Ideas are born of our very robust creative impulse, an ability to transcend merely being able to describe the world we see but as well to prescribe it, to understand it and analyze it, and ultimately make something different out of what originally existed. This ability itself is a product of our behavioral evolution as extremely social creatures, almost entirely dependent on our society and its continued ability to provide for our needs. We used to believe that we were the only animals with this ability but studies increasingly show that most mammals and other fauna we used to consider "dumb" -- some reptiles, fish, etc. -- have considerable reasoning capabilities, including this creative impulse. None, however, have it to the degree we do, and this likely derives from the reality that from a physical standpoint, when compared to almost any other animal, we suck. Physically, we only have a couple biological advantages (e.g., sweat glands, opposable thumbs, etc.), but just about any other animal on the planet can out-run us (on a sprint), out-swim us, and they all come with nifty teeth, claws, shells or poisons that make subduing them for lunch very difficult even when we can catch them. Our sole advantage -- though it's turned out to be a boon -- has been our ability to think, and not just observe.
Summary: Ideas come from brains that have been trained through 2 million years of evolution to see the world differently from other animals -- for sheer survival -- and most importantly, to take static concepts and combine their in unique ways to make something altogether new. One side effect of this evolutionary creative impulse is a powerful drive for modern people to express themselves by writing books, which we hope to the gods someone somewhere will want to read....