Physical Development and Need
In terms of physical development, the human infant, unlike some (but not all) animals, is not only helpless at birth, it remains physically dependent upon other human beings for a number of years (the exact number is not clear and varies from culture to culture).
If, for example, a human baby is neglected at birth, it will die. In this respect, one of the most important relationships that we experience in life is the one that exists between us and the people who help us to develop physically to the stage where we can start to look after ourselves.
In the usual course of events, these people are our natural (i.e. blood-related) parents, although it's debatable as to whether such care is best provided by our parents (or whether any adults can potentially do the job).
Whatever the arguments involved here (and they are many, varied and invariably fierce) the fact remains that the initial physical helplessness of the human infant is a significant factor in both its physical and, by extension, cultural, development.