Human languages are usually referred to as natural languages, and the science studying them is linguistics.
Making a principled distinction between one language and another is usually impossible. For example, the boundaries between named language groups are in effect arbitrary due to blending between populations (the dialect continuum). For instance, there are a few dialects of German similar to some dialects Dutch.
Some like to make parallels with biology, where it is not always possible to make a well-defined distinction between one species and the next. In either case, the ultimate difficulty may stem from the interactions between languages and populations. (See Dialect or August Schleicher for a longer discussion.)
The concepts of Ausbausprache, Abstandsprache, and Dachsprache are used to make finer distinctions about the degrees of difference between languages or dialects.
Scientists do not yet agree on when language was first used by humans (or their ancestors). Estimates range from about two million (2,000,000) years ago, during the time of Homo habilis, to as recently as forty thousand (40,000) years ago, during the time of Cro-Magnon man. The nature of speech means that there is almost no data on which to base conclusions on the subject.