Lets move away from the daft idea that a basic organism suddenly popped into existence with a spine. Thats a pretty complicated structure to come about in one fell swoop.
Suppose, on the other hand, an organism mutates a light-sensitive organ. Just a basic little thing. That basic eye enables to move towards light where theres more food (you can use your imagination to work out what it might do with it). More food = better chance of survival and the eye gets passed on to the population through breeding. Over time this eye gets more complicated.
Now, thats not so ridiculous is it? Maybe you would grasp the idea of evolution better if you didnt think basic cells suddenly grew the complicated organs that we, the most powerful life forms on the planet, have today.
Suppose, on the other hand, an organism mutates a light-sensitive organ. Just a basic little thing. That basic eye enables to move towards light where theres more food (you can use your imagination to work out what it might do with it). More food = better chance of survival and the eye gets passed on to the population through breeding. Over time this eye gets more complicated.