The thing is you can still understand the past without written evidence just we didn't live back in the day doesn't mean we can't know at least a little bit!
Just like in this situation. We don't understand what or why people drew on the wall. But we would understand it better if...
Phew, all of that cleaning made me tired. But back to the point. There's also such thing as a secondary source which is you are making an estimated guess or maybe someone you know(in this case it's his friend) told you! Just like the revolutionary war. We are secondary sources to that because we were not actually living during that time. But his friend was a primary source. Primary source means you we there when the specific event happened. Just like Covid-19 and how we are a primary source because we are living through it.
Wait...what's this? OHHHHH I forgot to mention WHY you can still understand the past without written evidence! There are also visual sources! They show you what happened with only a sentence or no words. That's how you can understand the past sometimes with visual sources. Just like how we use some of otzi's gear to think about history back then
Hmmm I wonder why it looks so dirty down here... It looks like a cave with cave paintings! What if they have a meaning?We need to clean this up anyway.
It's ok. You just scared me that's all! WAIT did you see what happened here? Can you help me clean it up?
I'm sorry miss. I was running and I didnt see you.
Actually YES and YES. My friend was part of this and he said that he was with his other friends and destroying the bridge!