@Ross phil,
First, the answer to the original question somewhat depends upon whether you accept the theory of evolution as a reasonable explanation; many people, arguing from various religious perspectives, might object that a deity created man full-blown with a rational mind.
Second, the definition of "reason" or "rational" seems important to any answer. If, for example, one would argue that understanding the means/ends relationship is at the core of being rational, then many locomotive animals exemplify rational action (a monkey uses a long twig to get to juicy ants in their hole). It may be that various animals do exhibit what we would call different
traits of rationality, just as they do memory or speech, but in very inferior ways to humans. This might help explain how humans achieved rationality through evolution by showing intermediate phases.
Yet another viewpoint might include the concept that genetically humans have a predisposition for society, and that the social interaction (the written word, for example) has played a significant role in forming human rationality.
I would think the last two perspectives seem most likely, and that therefore it was not the case that one day humans were animals and the next they were rational. While I am sure there are other likely scenarios to be contributed by other Members, much of what one argues will nevertheless depend on what is meant by being rational, and whether that term is as simple as we might initially believe.