An interesting question is : if only man has free will and has evolved from an animal which is not supposed to have free will ,it is supposed to have developed at some evolutionary stage .Of course, one never knows if man has free will at all because what we think is free will may have been a part of the grand design and it is likely that we are acting the way we are merely because we are being prompted to act that way. That is a side -issue.
An interesting idea is the difference in the way the desires function in the animals and in human beings -while both animals and humans have the same set of primary desires viz:food,sex etc. the animals are different in that they have no secondary desires-the desire to desire a particular thing e.g.the desire to desire to go to the opera or the desire not to desire cigarettes etc. According to this idea of Harry Frankfurt, free will requires the second order desires of the above kind .Animals have only desires of the first order i.e.food .sex etc.