Since the beginning of human migration, the Iberian Peninsula—home of modern-day Spain and Portugal—has been a place where the cultures of Africa, Europe, and the Mediterranean have mingled.

In a new paper in the journal Science, a group of 111 population geneticists and archaeologists charted 8,000 years of genetics in the region. They paint a picture that shows plenty of genetic complexity, but that also hints at a single mysterious migration about 4,500 years ago that completely shook up ancient Iberians’ DNA.

The team searched DNA evidence for clues to how and when various populations became part of the Iberian Peninsula’s gene pool. They sequenced the genomes of 271 ancient Iberians, then combined that information with previously published data about 132 other ancient peninsula dwellers.

The picture was more complex than they had anticipated.

The men from the steppes

Beginning in the Bronze Age, the genetic makeup of the area changed dramatically. Starting in about 2,500 B.C., genes associated with people from the steppes near the Black and Caspian seas, in what is now Russia, can be detected in the Iberin gene pool. And from about 2,500 B.C. much of the population’s DNA was replaced with that of steppe people.

The “Steppe Hypothesis” holds that this group spread east into Asia and west into Europe at around the same time—and the current study shows that they made it to Iberia, too. Though 60 percent of the region’s total DNA remained the same, the Y chromosomes of the inhabitants were almost entirely replaced by 2,000 B.C. That suggests a massive influx of men from the steppes, since Y chromosomes are carried only by men.

“It looks like the influence was very male dominated,” says Miguel Vilar, a genetic anthropologist who serves as senior program officer for the National Geographic Society.

Who were these men—and did they come in peace? Vilar, who was not involved with the study, speculates that the steppe men may have come on horses bearing bronze weapons, hence ushering in the Bronze Age to the area. He compares the migration to the one the indigenous peoples of North and South America faced when the first Europeans landed in the 1490s.

“It shows that you could have a migration all the way across the whole continent (of Europe) and still have a heavy influence on this far extreme,” he says.

Although bronze came into use in Iberia around that time, no other distinct traces of steppe culture have yet been found. The study did show that people in present-day Basque, who speak Western Europe’s only non-Indo-European language, carry genetic markers closely related to those of the steppe people. And unlike modern Spaniards, modern-day Basques don’t show the same amount of genetic mixing that happened on the peninsula over the centuries.

The team also found a single individual with North African DNA from a site in the middle of Iberia. His bones date to about 2,500 B.C.

“At the beginning I thought it was a mistake,” says Iñigo Olalde, a population geneticist who led the study.

When he replicated his work, it checked out. The presence of that lone African suggests early, sporadic interchange between Iberia and North Africa, making sense of archaeological discoveries of African ivory at Copper-Age Iberian digs. But the team thinks that North African ancestry only became widespread in Iberia in about the last 2,000 years.

Ice Age diversity

The study forms a complex picture of the genetic history of Spain—one that’s reinforced in a companion piece published in the journal Current Biology. In that study, researchers from Spain and Germany found that hunter-gatherers and farmers living on the Iberian Peninsula also were more genetically diverse than previously thought. They found evidence that different hunter-gatherer cultures mixed on the warm Iberian Peninsula, which they used as an Ice Age refuge 19,000 years ago. Newer farmers to the area mixed with the hunter-gatherers later.

”The DNA was a surprise,” says doctoral student Vanessa Villalba-Mouco, an archaeogeneticist who led the research for the Max Planck Institute for the Science of Human History in Germany and the University of Zaragoza in Spain. “Clues about what happened in that moment help us understand the evolution of the next period. We need to sample more individuals to know their history in a more accurate way.”

Ancient DNA work “is helping us deconstruct the idea that that we have distinct geographic populations like Africans or Asians or Europeans,” says Vilar. “Not only are people living in areas like Iberia heterogeneous, but they were the product of different waves of migration themselves.”

For Olalde, the work was an unprecedented chance to explore the genetic history of the place he calls home. “Being able to do this study was a dream for me,” he says.