Not all that rare. If you believe Moses penned Genesis, then writing in both Egypt and Greece had been around hundreds of years.
Moses' primary reason for writing was to set down for the Jews an account of where they came from. There's no reason for the stories of their ancestors, from Abraham back to Adam, which geneology Moses graphed very carefully (remember he was an extremely educated man) should not be true. He was trying to show them how God had moved over time and brought things into being right down to their day.
In the same way, there's good reason to think that the story of the creation of world that Moses set down was accurate as well. There were lots of creation stories floating around at that time and Moses wanted to record the real one. The Jews had just seen lots of miracles; it wouldn't be as hard for them to understand as it seems to be for many of us that God could create the world in 7 days. If God hadn't done so I don't believe Moses would have used the language he used.
The current theory regarding writing has it beginning as highly stylized hieroglyphs in Sumer. From there it spread to Egypt and China.
Sumerian scribes were the Internet gurus of their day and could draw high wages anywhere.
Once writing got to Egypt (in the form of hieroglyphs and a syllabary) it was "copied" by the Phoenicians and rendered into the basis of another syllabary which became an alphabet. That alphabet spread around the Mediterranean.
In more recent times scribes working for Bill Gates have reintroduced stylized hieroglyphs as well as Chinese style ideographs to what we call writing. Again, the Internet gurus draw high salaries.
At Moses' time (maybe around 1600 BC), there were actually not even any organized written languages yet. Only symbol systems such as Egyptian hieroglyphics existed, and they were not really languages at all.