"It exists for cultural reasons," Dr James said. "It is used in mourning, it is used around sacred objects and in dances. Everything has a dance, so everything has a sign. It is used at a distance so you can communicate without shouting, or in a noisy car or plane." It was also used in hunting so "you (can) creep up on your prey without them knowing you are coming. It was very likely how locals plotted to attack one of the early nagging missionaries," Dr James said. Over thousands of years, hand talking adapted to change, with a sign for everything important to those living on land and sea country. It includes an aggressive sign for a stingray, conjuring how the whip tail is gripped with the teeth, as a spear thrower is hooked around the marine animal's big barbed spike and ripped off. It has developed signs for iPads, Facebook and mobile phones. Air conditioning is depicted with a sign for a box plus someone shivering. Some signs involve a lip point, foot gestures and eye rolling.

In a last-ditch effort to save the language, Dr James has started a GoFundMe page to complete an illustrated handbook of 500 of the 1700 signs frequently used. His goal is to raise $38,000 by September 23, the International Day of Sign Languages in what the United Nations has declared the International Year of Indigenous Languages. A copy will be given free to every local school, homeland and library so everyone can learn the culture embodied in these signs. Traditional ways: Daisy Burarrwanga signs djulngi (dear, good, fine, nice); Sheridan Bukulatjpi signs djiwarr (above, sky, heaven); Fred Djamarrandji signs wanga (speak); and Michael Ganambarr signs marandjalk (stingray). Credit:Therese Ritchie "It is a way of life that is completely disappearing yet it is the greatest treasure that Australia ever had," said Dr James, who has been living and working in the Crocodile Islands, off the coast of Arnhem Land, for 25 years. "Australia leads the world in the destruction of Indigenous languages. We have no idea how many we have lost, but we will save this one."

When Dr James arrived in north-east Arnhem Land, only a handful of people spoke the traditional local Yan-nhangu language. Their leader was Laurie Baymarrwanga, known as Big Boss, who was born on Murrungga, the largest of the outer Crocodile Islands 500 kilometres east of Darwin, around 1917. Everyone, though, used sign language. Hand talking is dying as more Indigenous children are educated in English-speaking schools, more homes with walls are built and children spend more times on electronic devices. Big Boss spoke nine Indigenous languages. She forced Dr James to learn the languages - spoken and sign - by teaching him the culture. "[Bentley James] was unable to catch a fish, collect his dinner or even light the fire and so I had to teach him everything as he was helpless and illiterate," she said. Recognised in 2012 as the Senior Australian of the Year, Baymarrwanga dedicated her life and fortune - $500,000 received in back rent on a land title after she and Dr James proved her traditional ownership of her country - to preserving the language and culture. They first published a dictionary and an atlas. Big Boss' last wish was that every Yolngu child should know the sign language. But she died aged 97 before the handbook of sign was finished.

At the time of European settlement in Australia, there were around 250 languages, including 800 dialects. Of these only 13 are still learned by children, and another 100 are at risk as elders die, according to the Australian Institute of Aboriginal and Torres Strait Islander Studies (AIATSIS). Most people think of Aboriginal Australia in monolithic terms, according to AIATSIS chief executive Craig Ritchie. He is also co-chair of the UNESCO International Year of Indigenous Languages steering committee. "Far from Australia only being multicultural since the Second World War, for 65,000 years this continent was a multicultural place," he said. A 2014 survey on the state of Indigenous languages showed "things weren't good in pure numbers". Since then 22 language centres and language nests have been established. The government is also set to announce grants for Indigenous languages. Indigenous sign languages like Yolgnu were less well known and hadn't been included in the 2014 survey, said Mr Ritchie. He said this year's survey would focus on the "broader concept of language to include the sign languages".

Bi-modal bilingualism, the norm in Arnhem Land, is not well known. Unlike languages developed by people who were hearing impaired, some sign languages used by Indigenous communities here and overseas were developed by "speaker-hearers". Early Europeans saw Aboriginal people using signs. But often they were misinterpreted as an arbitrary gesture, said academics in Indigenous Sign Languages in the International Journal of Learning in Social Contexts. Dr James is the only non-Indigenous person to have learned the sign language and a dying spoken language called Yan-nhangu. When he arrived on the Crocodile Islands, he felt compelled to save the language and culture: "It was a little bit like when you see a baby about to fall in a well. You just have to reach out and grab it."