Generally speaking, if a person is left alone, he will not develop a language entirely of himself. However, if two or more people are put together at birth, they will inevitably invent a language of their own. A good example of this is young children, where, although they have no capacity for speech as yet, can communicate with each other. Thus, there might not have been an "original language" as such, but a host of languages each developing from a unique culture.
When a culture has a stable language, it gains in size. The sections branching out start to make their own dialects. When cultures spread out enough, languages become less and less like the "original". Enough that it becomes its own language. This is how we end up with latin based languages (i.e. Spanish, French, Italian etc.) and the many other languages that exist today.