Animals become extinct because they don't behave in way that fit the place where they live. The dinosaurs were okay for millions and millions of years, then a comet put a lot of dust in the atmosphere, and the world couldn't grow enough food to support them. Evil scientists put deer on an island where there hadn't been any, and where there weren't any animals that eat deer. The deer had it pretty good until there were too many deer, and they had eaten all the food.
Environmentalists believe that humans are like the deer on the island, sometime before all the food was gone.