There are currently around 6,000 different languages spoken around the world, but everything has to start somewhere. Using statistical techniques to analyse the rate at which words and dialects mutate, it has been calculated that it would have taken at least 100,000 years for a single language to have diversified that much. That would take us back to the middle of the Stone Age, around the time that Homo sapiens first emerged as a species. It’s possible that earlier human species such as Homo heidelbergensis (600,000 years ago) or even Homo habilis (2.3 million years ago) had language, but the evidence for this is much weaker.
If language evolved before the human migration from Africa, 120,000 to 150,000 years ago, it is possible that all the languages spoken today have evolved from a single root language, in the same way that all humans alive today have a common maternal ancestor. But even if this is true, we have no way of knowing if there were other, older languages in use at the time of that migration that have subsequently died out.
There have been several attempts to trace the family tree of our languages and find ancestral vocabulary and grammar. In 1994, Stanford University linguist Merritt Ruhlen suggested several root words that may have belonged to this ancestor language, including ‘ku’ (‘who’), and ‘ma’ (‘what’). But this is still controversial and many linguists regard the search for a ‘first’ language as pointless.
So in short it is impossible to determine which language was the first language spoken by human being.