There are many answers ranging from the biblical Tower of Babel to other theories, but this is the one makes sense to me:

Can I use this example to explain? Some years ago I organised an event in Brisbane, Australia at which a deaf lady was to receive an award. I found someone to translate the speeches in deaf sign language, and afterwards to my horror I discovered that there was confusion. It appears that deaf people on one side of the Brisbane River, without contact with deaf people on the south side of the river had developed their own signs. (At that time there was no official Australian Deaf Sign language – there is now, called AUSLAN) People develop their own language by communicating with their “own” people. I often laugh when I see sparrows here in China – they look the same as our Aussie sparrows, but their bird call is different. They developed their own bird call over generations. I hope that helps.

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