Spoken language preceded written language by many thousands of years. As a result, it is not known how language first emerged, or what the first language was; however there are a number of theories regarding this. Unfortunately, though, there is no way of proving any of these theories, and, as a result, they still remain something of a mystery even today. In fact, in 1866, the Linguistic Society of Paris actually banned any discussion of the topic in their meetings, as it was a constant source of frustration.
There are several theories highlighted by Danish linguist Otto Jespersen. One theory suggests that humans first started speaking by imitating the sounds that animals make, as well as those sounds heard in their environments. Another theory puts forward the idea that the first words were spoken to express emotions such as anger or pain. A third theory suggests that speech came about as a result of spontaneous reactions to surrounding stimuli and the sounds corresponded to that which they represented. Yet another theory proposes that through working together, people produced sounds with a rhythm to them, which may have expanded into chants and consequently language. The last of these theories suggest that language initially arose from love, play, poetic feeling, and song.
In addition, the fossil record has been examined in the hopes that it would help shed light on when language occurred, if not how it did so. Humans have the capacity for language as a result of several factors, including intellectual or cognitive ability, as well as the development of the vocal tract.
Brain size could perhaps be considered as a factor, which might suggest that both Neanderthals and the Cro-Magnons may have had some language ability. Both sub-species were around or before 30,000 BC. However, this is not entirely plausible, as gorillas have a similar-sized brain to modern man, and yet while they do have some form of communication, they are not capable of human speech.
Fossils show the shape of both the jaws and the oral cavity; however, what they don't show is the the size and shape of the organs necessary for speech, such as the tongue, pharynx, and larynx, as well the extent to which earlier hominids were able to move them. But some of this information can be obtained from reconstructions using plaster casts. A comparison of a Neanderthal vocal tract with that of a newborn baby shows several similarities, which would suggest that Neanderthals had limited vocalisations, rather than actual speech.
It is possible, though, that speech arose as a result of a gesture system being in place. Once human ancestors began to walk upright, their hands were no longer required for movement. As a result, they could use their hands for other purposes, such as working with tools. One way to convey information with regards to the usage of tools and even aspects of culture would have been gestures. In time, it seems likely that sounds, and, later, words would have accompanied these gestures.
As for the possibility of there being one original language, this isn't conclusive either. There are three theories surrounding this. Monogenesis refers to the idea that there was a common ancestor language from which all other languages diverged. According to this theory, the differences between various languages resulted from groups of people spreading out to different parts of the world. Polygenesis, on the other hand, has the view that language emerged in a number of different places around the same time. Similarities between different languages are explained by people having similar constraints posed by the environment or human physiology. Last, the third theory suggests that all languages in existence today branched off from a common ancestor, much like monogenesis; however, this particular line of descent is just one of several others. The other lines of descent have since become extinct.
Crystal, D. 1997. The Cambridge Encyclopedia of Language. Cambridge: Cambridge University Press.