According to the article, those receptors are involved with exactly what contemporary opioid abusers are after. They are involved with a sense of intimacy, closeness, connection with another human being. Apparently there have been enough of the odd opioid to do that long, long before humans came along.

Watching a mother cow, or dog, or cat take care of their young it's not hard to imagine that they have used hormones to stimulate the production of some chemicals which will not only ease pain but produce a sense of closeness with their young. And these chemicals seem to have come along about the time of live births in land animals.