Findings on nonprimate mammals place the issue of mother-infant lateralized relations in a broader context, demonstrating that humans are one of many species showing this feature. The remarkable interspecies consistency in the direction of lateralization points to a continuity between lateralized mother-infant interactions in primates and nonprimate mammals and suggests ancient evolutionary roots of human cradling bias. The results from species which, in contrast to primates, have no direct involvement of forelimbs in mother-infant spatial interactions clearly support the perceptual origin of this type of lateralization. A right hemisphere advantage for social functions relevant to mother-infant interactions is the most probable background for the left-sided biases in the behavior of mothers and infants. Recent findings suggest the contribution of lateralized mother-infant interactions to biological fitness. Mother and infant both can gain advantage from keeping the other on the left side.