The main differentiating factor that turns humans to thinkers is language. There are very few other species who have some form of communications and none have the capacity to express complex abstract syntactic structures like we do. It has obvious evolutionary benefits, and as a side effect it gives us the ability to reflect on the world. There are other faculties that we miss, that might also have interesting side effects (e.g. magnetoreception), but based on the size of the achievements of the human race, language is more important.