Global Oxygen Content

One day, the global oxygen content suddenly increased. Animals and plants grow crazily, slowly taking over the towns where humans live. Therefore, human beings established warm nests to continue to survive. However, as humans' own resources were exhausted, they could only leave the warm nests at risk and search for precious resources. Therefore, humans established pioneers to find resources for humans, protect humans, and create a home for survival. A later operation accidentally discovered why the global oxygen content was increasing. Is this intentional on the part of the earth, or is it done by other civilizations? Can humans continue to survive when other civilizations come?