Our tradition states that Hebrew was the language with which Godcreated the world (Rashi commentary, Genesis 2:23, quoting themidrash); and it is the language with which He gave the Torah.
Since it was considered a holy language and was used for prayer andthe teaching of religious tradition, it was not spoken in mundanecontexts and wasn’t taught to just anyone. It was handed down fromindividual teachers to disciples as part of the original tradition;and the same goes for the art of writing (letters on parchment, asopposed to cuneiform or heiroglyphics). Thus, certain Hebrew Psalms(92 and 139) and teachings are attributed to Adam, the first man.The wider public, most of whom descended relatively quickly intoidolatry and sin, were not given access to the treasures of theoriginal tradition, since by their actions they implicitlyrepudiated it.
After the Flood, the Hebrew language had a brief period in which itwas generally known, thanks to Noah (see Rashi commentary onGenesis 11:1). This is why many hundreds of Hebrew words havecognates in languages as diverse as German and Japanese. Thealphabet, which secular scholars trace back to the Phoenicians, isaccording to our tradition actually one step older than that: it isfrom the Hebrew aleph-bet, which those of the Phoenicians andGreeks closely mimic.
After the Flood also, the knowledge of Hebrew eventually declined(see Genesis ch.11) and was preserved only among the WesternSemites, the ancestors and cousins of Abraham. Eber, from whom ourword “Hebrew” (Ivrit) is named, was a Semitic descendant of Noahand ancestor of Abraham. He was one of the major transmitters ofthe original traditions. He is credited with having broadened theHebrew language, and some Hebrew grammatical constructs areattributed to him by certain Jewish researchers.
As time passes, languages grow and adapt. Thus today we canidentify words and types of usage that go all the way back (andthese are the ones that are most likely to have widespreadcognates). And then there are Late Biblical Hebrew; the Hebrew ofthe Mishna; Medieval Hebrew, and so on. All of these have a broadoverlap, but each has introduced its added vocabulary words andusages. Today, Torah-Hebrew includes some words that were borrowedfrom the Persian, some words taken from ancient Greece, Aramaicwords, etc.