See:
https://www.sciencemag.org/news/2019...gins-monuments

"A new study suggests these megaliths weren’t created independently but instead can be traced back to a single hunter-gatherer culture that started nearly 7000 years ago in what is today the Brittany region of northwestern France. The findings also indicate societies at the time were better boaters than typically believed, spreading their culture by sea."

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What she did was sift through radiocarbon dating data from 2410 ancient sites across Europe to reconstruct a prehistoric archaeological timeline. The radiocarbon dates came mostly from human remains buried within the sites. The study looked not just at megaliths, but also at so-called premegalithic graves that featured elaborate, earthen tombs but no huge stones. Schulz Paulsson also factored in information on the sites’ architecture, tool use, and burial customs to further narrow the dates.The very earliest megaliths, she found, come from northwestern France, including the famous Carnac stones, a dense collection of rows of standing stones, mounds, and covered stone tombs called dolmens. These date to about 4700 B.C.E., when the region was inhabited by hunter-gatherers. Engravings on standing stones from the region depict sperm whales and other sea life, which suggests the precocious masons may also have been mariners, Schulz Paulsson says.
Northwestern France is also the only megalithic region that also features gravesites with complex earthen tombs that date to about 5000 B.C.E., which she says is evidence of an “evolution of megaliths” in the region. That means megalith building likely originated there and spread outward, she reports today in the Proceedings of the National Academy of Sciences."