Correct, we don't know for certain. (I'll preface the following by saying that human prehistory is not my field, so I'm certain to be missing the subtleties and may even have some of the big pieces wrong.) Language, being a symbolic construct, is thought to be associated with other manifestations of symbolic thinking. The only other hominid species around at the time of the Great Leap were the neandertals, and there is still some debate as to whether or not they were capable of human language, the general consensus being that they were not.
There is no archeological evidence that they engaged in any symbolic thought: no art, no definite burials, no evidence of ritual, etc. There are discoveries that suggest that neandertals, just before their extinction, interacted with modern humans and may have borrowed some of the non-language, symbolic behaviors that they saw in modern humans. A seashell, recently discovered at a neandertal site, shows traces of paint on it, for example. Because of the dating of this type of discovery, viz., only after the two species were in contact with one another, adornment of this sort is considered by most to be borrowed and not indicative of symbolic thought.
