The use of arms in humans and apes began to play a fundamental role a long time ago.
It is true, nowadays, between strollers and high chairs, mothers no longer hold their children in their arms, but every time we see them holding them in this way, we can observe that preferably they do so with their left arm resting them on the left side of the chest. We often do not notice, maybe even moms don't realize it, but this is the trend. What about animals? For apes, which on the other hand are the ones that are evolutionarily closer to human beings, how are things? For example, gorilla moms hold their offspring in this way about 80% of the time, those of the orangutan more than 60%. Chimpanzee mothers do this with a percentage that is the highest of all monkeys and very close to that of humans, that is, about 85% of the time. On average, in all anthropoid monkeys these percentages exceed 75%. Basically, all mothers tend to hold their offspring with their left arm. One might think that this is due to the fact that almost all of them are left-handed, but, as we know very well, this is not the case, then left-handedness, in these cases, has nothing to do with it.
If, however, hypothetically, this were true, how could left-handed mothers manage all the tasks they carry out during these moments, for example, while cooking, preparing meals or doing household chores? They would be strongly prevented and would spend all their time moving their little one from left to right and vice versa. The question has been much debated among psychologists and not only, and for a long time, coming to the wrong conclusions, however, that right-handed mothers, not left-handed ones, preferentially hold the baby on the left to keep the right hand free. It would seem an obvious explanation, but that is not the only reason why they do it. Mothers preferentially hold their baby on the left for another reason (we will soon say exactly what). Then, other researchers came to another very interesting explanation, that mothers prefer to keep their babies on the left because it is the part where the heart is and, in this position, the babies would feel it beating. This would calm them down, insure them, feel closer and more reassured to the figure of those who take care of them, that is, the mothers or those who take their place. We will see that even this idea, however convincing it may seem, is not the right one, also because, for example, African mothers who work at home and in the fields, hold their children with a cloth tied on their back and it is not that these mothers are less reassuring and affectionate for their children than those who hold them on their chest. The hypothesis of the heart was probably influenced by an experiment in which it was shown that the heartbeat can be reassuring for a newborn baby because, when it was replaced with an artificial sound similar to that of the heart, the babies reacted positively and fell asleep calmly. On left-handedness, finally, there is another non-secondary issue on which there is no agreement among scholars.
Some say it is a very precocious phenomenon, that is to say that it manifests itself from 6 to 10 months of the child's life, while others say after four years of age. The truth lies perhaps in the middle, that the predisposition to left-handedness begins to manifest itself from an early age, this is true, and then proceeds until it is completed in 4/5 years of age.
Right-handedness and left-handedness
On right-handedness and left-handedness, therefore on the preferential side in which mothers hold their offspring, another hypothesis has spread, (I do not know to what extent it has been verified and it is not even known the size of the sample), in which it is claimed that the children, male and female, of those giving birth over 40 years of age, tend to become left-handed, so do the grandchildren, and that these women during delivery have more difficulty in giving birth and with more respiratory complications. This might be true, but it has been added that these mothers became emotionally weaker because they were older and faced much more stressful situations than the young ones and that this could then affect the right-handedness and left-handedness of their children and therefore the way they would then hold their offspring in their arms. Even this hypothesis, however seriously it was taken at the time, can never be proven. To complete the picture, there have also been those who have argued that if both parents had been left-handed the probability that their children would have become left-handed would have been higher than normal, but, once more, without adequate statistical evidence.
While the above hypotheses are all to be demonstrated and some of them are very weak, we have just to take a look at another fundamental aspect. It would concern the process of lateralization of the brain and its evolution, especially in higher mammals. However, this would be useless if it were not evaluated according to the mother-child affective behaviour and therefore attachment behaviour, in essence the cortical areas in charge of controlling these psychological phenomena which, incidentally, are under the control of the right side of the brain.
The lateralization of the brain has certainly accompanied the evolution of our species and of many other animal species since its origins; probably, dinosaurs were also lateralized. Returning to mammals, for example, monkeys, it has been observed that they prefer the left hand to eat, to pick fruit and rip leaves from trees, while chimpanzees prefer the right hand to use sticks to put in termite mounds to extract insects (lateral dominance), therefore different ways, although both have to do with feeding.
In humans, where 90% are right-handed and, as we know from the time we studied at university, the left hemisphere is the dominant one. It controls logical-linguistic functions, reading, writing and analytical thinking, while the right controls visual and spatial functions, imagination, prosody, holistic thinking and affectivity.
Starting from these assumptions, when can we trace the preferential use of the left arm in the support of children? Probably about 8 million years ago, so when we were not yet Homo sapiens, but all anthropomorphic apes, orangutan chimpanzees, gorillas and all other apes whose origins can be traced back to about 60 million years ago. In fact, macaques, of which we know a lot about mother-child relationships, the development of affectivity and attachment behaviour, tend to use their left arm to support their young on the left side of the chest.
The conclusion we can draw is that the use of arms in Human Primates (humans) and non-human Primates (apes) began to play a fundamental role, a long time ago, millions of years ago. It has been a long adaptive process that has enabled us to properly care for our offspring, the best, although we must not forget that in some animal species, newborn babies are abandoned by their mothers at birth, although this is not a kind of parental alienation as in humans, but a natural fact. In fact, most snakes do it. This would mean death for humans who have a longer neoteny than all living mammals. Neoteny is the phenomenon for which infantile morphological characteristics remain in children for a long time and the brain too has yet to reach its maximum development, so they must be protected, cared for and nursed for a long time after birth. Human infants are immature to behave independently immediately after birth.
The use of the arms has obviously undergone a process that must also be analysed in relation to sensitive functions, such as sight, hearing, touch, smell and taste. What is primarily the area of the brain that controls them? It is the right hemisphere, although not 100% of the entire human and animal population. All these functions, however, must have evolved in connection with the emotional and affective processes between mother and child. It is therefore no coincidence that external stimuli coming from the left when holding a newborn baby are processed by the right hemisphere. In fact, if, for example, we held a newborn baby on the right for a long time, we would get tired very soon, we would not be emotionally involved and we would soon move the baby to the left holding it with the left arm. In the support of a baby, the cortical areas that are positively involved must be the right ones, i.e. the ones that are in charge of the above mentioned tasks, not the other way around. In conclusion, for at least 90% of the human population, the most reassuring maternal arm for a newborn baby is the left arm, not the right one.