Every animal has a spirit, separate from their bodies.  In the Hindu religion it is taught to kill no living thing because if you do Karma will bring you back as that living thing.  (Think that's their belief.)  I don't believe that per se though I have seen people with past lives as animals a horse, as an example.  As we incarnated in other forms of bodies - including the very early hominoid bodies, we as spirit raised the energy levels until the bodies progressed into what we now call Homo Sapiens.  Also remember that there are many who have inhabited Homo Sapiens bodies who have learned all their lessons and no longer need to incarnate in this plane.  We are here only to learn and when we have learned our lessons we progress to higher planes on our return to God.