We could use modern cities as our analogy. If you shut down electricity in any major city it will stop functioning. There will be riots, looting, lack of food . . . you name it. So does this mean that there were never cities without electricity?
Obviously not. When electricity was first introduced to major cities it was a luxury. You could run fans or other gadgets off of it, but if electricity was suddenly taken away the city would keep on functioning. Over time there were new technologies that emerged, and those technologies depended on electricity. As the city became more and more dependent on these new technologies and got rid of the older technologies it created a situation where the city was not dependent on electricity.
The same happens in biology. At first, a symbiotic relationship is simply a luxury. It increases fitness, but if the relationship suddenly ended both species could go on surviving on their own. Over time the two species began to evolve in such a way that the symbiotic relationship was not necessary. They will lose features that allowed them to survive on their own because they no longer need them. They will also evolve features that are dependent on their symbiotic partner. This causes two independent species to evolve a dependence on each other over time.
Therefore, these species didn’t have to be created all at once.