Children who have experienced long periods of isolation, such as the example of anna, illustrate how:

It illustrates how "human nature" is the result of a complex relationship between nature (biology) and nurture (the social environment).
If this is not the case, Anna would end up to grow up into the ordinary woman just like other children in her age.
But instead of this, Anna grew up into a socially incapable child that could barely mutter words that other people could understand.