

Detailed trait descriptions and the availability of plaques for many traits has significantly reduced observer error, a problem in pre-1970s research (Scott, Maier, and Heim 2016; Scott and Pilloud 2019).

Most dental morphological traits are either absent or exhibit a range of expression from slight to pronounced when present. Although twin and family studies show dental traits are under strong genetic control (Hughes and Townsend 2013; Paul et al. 2020; Townsend et al. 2009), they do not have simple Mendelian modes of inheritance like blood group antigens, serum proteins, and other simple genetic markers (cf. Roychoudhury and Nei 1988).

Therefore, they have been assumed to be quasicontinuous or threshold traits with polygenic modes of inheritance (Harris 1977; Scott 1973). For that reason, nonmetric dental characters are characterized by “trait frequencies” rather than gene frequencies. Trait frequencies represent the total frequency of presence, but in some instances, trait frequency is defined by “frequency above a breakpoint,” and consequently are studied as though they are a presence/absence phenomenon.

Although the identification of major gene effects on crown trait expression met with limited success in early studies (e.g., Kolakowski, Harris, and Bailit 1980; Nichol 1989, 1990), researchers have demonstrated that shovel-shaped incisors are partly controlled (ca. 19%) by the EDAR V370A allele (Kimura et al. 2009) that has effects on hair, skin, and teeth. Other dental traits, including lower molar cusp number, are also linked to this allele (Park et al. 2012).

Increasingly, dental trait evolution is being investigated as part of a pleiotropic milieu (e.g., Hlusko et al. 2018). Although population variation in crown and root traits has been dictated primarily by random factors (genetic drift, founder effect) associated with time since divergence and geographic isolation (Monson, Fecker, and Scherrer 2020; Rathmann and Reyes-Centeno 2020), some traits may be indirectly affected by natural selection acting on variables critical to survival and reproduction (i.e., they can act like “genetic hitchhikers” (Scott et al. 2018a)).

Researchers have recently used genomic data in concert with dental morphological data to reconstruct population history (Hubbard 2012; Hubbard, Guatelli-Steinberg, and Irish 2015; Irish et al. 2020; Rathmann and Reyes-Centeno 2020; Rathmann et al. 2017; Reyes-Centeno et al. 2017). Although there is no one-to-one correspondence in results, these analyses show dental morphological (nonmetric) traits produce high and significant correlations with neutral genomic data. Rathmann and Reyes-Centeno (2020) note some traits are closely in line with neutral genomic markers while other traits show patterns of variation consistent with the action of selection. Irish et al. (2020, 25) conclude:

Additionally, crown and root morphologic traits are highly resistant to change when populations migrate from one area to another, no matter what new climatic, dietary, and disease factors are encountered. When Europeans colonized the Americas, Africa, and Australia, their dental traits continued to exhibit a Eurodont pattern (Scott et al. 2013), one that provides a stark contrast to the teeth of the indigenous populations of the other three continents.

This is taken from a long article. Read the rest here: tandfonline.com

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