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Climate Change May Not Have Caused Population Migration in Ancient Mexico, Genomic Study Finds

Copper Canyon Tarahumara Sierra Mexico

NEW YORK — A new genomic study of samples from pre-Hispanic times in Mexico have disproved the theory that a global warming episode about 1,100 and 900 years ago led to a population replacement in the northern frontier of Mesoamerica.

Following a period of severe droughts, the border between Aridoamerica in the north of Mexico and Mesoamerica in the central and southern regions of the country shifted southwards. Up until now, researchers had believed that this change in climate had led to the migration of people from northern Mexico, who were mainly hunter-gatherers, into regions of central and southern Mexico, where they replaced agriculture-based civilizations. 

"However, this hypothesis relies solely on archaeological data," corresponding author María Ávila-Arcos, a researcher at the International Laboratory for Human Genome Research at National Autonomous University of Mexico, and colleagues wrote. "Whether this change [in culture] was the product of migration or acculturation has been debated by archaeologists for years," they added.

For their study, published in Science on Thursday, the researchers generated genome-wide shotgun sequencing data from samples ranging in age from about 700 to 1,450 years from 10 individuals within Mesoamerica in central Mexico, and two from Aridoamerica, in the Sierra Tarahumara. Additionally, they reconstructed the mitochondrial genomes for 27 individuals across these sites, ranging in age from 2,300 to 700 years.

The findings showed genetic continuity in the ancient individuals from before and after the climate change episode.

"This contradicts the hypothesis of population replacement by Aridoamerican groups in this region and suggests that the local population stayed in their homeland despite the longstanding droughts" the authors wrote.

The researchers concluded that the pre-Hispanic population structure closely resembles the present-day Indigenous population structure in Mexico that clearly differentiates northern and central populations, and that the genetic structure of the populations inhabiting the Mexican territory has been conserved for at least 1,400 years.

It is possible that some populations may have resorted to a mining-based economy during the climate change period when the border between Aridoamerica and Mesoamerica shifted southward, the authors noted.

Meanwhile, the researchers also identified a contribution to pre-Hispanic populations of northern and central Mexico from two ancient unsampled "ghost" populations. This finding made it clear that the demographic events that gave rise to Aridoamerican and Mesoamerican populations are more complex than previously thought, according to the authors.

Ávila-Arcos told GenomeWeb that in the future, she would like to conduct similar genomic analyses on more pre-Hispanic samples and collaborate further with archaeologists. "I enjoy doing this kind of research in which I first start with the questions archaeologists already have, and then see what I can answer with the genomics data," she said.

In their paper, the researchers acknowledged "the need to involve Indigenous perspectives in delineating the regulations for these types of studies," as current Mexican law does not require consultation with Indigenous populations for projects that destroy archaeological material, and said they engaged communities in the area by giving public lectures about the genetic results.

In a perspective accompanying the paper, Bastien Llamas and Xavier Roca-Rada of the University of Adelaide in Australia, who were not involved in the study, noted that the work was a remarkable example of how research on a region in the Global South was led and conducted by predominantly local researchers.

"This is a considerable departure from collaboration between local scholars and laboratories from the Global North, which requires the export of samples and often leads to the relocation of local students and early-career researchers to the Global North for training," they wrote.