From its founding in 1640 through the end of the 1800s, people who were born in Chilmark, a small town on the western end of Martha’s Vineyard, also tended to die in Chilmark.

Two of those people were the children of Jonathan Lambert, a man who had come to Chilmark from Kent, England, in the late 1600s. According to island records, Lambert was deaf; his children, born after his arrival, were the first congenitally deaf residents of Martha’s Vineyard. They were also the beginning of a language and deaf culture unique to the island—one that used to thrive, but is now extinct.

For its first couple centuries, Chilmark was a rural fishing village, isolated from the other towns on the island. Getting to Chilmark from anywhere else “was probably a solid day’s journey over bad roads on a horse,” said Bowdoin Van Riper, the librarian at the Martha’s Vineyard Museum. Other towns had their own ports, which meant a consistent flow of outsiders coming in and out on ships; Chilmark had none. In seclusion, its residents married and had children almost exclusively with one another, and the Lamberts’ hereditary deafness soon spread throughout the town. By the middle of the 19th century, one in every 25 people in Chilmark was deaf. In the U.S. overall, by contrast, that number was roughly one in 5,700.

But Lambert’s legacy was more than genetic. He came to the U.S. speaking what historians presume was a regional sign language from his home in Kent; over the years, it evolved and spread into what would become Martha’s Vineyard Sign Language. And while one in 25 people were deaf, something closer to 25 in 25 knew how to sign. Long before the development of American Sign Language, they used sign as naturally as spoken English, and in every combination: Between deaf people, between deaf and hearing, and even from one hearing person to another. The language didn’t belong to the deaf community; it belonged to the town.

“People tended to think of the deaf folks in Chilmark as individuals first,” Van Riper said, “and not about their disabilities, except in a peripheral way. No different than someone who’s very tall or only has one eye.”

Mainland scientists who heard of Chilmark were puzzled by it, including Alexander Graham Bell, who conducted genealogical research on Martha’s Vineyard in the 1870s in an attempt to isolate the cause of the deafness. “If you look at his research notes … it’s basically page after page of, ‘This person was this person’s grandfather and this was his great-grandfather,” Van Riper said. Mendelian genetics hadn’t yet gone mainstream, “so the actual details of how conditions like deafness were inherited from one generation to another were very poorly understood.”

To the people of Chilmark, though, the remarkably high concentration of deafness wasn’t something that needed to be understood, because it wasn’t remarkable at all. Largely cut off from the rest of the world, they didn’t know the difference.

“These were small-time farmers and coastal fisherman, and by and large they didn’t leave written records of their thoughts behind. It may well have been that they sat around the fire at the end of the day shooting the breeze and said, ‘Why do you suppose it is?’” Van Riper said. “On the other hand, there’s reason to think that it never occurred to them to ask the question.”

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Among the islanders, signing was considered a life skill, like knowing how to fish, more than a formal language to be taught. “It was passed on to kids as part of, ‘Here’s the stuff you need to know to make a living in this corner of the world,’” Van Riper said. Children picked it up from their parents; no records indicate that it was ever taught in schools.

No records, in fact, indicate much of anything about Martha’s Vineyard Sign Language. Nearly all that’s known about it has come from oral histories from people who lived during the peak of the island’s deaf population. The last known person with Lambert’s hereditary deafness, Katie West, died in 1952, and the pool of hearing people who still know the language is dwindling. There are no photographs, videos, or diagrams that preserve it.

“That’s the problem with the history of sign language: It’s ephemeral,” said Nora Groce, a medical anthropologist at University College London. “It’s not like a written language where you can go back 3,000 years.”

In the 1970s, Groce, then a graduate student at Brown University, spent three years traveling back and forth from Martha’s Vineyard to interview the island’s oldest residents for her book, Everyone Here Spoke Sign Language. Even then, much of the language was already lost: “We had vestiges of it, a few gestures,” she said, but “I couldn’t have reconstructed it.”

At the time Groce began her research, Martha’s Vineyard Sign Language had been in decline for nearly a century. In part, it was because the isolation of Chilmark, and the island overall, had eroded—infrastructure improvements made it easier to travel from town to town, and the tourism industry, which took hold in the late 1800s, had broken down barriers between the island and the mainland.

It was a change that allowed for new genes, but also for new ideas—and on the mainland, the concept of deaf education was beginning to take hold. In 1817, the American School for the Deaf—the first of its kind in the U.S.—opened in Hartford, Connecticut, laying the groundwork for the development of a national sign language. The school in its earliest years was a Babel of gestures: Some students came from communities like Martha’s Vineyard that had developed their own languages; others had made up systems of signing within their households; the school’s first teacher, Laurent Clerc, used the standardized French Sign Language. Eventually, these three strains merged into what would become American Sign Language, supplanting the regional languages.

Most of those languages vanished, but the formation of American Sign Language gives some clues as to how they had developed in the first place. In the 1910s and ‘20s, the National Association of the Deaf recorded sign language on film in response to the threat of oralism, the idea that deaf people should be educated primarily in spoken language.

“When moving-picture technology came out, the deaf community rallied around that and saw that as the perfect way to preserve sign language,” said Patricia Clark, an American Sign Language professor at the University of Rochester. The nascent national language survived, but the precautionary effort left researchers with a firsthand look at its evolution. “We have examples of signers from the second generation of ASL to the fourth generation on the films, records from way back when, so we’ve been able to study how the language changed between those generations”:

The patterns go from people using lexical phrases, where you combine three or four signs together to express one concept or one word. These lexical phrases become reduced over time through generations, become more compact, and eventually look as if they are a single word or a single sign.

Worldwide, there are still some insular communities that have retained their own sign languages, one of the most well-known being the El-Sayed Bedouin tribe in Israel. But by and large, Groce said, today’s regional sign languages can no longer be studied as systems that evolved in isolation; often, they’re hybrids of older community-sign systems, emerging standardized national languages, and sometime global influence.  “For example, in a country like Mozambique, which is Portuguese-speaking, you’d think the sign language there would be related to Portuguese sign language,” she said. But the deaf in Mozambique speak something closer to Finnish sign language, a relic from the presence of Finnish missionaries in the mid-1900s.

“Signs are not arbitrary. They have a history to them,” Groce said. But “the history of a sign language is going to be complex and unknown. And a lot of it is going to be unknowable.”