I have had prospective parents say to me that they could never love a child who was not their own. In human cultural development, great emphasis has been laid upon the importance of paternity. In many Arab cultures, it is the duty of a brother to kill a sister if even the slightest hint of possible loss of virginity before marriage occurs. And the courts tend to treat such a crime very leniently. Recent investigations have shown that our notion of the constancy and fidelity of coupled birds is pathetically na ve.
Close observation indicates plenty of hanky-panky.
As devotedly and persistently as some adoptees seek their parents, some couples seek adoptees, even if they already have healthy progeny of their own and could have more. They spend considerable money and effort in obtaining an adoptee locally, or, most often, from a third-world country. Such experience is related in the legend of Moses in the bullrushes. Perhaps in the remote days of the human band or tribe, when child mortality was frequent, the survival of a child was considered even more important than its lineage and adoptive behavior was culturally encouraged. So, in fact, every human is closely related to every other human.