The stones have stood silently for thousands of years, arranged in rows and circles or balanced atop one another, often oriented to face the rising sun. Some 35,000 symbolic arrangements with similar architectural features have kept watch over ancient graves and sites across coastal Europe, from a snow-swept Swedish hilltop at Haväng, high above the Baltic Sea, to the sun-drenched shores of the Mediterranean.

Because their Neolithic and Copper Age creators—and their motivations—are lost to the mists of prehistory, the stones have invited speculation for centuries. Who built them? Is some single group of people responsible for launching this type of striking stone architecture? Or did multiple cultures separated by hundreds or thousands of miles develop the practice independently?

A sweeping new study of megalithic monuments across Europe suggests that such burials originated in northwest France, and the practice of building them spread along the continent’s coastlines in several migratory waves.

Bettina Schulz Paulsson, an archaeologist at the University of Gothenburg, reexamined some 2,410 radiocarbon dating results that have been assigned to Europe’s megaliths and put them through a Bayesian statistical analysis. Based on the picture the data present, Schulz Paulsson believes that the megaliths were first constructed by dwellers of northwest France during the second half of the fifth millennium BC. From this single origin, her analysis suggests, the practice of constructing standing stone monuments spread during three major periods via what may have been surprisingly robust maritime travel routes.

Soon after their initial appearance, the stone structures spread in France and to parts of the Iberian Peninsula and Mediterranean. During the first half of the fourth millennium BC, thousands of passage graves appeared on the Atlantic coasts of the Iberian Peninsula, the British Isles and France. Finally, during the second half of that millennium, megalithic architecture began to appear as far north as Scandinavia and modern Germany.

Europe’s prehistoric stone structures have long been the subject of legend, attributed to everything from giants to aliens to Satan. Early modern scientific theories from the 17th and 18th centuries also held that the megaliths originated with a specific group of people. These ancients, thought to be from the Near East, the Mediterranean, or elsewhere, depending on the theory, were believed to have spread their monument building practices over sea routes around ancient Europe.

But such theories began to change with the proliferation of radiocarbon dating in the 1970s. Based on the dates assigned to scattered megalith sites across the continent, scientists began to create a new map of their origins. Rather than a single group of people spreading the practice, it seemed that different people in different regions must have independently taken to expressing themselves with stone structures around the same time. Portugal, Andalusia, Brittany, England, Denmark and Ireland were all suggested as locales where the practice developed independently based on the timing of the sites' construction.

Durham University archaeologist Chris Scarre explains that this theory has gradually lost much support over the past 20 years due, at least in part, to questions about the accuracy of past dating.

“I think as more dates have become available, and people have become more critical about which dates are really reliable, it has come to seem that the sites in northern and western France are indeed older than the other groups,” says Scarre, who wasn’t part of the new study.

Schulz Paulsson’s research suggests that not only was northwestern France the origin of such megalithic structures, but the practice spread out from the region, likely borne on ancient boats navigating the Mediterranean. Though multiple population centers made the decision to build such structures, the findings provide more evidence that the idea for the European megalith style of burial has a singular origin.

“It’s not quite 100 percent pinned down, and there’s always other research to do, but that [theory] seems like a very plausible scenario,” Scarre says. “This study falls in with the more accepted idea that there are links between these different regions with megalithic monuments. The challenge is to understand how those links worked.”

Schulz Paulsson spent ten years travelling around Europe, meeting with scientists and devouring megalith research studies in 11 different languages to try to paint a broad picture of how and when the monuments appeared on a European scale. “People tend to focus on work in their own regions,” she says. “To bring all this together was a lot of work, and some people said I was a bit crazy to take it on.”

Using modern techniques, she analyzed 2,410 existing radiocarbon dates, recalibrating them to be more precise and looking for data that may have been sampled in error.

“The problem was that if you build a megalith, it’s an intrusion into the ground,” she explains. “We know now today that often megaliths are built on old settlement layers, so we have pre-megalithic layers and then the megalith. So some past researchers were mixing up older sample materials, and their data had nothing to do with construction of the megalith itself because it was too old.”

The materials typically used to date these megalithic tombs are human bones or charcoal. Generally speaking, human remains found in chambers are a safer bet to reveal the date of a tomb’s construction than remnants of fires which might have been burned at the site during other eras. But sometimes human remains aren’t present or appear to have been disturbed.

Fortunately, many other clues exist to supplement dating efforts. Schulz Paulsson also delved through excavation reports from megalith sites across the continent, looking for crucial contexts that could help make the dating more precise. “If you date a megalith it’s really difficult, you have to look at the whole package. So I was looking not only at the [Carbon-14] reports, but I was looking at the cultural materials. I was looking at the funeral rites. I was looking at the architecture. All this together, the whole package, is giving you the real idea.”

In ongoing studies, Schulz Paulsson is also comparing the art associated with such sites, looking for patterns among engravings, symbols and images that might help to recreate the ancient movements of people and ideas—and maybe even yield some new insights about the intent of the megaliths.

There’s been no shortage of speculation as to why the practice spread, Scarre says. “People as far back at the 19th century said this could be seen as a ritually related thing, so maybe what it’s spreading is a kind of religious idea. It may have something do to with social structures. These are very impressive monuments, so maybe it’s something to do with patterns of prestige or social emulation.”

It may be that some of these ideas spread among peoples, to be adapted locally by cultures in different regions. That scenario would match the findings in the field.

“One of the puzzles about it all, whether or not you’re entirely convinced by the dates, there’s still the issue of why is it that the monuments are built in strongly regional architectural traditions,” Scarre says. “Iberian tombs fall into several series, but they are a bit different from the ones you’d find in France, which are a bit different from those you’d find elsewhere and so on.”

Schulz Paulsson hopes it might be possible to untangle more of these ancient interchanges of people or ideas by an ongoing study of megalithic art, the engravings and paintings found adoring these ancient sites. “We’re collecting the symbols and images we have in Europe, and comparing the patterns and combinations we see in different regions,” she says. “The interesting thing so far is that only in northern France, which I am showing as the origin of the megaliths, do we have boats depicted. So I find that really fascinating.”