I am not a buddhist and so I cannot answer this with authority.
But if I am living in those times and stand back and observe the workings of Man and Woman in the course of their daily lives, I will be faced with the question of why the wicked prosper and the innocent suffer. The theology of those times would have told me that the reason why we are in different castes is because of the good and evil we did in our lives. Thus the basis for our salvation is to accept our lot and in acceptance, we would have evolved to the next step.
In answering this question, this model of rebirth will be a logical extension of my thesis.
At the end of the day, my answer will be a product of my thinking and the culture I was born into. Thus the answer of rebirth into an animal is the Buddhist and Hindu answer to injustice. Man therefore holds the keys to his own salvation. He did wrong, he pays for it. Once paid, it is over until the next injustice is done.
In contrast, the Muslims, Jews and Christians view injustice as an act that will be punished by God Himself or as an act where they are called to execute justice. The progression is linear. Man was born and Man will die. Once dead, that is the end. Judgement awaits. Justice will be met.
Thus since you asked me, perhaps rhetorically, my answer is to be respectful of others' belief system. All I will say is that this is my belief system......
