I'm gonna also butt in here to say that life most likely started from a 'primordial soup' and a small bit of electricity. Or something akin to that. The point is, fish weren't always fish. They used to be cells, but those cells had to adapt, they became larger, and after a long time, you have a really basic algae. Those reproduced, but some of the other cells became different, they became really basic fish, who fed off the algae and the algae fed off of the sunlight and CO2 in the air. Eventually, there is more oxygen, gene mutation, some environmental changes, other cells became carnivorous, those evolved, and you have a basic ecosystem. As for why the Gar still exists, some of those very basic cells still exist in Yellowstone National park.