April 2021

Admixture models in the Mexican northwest

To study human admixture from the genetic point of view, America constitutes a benchmark. Within this space, regions of specific interest stand out, such as north-western Mexico, which includes the current Mexican states of Baja California, Sinaloa, Sonora, and Chihuahua. The first American settlers traveled through this region, in addition, it has been a collecting space for populations from different regions of the world. In fact, it is currently considered that the genetic composition of New World mestizos, among them those of the Mexican northwest, conforms to a tri-hybrid model, which postulates that European and African populations overlap on an ancestral aboriginal substrate.

On the other hand, and for a few decades, populations around the world have been genetically characterized through the HLA system (Human Histocompatibility System), responsible for human defence against pathogens and related to the rejection, or acceptance, of transplanted organs. This information, together with the development of specific software, allows the exploration of the miscegenation models beyond the current tri-hybrid model.

For example, the tetra-hybrid model, which proposes that in addition to the three populations involved in the tri-hybrid model, a fourth should be considered, the Asian one. Or even a penta-hybrid model, in which a fifth reference population, the Australian, would come into play. The methodology consists of determining which admixture model, taking into account different numbers of parental populations, supports the current genetic diversity of the mestizo populations of Baja California. For this, three Amerindian groups from the states of Chihuahua, Durango, and Sonora were used as indigenous reference populations; the Rarámuri, or Tarahumaras, who inhabit the wild regions of the Sierra Madre Occidental; the Seris, a population settled mainly at the mouth of the San Ignacio River and on Tiburon Island, in the Gulf of California; and finally, the Mayos, who inhabit the coastal regions between the Mayo and Fuerte rivers. For the remains of reference populations, individuals from the Iberian Peninsula have been used, as European representatives; populations from the Horn of Africa, which will mark the African ancestry; the Han ethnic group from mainland China, as Asian representatives; and, finally, the Australian Aboriginal peoples as parental populations of oceanic provenance.

Applying these principles to the Mexican northwest, the results fit unequivocally to a penta-hybrid admixture model, that is, at least five parental populations intervened to configure the genetic structure of the mestizo populations of Baja California. But these contributions were made in different periods of time, which allows us to propose, explore and reconstruct the chronology of historical and demographic events.

It starts from an ancestral Amerindian substrate from Asia that reached the new continent, crossing the Bering Strait, about 15,000 years ago. On this substrate, European populations that arrived in the region at the beginning of the 16th century overlapped and dragged African populations through the slave trade. From the 17th century, the fourth reference population was incorporated, the Asian one. The presence of this last component could be related to mainly Filipino populations that reached the shores of Baja California through the so-called «Galeón de Manila» («Manila Galleon»), a maritime trade route opened by the Spanish between America and Asia. Although there is another alternative, that this group came from China and arrived in the 19th century, during the industrial development of the region.

The presence of a tiny Australian substrate constitutes an interesting anthropological problem, with three possible explanations that are not necessarily exclusive. On the one hand, the contribution may be recent and would be made through mestizo individuals from that region. One of the alternative proposals forces us to re-evaluate the hypothesis of the early settlement of America. This theory defends the idea that the first settlers of the continent crossed the Bering Strait between 40,000 and 45,000 years ago. These would be groups belonging to the same genetic background as the humans who colonized Sahul (Australia and Papua New Guinea). This archaic group would be represented in America by the Pericúes, a human group characterized by an elongated skull or dolichocranium. This town was distributed in the southern regions of Baja California and disappeared in the 18th century. An interesting detail to highlight is that the Jesuits who accompanied the first European explorers of Baja Californiaincluding Hernán Cortésdescribe them as a group that is culturally, linguistically, and socially far removed from other groups on the peninsula.

But there is a third explanation, the possibility of a transoceanic contact between the Pacific islands and the new continent. This idea was raised by the Norwegian Thor Heyerdahl in 1947 on the now mythical Kon-tiki expedition. The Nordic explorer built a raft according to traditional indigenous techniques and in 101 days of navigation completed the route from Peru to the Tuomatou atoll. This hypothesis was rejected for decades, but the most avant-garde genomic studies have forced it to be recovered. These studies have made two surprising contributions. On the one hand, the presence of Amerindian genetic material in the autochthonous inhabitants of Rapa Nui (Easter Island), although in this case, the authors propose that this contribution was made in postcolonial periods, during the 18th and 19th centuries. What is difficult to justify is the unequivocal presence of Polynesian genetic traces in indigenous populations of the Amazon rainforests, this evidence could only be justified by an ancestral transoceanic contact between the islands of Polynesia and the South American subcontinent.

Today the debate is still open, but the proposal confirms, first, the complex history of our species and, second, the importance that the phenomenon of miscegenation has had in shaping current human populations.

 

Amaya Gorostiza and Antonio González-Martín are members of the ‘Evolution and Conservation Biology Research Group’ of the Department of Zoology and Physical Anthropology, Faculty of Biology, Complutense University of Madrid (Spain)