Who created the trop (cantillation) for leining? Is it a Mesorah from when the Torah was written?
The te'amim (trope) symbols are about 1,000 years old and are pretty much universal within the Jewish world. They replaced earlier (and probably less efficient or more confusing) systems of notating the grammatical breakdown of verses. The actual parsing of each verse into clauses and sub-clauses is, of course, a much older oral tradition.
The melodies we assign to the trope symbols are harder to track down. They are certainly old, but have clearly changed over time, and differ from community to community. Ashkenazi, Sephardi, Mizrachi and Yemenite Jews all have different sets of melodies and use them in very different ways, and there is a huge amount of diversity within each of these major groups.
There are a few older examples (perhaps 400-500 years old) of the tropes written out in something like modern musical notation. These come from a German Christian work of that era. From what I recall, the melodies indicated sound very little like either our modern day Western Ashkenazi or Eastern Ashkenazi trope.
In all systems of trop there is a typology regarding the functions of the trop. Some are full stops, partial stops, assistants, etc. that all lend assistance in understanding the text. There is no need for a source to prove that we as Jews have known how to read the Torah and understand it since we received it.
What is not clear is the various styles of trop and the way they tend to sound like the musical cadence and scales of the host countries of the Jewish communities that they stem from.
A fair assesment could be that the style, inflection, and actual musical intonations always changed depending on the native language of the Jewish community. The pauses and other tones used may have made sense to those who spoke the language of those host countries.
Wasn't it Moshe Rabbeinu who instituted reading of the weekly Torah portion every Monday and Thursday so we not abandon Torah for more than two days? Wouldn't you expect therefore that "trop" was devised back then and its מסורה handed down -- since then? And just like trop, wasn't it since then that also handed down were the "psikim" (or whatever they're called), those bar-like separators between two words (looks like this: | ), which tell us that a separation between those two words is needed?