All animals show motherly attachment to their siblings, its a natural process inevitable for the survival of the sibling in this tough world. There is not a straight answer to why a mother is called Mom or Mamma. It wouldn't be easy to triangulate when and why this 'ma' sound is assigned to the 'mother' in linguistic process. Does this commonality in sound show the single root of all languages or does it bcoz, humans think alike everywhere. What ever may the reason be, we must've noticed, the first sound a baby can easily produce is 'aa', then after a little, more refined 'maa' sound. A baby usually use such sound to call out, the person who is always in contact with it, that would usually be it's mother, for getting attention. So there is possibility that the child could be the reason for the assignation of the sound, to the person who cares and manages them. Before the linguistics were developed, before the grammatical intricacies of the languages, a small helpless child looking at the person, extending it's arms to whom it trusts, produced a sound which only it knew to make, revolutionized the entire philosophical and religious fraternity for centuries to come.