"If we hadn't a voice or a tongue, and wanted to express things to one another, wouldn't we try to make signs by moving our hands, head, and the rest of our body?" Those were Socrates words, which is also one of the earliest written records of sign language from 5th century BC.
Sign language was mainly created and developed for mute people, to express their thoughts and feeling to others. Just like English, Tamil or any other language, sign language was also developed naturally out of groups of people interacting with each other. Initially, in the 80s, a school for the deaf was opened in Nicaragua, where students brought the gestures they used at home, created a sort of pidgin sign with each other, which eventually evolved in time, and stabilised into a full-fledged linguistic system.
Sign language differs from each country, even if the spoken language is the same, as it was developed within deaf communities, making it independent of the surrounding spoken language. Even though sign language does not represent spoken language, there is a lot of contact between them, where deaf people read and write or lipread in the surrounding language, and the spoken language, English for example, can be represented through finger spelling. Sign language has its own grammar as well.
Children acquire sign language in the same way they acquire spoken language. Babies start by babbling with their hands, and as they start producing words, they substitute easier handshapes for more difficult ones. They start making sentences by stringing signs together and only later get control of all the grammatical rules.
Brain damage affects sign language in the same way it affects spoken language. Just like how some would forget certain words or its meaning, fluent signers might also lose the ability to sign.
Ultimately, sign language is just like spoken language in many ways, but it’s also different. It can be very straight forward and formal, but can also be expressive and artistic. It is amazing not because of what sign language can do, but because of what language does.