By a strict definition of writing (e.g. what we're doing now), people have only been able to write for a few thousand years, and much of the adult population was illiterate until recently. Define it widely enough (visual communication of information), and some other animals (e.g. tigers) also "write": https://animalresearcher.com/why-do-tigers-scrape-trees-at-s...
You could reasonably argue though, that we have passed on information over long periods of time orally, e.g. in epic poems, and that (as far as we know) no other animal does that.
Re: defining it broadly... The definition I've landed on thru defending my stated belief in these comments is, "Conveying information symbolically by etching it into or onto a durable medium so it can be referred to later."
But I guess an important quality of "writing" is that it's information that could otherwise be communicated with spoken language, and that the written and spoken languages are isomorphic.. maybe?
Honestly this is the first time I've put this idea out into the world for criticism so I'm still working thru these things :)