For now slavery had also been invented. To the barbarian of the lower stage, a slave was valueless. Hence the treatment of defeated enemies by the American Indians was quite different from that at a higher stage. The men were killed or adopted as brothers into the tribe of the victors; the women were taken as wives or otherwise adopted with their surviving children. At this stage human labour power still does not produce any considerable surplus over and above its maintenance costs. That was no longer the case after the introduction of cattle breeding, metalworking, weaving and, lastly, agriculture. Just as the wives whom it had formerly been so easy to obtain had now acquired an exchange value and were bought, so also with labour power, particularly since the herds had definitely become family possessions. The family did not multiply so rapidly as the cattle. More people were needed to look after them; for this purpose use could be made of the enemies captured in war, who could also be bred just as easily as the cattle themselves.

— The Origin Of The Family, Private Property, and The State by Friedrich Engels (Page 65)