I'm not studied in linguistics so bear with me here. Phrasing the question was also difficult, but to elaborate it's something like this:
We take a modern hunter-gatherer tribe that hasn't been contacted by Western civilization. Say in theory we can gather the complete scope of their language and understand all of the objects and concepts that have been symbolized within it. Now lets go further and state that their lifestyle has been fairly constant for an arbitrary period of time - let's say 50 thousand years. Can we say that the objects and concepts that are symbolized in modern times, may be a good indication of those which were symbolized 50 thousand years ago?
Obviously the structure and phrasing of the language/words would have changed significantly, and the represented objects wouldn't be an exact replica, but can we say that their divergence may be equivalent to the degree that their lifestyle and lifeways have changed?
Additionally, if there is a sub-field of linguistics that approaches this subject that would be great to know.