I feel this is a stunning demonstration of the unity of all life. We humans share these emotion molecules with the most modest forms of microscopic life, even though evolution has caused us to develop into trillion-celled creatures of extraordinary magnificence. If emotions such as fear, love, joy, sadness, and sorrow are based on opiate receptors, how can we be so species-centric to think that only in humans can the results of these molecules be interpreted as emotions?

Allen M. Schoen, Kindred Spirits