E.T. challenges the moral significance of the species boundary both because we recognize in him a being with feelings very like ours, and because we have no prejudice against him based on a history of eating his kind, putting them in circuses for our amusement, or using them as beasts of burden. 

It might, however, be difficult to tell whether extraterrestrial life forms are capable of suffering or experiencing happiness. We recognize the suffering of other beings by analogy with our own. Where there is a nervous system sufficiently like ours, and behavior similar to our responses to pain, it is reasonable to assume that a nonhuman animal is experiencing pain. This reasoning is strengthened by our knowledge that we and nonhuman animals have a common evolutionary origin, because it is plausible that the mechanisms that we share with other animals work in similar ways.