Swamp

At some point, spores appeared on Earth that, when they got into the lungs of a person, caused mutations. Gradually, a person turned into a tree.
On average, it took a person 2 to 5 years to go through the entire cycle and become a plant. Humanity died out, and trees took over the world again. However, these doomed people still had children - children with mutations, half plants, half people. These new creatures began to build their own society.