Many religions consider language to be a gift from God. Thus, it is natural that every religion will have a theory of its own.
In the biblical tradition, God created Adam, and whatever name Adam called every creature became the name of that thing.
Following a Hindu tradition, it is Sarasvati, wife of Brahma, who is credited with bringing language to humanity.
In most religions, there appears to be a divine source who provides humans with language.
The basic hypothesis seems to have been that, if human infants were allowed to grow up without hearing any language around them, then they would spontaneously begin using the original God-given language.
If human language did emanate from a divine source, we have no way of reconstructing that original language, especially given the events in a place called Babel, “because the Lord did there confound the language of all the earth”.