For millennia, couples have had to decide where to live.
"For the vast majority of human history," says Lara Cassidy, a geneticist at Trinity College Dublin, "societies were centered around ties of kinship, so you're deciding whose family you're going to live with."
In Britain, during the Neolithic Period (characterized by the introduction of agriculture) and the Bronze Age, which dated from about 4000 to 800 BCE, prehistoric human societies tended to be patrilocal. That's "where women move," says Cassidy. "They leave their home upon marriage, and they go join the village, the community of their husbands."
This is why Cassidy and her colleagues were surprised to find remains of a Celtic tribe that lived during the Iron Age in Britain from around 100 BCE to 100 CE where it appeared, after studying their DNA, that women were at the center of their social network. The research is published in Nature.
"It's a really rare pattern." says Cassidy. "Never seen before in European prehistory to have so many people all related through the female line. It's adding to that pile of evidence that women were able to wield quite a lot of social and political influence in these societies."
A frequent challenge for women
Patrilocality tends to be a more difficult arrangement for women. "It separates them from their families," says Cassidy. "You've lost your childhood support network. You're coming in as a relative stranger."
Researchers know that women joined their husbands' communities by examining the ancient DNA of groups that were buried together from the Neolithic Period and Bronze Age.
"When most of the men are sharing the exact same Y chromosome," says Cassidy, "which is passed from father to son to father to son, that means they all descend from a recent male ancestor through the male line."
After the Bronze Age came the Iron Age. In Britain, it started around 800 BCE and it's a period where relatively little is known about how societies were structured, though there are hints that at least some women in Britain did have a degree of influence.
"Sometimes there's a knee jerk reaction, whenever you hear about a female ruler or find a very wealthy burial to assume it's an exception or that she just must have been somebody important's wife rather than important in her own in her own right," says Cassidy.
A special burial site
Cassidy was eager to collaborate on studying the remains of the Iron Age burial site of a Celtic tribe called the Durotriges dating from about 100 BCE to 100 CE in what is now southern England. "Cemeteries of unburnt burials are quite unusual," she says. "The funerary customs in Iron Age Britain seem to have been quite diverse, but there's a lot of cremation and scattering of remains in the landscape. So it was quite a unique opportunity to get to sample loads of members of the same community."
Cassidy and her team sequenced the ancient DNA of more than 50 individuals from the dense bone that surrounded their inner ears.
"When we finished the first quite big batch of samples, we immediately saw that there was something special about this site on the screen," she recalls.
The men in this community didn't share a Y chromosome. Rather, individuals shared mitochondrial DNA, which is passed to children from their mothers.
"And that's when I was like, 'Oh my God,' " Cassidy says. "This is a community where a lot of people are related along the female line. This is matrilocality and it's rare. I was not expecting that."
Matrilocality is the opposite of patrilocality—men left their families to live with their wives. The women stayed put in their communities.
"So they're not just mothers and wives," explains Cassidy. "They're also daughters and sisters and cousins. They're embedded in a much wider network of relatives and people they've known since birth. So they have a much greater support system."
This wasn't a matriarchy where women necessarily dominated positions of authority, but Cassidy says they had status and influence over family finances, decisions, and property.
She and her colleagues found the same thing when they inspected hundreds of Iron Age genomes from cemeteries across Britain. "The majority of people buried there were the descendants of a small number of women," Cassidy says. "That's when we're like, 'This is widespread on the island. This is probably a custom that stretches back many centuries.' "
"The results are intriguing because they show a very, very different scenario in comparison to other regions of Europe," says Marta Cintas Peña, a prehistoric archaeologist at the University of Seville who didn't contribute to the research. Still, she cautions that the data record for this time period is sparse. "I wouldn't say that before it was patrilocality everywhere and after that, there was a change," she says. "I think that we need more data to confirm or reject that."
Of language and war
"I was impressed by the article," says Carol Ember, a cultural anthropologist and president of the Human Relations Area Files at Yale University who wasn't involved in the study either. "But I'm not surprised that there were matrilocal societies in the past."
"That's because although matrilocality isn't common, it does make up some 15% of the anthropological record," including in Central Africa and among certain Native American communities, she says.
Ember says there's something else that's interesting about matrilocal societies. They often speak a different language from their neighbors (in this case, Celtic) — evidence they probably migrated from somewhere else. "If you intruded successfully into other people's territory," she says, "it means that you would have to be pretty successful at war."
Matrilocal societies, it turns out, tend to do well at fighting external wars. Ember, Cassidy and others suggest this is presumably due to a lack of internal feuding and broader tribal unity. "It is not definite but it's a good theory," says Cassidy.
Still, much remains unknown about this Celtic tribe, as well as the many groups of prehistoric humans scattered across space and time., but Cassidy is convinced that ancient DNA will help unravel some of that mystery.
"I think there's going to be loads more surprises," she says, "and we need to stay open minded as we start to delve into it."
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