We know what instinct is, but we can not see it. We know instinct is present, but have no explanation as to how it originated. My question is how did instinct come to originate in different forms inside of animals and humans? Scientific explanation only

Study the theory of evolution. Charles Darwin. Also, study Richard Dawkins'"The Greatest Show on Earth" which is his new one, and also read Dawkins' "Selfish Gene." Realistically I don't think providing an adequate "nutshell" scientific explanation for the development of human/animal instinct can do justice on a forum like this one. Read for yourself. Head to the library and read the various theories on instinct theory, which I would venture to include reading on Freud and the arguments for and against him, and also Erich Fromm who provides a wonder analysis into human nature. Read Read Read for yourself.

Read up on Carl Jung's theory of the 'Collective Unconscious'. Our major portion of human instinct has its roots there. I think the picture will be clear, once you read it. Otherwise I would be glad to elaborate on whatever I know.