It has been observed that 66-72% of human mothers cradle their babies on the left side. Whether left side cradling is unique among humans only or has it a similarity with other great apes also? John Manning and Andrew Chamberlain of the University of Liverpool studied this behavior among gorillas, orangutans and gibbons in zoos and found a left side bias in baby-cradling similar to humans. One of the most discussed reasons is the soothing effect of heartbeat of the mother on the baby, when cradled on the left side. But Manning and Chamberlain disagree with this explanation and question the heartbeat being localized on the left side stating that the sound of heart valves is in the sternum and in the middle of the chest. Moreover it’s not only the heartbeat of the mothers which quietens the baby but there are many other sounds doing the same trick. They hypothesized that right side hemispheric dominance for emotions may be helpful for the mother to keep a check on the emotional stress of the baby when it is cradled on the left side. This specific cradling behavior common to humans and great apes probably originated from a common ancestor some 6-8 million years ago which marks an important milestone in the human evolution.
Taking this point further Ambroise and associates opined that this way the mothers can see their babies face with their left visual field which is projected to their right cerebral hemisphere (Nature scientific reports of 3rd July 2020). Right side cerebral hemisphere is the dominant emotion processing hemisphere. Obviously mothers want to keep themselves informed about the emotional state of their babies therefore left side cradling alerts them on the emotional stress of their babies. They observed the cradling behavior of 44 baboons and found a similarity with humans in cradling the baby on the left side. This similarity in cradling behavior seems to be a phylogenetic continuity between the Old World monkeys and humans in right side hemispheric specialization for emotional processing inherited from a common ancestor thus stretching this behavior to about 25-35 million years ago on human evolutionary line.
Professor S. P. Singh, Ph.D.
Editor-in-Chief, Human Biology Review
Former Dean, Faculty of Life Sciences,
Punjabi University, Patiala, India