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Ancient DNA maps how prehistoric populations shifted socially and politically

Researchers tested the DNA of nearly 300 ancient Iberians to study the link between dynamic shifts in population and the major social and political changes of the third and second millennia B.C.

(CN) — By analyzing the DNA of ancient Iberians who lived during the Copper and Bronze ages, a new study shows how the populations of prehistoric cultures in Europe and western Asia changed socially and politically over time.

The Copper Age is a transitional period from the late Stone Age to the Bronze Age. During the Bronze Age, beginning at around 3000 B.C., humans made advances in metalworking after discovering bronze, a copper and tin alloy. That led to many other advances in architecture, art, textiles, as well as warfare and organized government and law. 

These advances made it a period in which large-scale social and political changes came about, and with a large diversity of settlements and fortifications, there was substantial demographic growth.

By around 2200 B.C., the so-called El Argar culture emerged in southeastern Iberia. The culture is one of the first state-level societies on the European continent, characterized by large, central hilltop settlements, distinct pottery, specialized weapons and bronze, silver and gold artifacts. The culture also held an intramural burial rite, in which they buried their dead in a variety of tombs.

A team of researchers set out to test the DNA of nearly 300 ancient individuals discovered during archaeological explorations to study the relation between dynamic shifts in population and the major social and political changes of the third and second millennia B.C.

The study, published Wednesday in the journal Science Advances, found that the bulk of the data shows that Bronze Age individuals are a mix of local Iberian ancestry from the Copper Age, or Chalcolithic period, with a smaller part of incoming ancestry from the European mainland. But the paternally inherited Y chromosome lineages show a complete turnover, which may be linked to the movement of steppe-related ancestry that is also observed in other parts of Europe.

“While we knew that the so-called ‘steppe’-related ancestry, which had spread across Europe during the third millennium B.C., eventually reached the northern Iberian Peninsula around 2400 B.C., we were surprised to see that all prehistoric individuals from the El Argar period carried a portion of this ancestry, while the Chalcolithic individuals did not,” Wolfgang Haak, senior author and principal investigator of the study, said in a statement. 

Vanessa Villalba-Mouco, postdoctoral researcher at the Max Planck Institute for Evolutionary Anthropology and the Institute of Evolutionary Biology, said the team also found ancestral lineages that were traced to central and eastern Mediterranean and western Asia.

“We cannot say exactly whether these influences arrived at the same time as the steppe-related ancestry,” she said in a statement, “but it shows that it formed an integrative part of the rising El Argar societies, attesting to continued contacts to these regions.”

But, whether the genetic shift was brought about by migrating groups from north and central Iberia or by climatic deteriorations that affected the eastern Mediterranean around 2200 B.C. is what researchers are still trying to determine.

“It would be foolish to think that it can all be explained by a simple, one-factor model,” Roberto Risch, professor at the Autonomous University of Barcelona and senior author of the study, said in a statement. “While the temporal coincidence is striking, it is likely that many factors played a role.”

One of those factors could be pandemics, such as early forms of the plague, evidence of which has been found in other regions of Europe around the same time. 

While not found among the tested individuals in southern Iberia, the plague could be a cause or driver for the movement or disappearance of other groups in the region. Though, evidence of infectious diseases and other illnesses have been found in the bones of El Argar people, whose demographic profile was “remarkably young,” according to exhibits in the Museo de Almería.

Examining skeletal remains, scientists have determined that half of the population died before the age of 6, according to the museum, likely due to chronic diarrhea and other infections, as well as anemia.

Only a quarter of the population reached adulthood, though some people lived to be over 60 years old. Life expectancy for men was about 40, and slightly shorter for women, likely due to complications with pregnancy and childbirth.

Those who made it into adulthood suffered from diseases associated with workload and diet, such as degenerative osteoarthritis, osteoporosis, traumatisms, enthesopathy, caries, periodontitis, abscesses, and dental calculi, the museum exhibit details.

Even with short life expectancies, the archaeological record of the El Argar group shows a clear break with previous Chalcolithic traditions with early state-like formations and an increase in social stratification, patterns of which are observed in other parts of early Bronze Age Europe, researchers note.

"This suggests a structured restart or resetting following some form of crisis or unstable, highly dynamic times,” Haak said.

For example, burial rites changed from communal to single and double burials, and elite burials indicate the formation of strong social hierarchies. 

Additionally, while earlier studies have found that elite women of the El Argar culture may have held significant power and even been rulers, Wednesday’s study found that males are on average more closely related to other people at the site, indicating that the group was likely patrilineally structured.

Such a social organization could explain the stark reduction of the Y chromosome lineage diversity, as Y chromosomes are passed down from father to son, researchers noted.

This lines up with the earlier study, which suggests that El Argar dominant men might have played an executive role in the culture, even though the women were likely in charge of the ideological ruling and possibly the government.

That theory is evidenced by the funerary goods with which the people were buried. The women of the dominant class were buried with silver diadems on their heads while the men were often buried with a lower quality sword and dagger — representations, researchers noted, of the most effective instrument for reinforcing political decisions.

Specifically, researchers found two individuals found in a grave — called grave 38 — were determined to be genetically unrelated, but they did share a daughter who was found buried near them. Next to them was a range of about 30 valuable and prestigious objects, many of which were made or embellished with silver and almost all belonging to the female. They included a complete repertoire of jewels and personal objects, such as bracelets, earlobe plugs, necklaces, spirals and containers with animal offerings. The most outstanding item was a silver diadem found on the head of the female.

The couple appeared to have died simultaneously or close together, with the woman having several congenital abnormalities and markings on her ribs that could indicate she had a pulmonary infection at the time of death, the study found. The man, on the other hand, had wear and tear on his bones indicative of extensive physical activity, possibly horse riding.

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