Alien Self-Eating Planet (Teen)

As I was drawing doodles in my Biology class, I imagined the possibility of alien life existing, not as individual units, but as a singular organism covering the entire planet. And since every organism must eat and generate waste, it functions as a gigantic inside-out stomach. It grows plant-like tentacles on the outside; they absorb sunlight and nutrition from the air, then eventually break off and fall through the cracks into the stomach to be dissolved and consumed. Since all feeding is local, there’s no need for a central cardiovascular system and no single heart to pump the nutrients through the entire body, which on such a scale would be impossible. Accordingly, this organism can’t be killed by destroying its vital central organ, since there is none. The pancake-shaped waste flies out of what looks like a big mouth and goes up in the air to form the brownish clouds above (you don’t want to be outside when it rains.)

How could such an organism evolve if, according to the theory of evolution, species develop only when individual organisms can compete and those who survive get to procreate? It is possible that this planet had other species in the past, but this one happened to be the most successful, spreading itself farther and farther until it engulfed the entire planet. Unfortunately, this can only be proven by means of archaeological excavation, which is impossible given that the planetary organism has most likely digested everything that’s underneath it and wouldn’t let anyone cut through its body anyway.