More on complex novelties

How do complex adaptations evolve when the initial stages of that adaptation do not immediately seem to be adaptive? As mentioned earlier, complex adaptations may arise through intermediate stages that do not initially seem to be adaptive but actually are. In addition, adaptations that evolved in one context may be co-opted for another. However, there are other possibilities:

Lifting functional constraints through duplication
Even when a feature is absolutely necessary for survival it can be modified by natural selection for a different function if it is duplicated. For example, globin is a truly ancient protein. Billions of years old, it was present in the common ancestor of bacteria, plants, animals, and fungi. Globin performed an essential job: binding and carrying oxygen. You might imagine that natural selection would lock globin into that one job; however, through duplication and divergence, different copies of the globin molecule were adapted for different roles. Vertebrates rely on several different globin genes: hemoglobin carries oxygen to body tissues (though a separate globin performs this function in fetuses), myoglobin keeps a reserve supply of oxygen for muscle cells to use, and neuroglobin and cytoglobin do jobs that we don't yet fully understand. Multiple globin genes are found all across the tree of life. In fact, some globins in deep-sea-dwelling worms are adapted for carrying both oxygen and hydrogen sulfide.

Myoglobin and hemoglobin are similar, but slight differences in structure let them perform different functions.