The contrast between Linda and John and the savages with whom they have lived for so long is created through Linda's experience of what "true life" as she thinks of it is all about. Having grown up and lived for so long in the "brave new world" where the majority of the action is set, she finds the life of the "savages" to be repulsive with its marriage, childbirth and alcoholism. Linda is defined by her early experience, and we can see her appearance is a testament to the way in which she turned to alcohol and sex as ways of numbing her incredulity at the kind of world she was trapped in with the savages. Note what she says to Lenina:
But it's all different here. It's like living with lunatics. Everthing they do is mad.
John, likewise, although he has never experienced the home of his mother's early years, has grown up with a fierce desire to see this world and has clear ideas of what it looks like in his imagination. The key contrast then is that both John and Linda are unsatisfied with their existence amongst the savages and feel that their lives are missing something.