The branch of linguistics
that studies the history of language.
This is similar to paleontology: like biological species, languages evolve (only much faster), and their evolution can partly be reconstructed from evidence.
One source of evidence are historical records. These are fragmentary and only
take us back 0-6000 years (depending on the language).
A more powerful technique is to compare existing languages and look for patterns.
To do this well requires extensive study.