From what I have understood, the "intelligence" of humans has happened because it came under both natural and sexual selection - directionally - for increased intelligence. Intelligence, from an anatomy standpoint, being the increase in density and proportion of the frontal lobe. More to the point - until scientists can agree on what constitutes intelligence - than you cannot answer the question. Many birds, fish, small mammals, etc. exhibit quite a bit of "intelligence." (If you saw a two-year old human doing the same activities you would be startled at the level of understanding. There are adults that cannot build the convoluted nests of some birds with or without tools.) So, what is intelligence? Is it intelligent, for example, for a dog to know when to back down to a stronger alpha male? If the use of tools is intelligence - that is done in many other animals. Same for problem solving. Is the presence of language intelligence? Many other animals have some form (verbal or non-verbal) of communication that could be called language by definition. Is it intelligent to have some sort of social order? Again, present in many animals. Is self-awareness intelligence? Can you measure that awareness and determine if it is not in other animals? Who are we to say that we know all of the subtleties that go in to any one action. As for complexity of any of the above traits - the scientist who thinks that they have observed all there is to know about a phenomenon is neither wise nor very good at their job. Sorry....off the soap box.