How is it possible for something as complex as the human eye to have evolved? It wouldn't work as well with one part missing. What good is a half eye while we wait for millennia for the rest to evolve? Darwin’s great realization was that if you have Variation, Selection and Heredity, you must get something that looks like intelligent design. Animals bear offspring with small genetic mutations. A great many of them die. The ones that survive long enough to reproduce, pass on their genes to their offspring. That’s all you need for complex design to arise over long enough time scales. Luckily life has had 3.5 billion years so far to make these evolutionary leaps.
What started with light-sensitive cells swimming to the sunny side of primordial goo has resulted in the human eye. Computer simulations show it takes 1,829 steps to go from a light-sensitive cell to a complex eye, which can happen in 350,000 generations (though in reality it took a bit longer). We see these intermediary steps happening everywhere, as eyes develop independently over and over again. For example the Chiton mollusk, which has developed simple eyes made of stone, to tell if predators are swimming above it — proving intermediary steps are useful. Our eyes aren’t perfect: Mantis Shrimp can see millions more colors than we can. Cave dwelling crustaceans lose their ability to see (why waste the energy in the dark), including the brain structures that support sight too.