From "Collapse" - Martha's Vineyard Island

I read Jared Diamond's "Collapse" everyday. Every part of the book is cool and interesting. Not only its main contents: history, geography, archaeology and biology of many countries and areas in the world but also its sub topics. I learn much information from such sub topics of this book as follows:

Main and sub topics
Diamond contends some reasons why ancient people destroyed their environments and their societies collapsed. One reason, in particular, is a society that relies on another specific society ends the trade relation. He gave us an example of Pitcairn Island in Polynesia. When the island was "found" by westerners in 1790, there were no people on the island, but some ruins showed them ancient human habitation. Why did the islanders vanish? The answer is that Pitcarn was totally depended on the nearby Mangareva island for trade. A massive increase in the population of Mangareva island eventually led to the destruction of the island environment, thus, ending its trade with Pitcarn island and other island. Pitcairn Island is so small and has little natural resources except fine stones for stone tools. For example, it is hard to catch fish because the surrounding sea bottom falls of steeply. Thus, people couldn't live without trade. That's the main topic of the chapter.

As for sub topic, it is the sentence about the small world like Pitcairn Island: "If the small population did ignore incest taboos, the resulting inbreeding may have caused congenital physical anomalies to proliferate, as exemplified by deafness on Martha's Vineyard Island of Massachusetts or on the remote Atlantic island of Tristan da Cunha."
I didn't know of Martha's Vineyard Island and Tristan da Cunha. I felt they were interesting so I searched and visited some websites.


Martha's Vineyard Island
The story of this island began in the mid-1600s. Many puritans went to this island from Weald, Kent in England, which was famous for many deafness caused by a genetic mutation. In 1854, according to a survey, the United States national average was one deaf person in 5728, but in Martha's Vineyard it was one in 155. Because the gene for deafness was recessive, such a large number of deaf people meant their parents must have had a common ancestor.

In 1881, a scholar studied deaf people in Martha's Vineyard Island and concluded that the deafness was caused by genes. So he recommended that deaf men don't marry with deaf women in the view of eugenics. His name is Alexander Graham Bell, the inventor of the telephone.

But the people of the island were well adapted to such a situation. People had original sign language and all residents, whether normal or deaf, could use it. The last person who could use this language died in 1952 and the language vanished, but this case is still appreciated as a successful barrier-free approach.