If that’s how you pictured written language being invented, then you are not so far off. Yet the story is much more interesting and Garry’s invented signs made a huge dent in the world, but they also have since mutated so much that it can compete with the evolution of biological creatures in terms of diversity. The fact that you can now look at these signs and make sense of them would be a major source of wonder to Garry or Sir.
If the entirety of human history was represented by a 24-hour day, written language came into play at around 11:30 pm. That’s how new of an invention it is, even though 5,500 years in the making (the passage of time since the first written language was invented) feels more like a huge amount of time to us humans.
The early written systems were basically represented by signs of known objects: man, dog, fire, mountain, spear, etc., made in clay. You need to keep in mind that these people were talking :). They had a voice and a vocabulary, they had ideas, and they communicated. It may be extremely hard for us to imagine that some people could communicate just fine without imagining words, without imagining a sentence, structure, verbs and nouns. But that was the norm for all human history.
Let’s see how they managed to map sounds and create a written language. Here are one of the first such signs:
This is the sign for female
(quite obvious I suppose)
The sign for male?
obvious again (unless that’s a rocket)