I would like to write a science fiction novel where, as of today (30 April 2023) all humans disappeared from the universe. All other animals, plants & fishes exist, just not humans.
What would be the impact of this?
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The impact of no humans on Earth is already known, there are several scientifically oriented TV shows that have tackled what happens after humans go extinct. Nature reclaims the planet, everything we've made decays into unrecognizable rubble in a few thousand years, maybe ten thousand at the most, although, as we see on ancient sites of extinct civilizations, some natural stone works (not concrete) can last nearly indefinitely (like Mount Rushmore, marble or granite sculptures, thick block stone buildings, like the Pyramids, or many stone works from Roman times like the Coliseum (in disrepair because of an earthquake).
The forest will pretty quickly creep back into cities, take over roads, vegetative growth (and earthquakes and floods and winds and lightning strikes and heavy snowfalls and decay) will break apart buildings and obliterate roads and highways.
One of the hardiest materials we make is stainless steel (like many sinks).
Except for those and the stones, look back to Earth about 10 million BC. If you don't mind the stone artifacts and some stainless steel, look to about 40,000 BC (e.g. Gobleke Tepe era). Or many other megalithic sites built by humans without any extraterrestrial help you can see on the travelogues called "Ancient Aliens", that are really about "Ancient Quite-Intelligent-All-On-Their-Own-Thank-You Humans".