I can't see what difference it makes. It's like knowing how old you are--you just have to take someone's word for it. If you are off by a year or two, who cares? What's important is that you always know what you are doing and why.
Per Rand, it - the Creator - is nature, nature made us rational. The only way we can live at all is by our mind, and by using our mind we can live exceedingly well.
Let me digress. In the Jewish perspective both we and the animals are animated or move, hence we both have souls. But, we have a distinctly human soul. The missing link is not to be found in bones. It is all around us. The smartest animals register something like an IQ of 40. An IQ of 60 would be an imbecile amongst us humans. We are amazingly off the charts. There is a huge enormous gap or missing link between us and the closest thing to us. So, simply on a scientific basis, without invoking any religious reference, we are amazing. It appears, in particular, that we are pre-wired for language and for reasoning. Now, I will interject that in addition to our wonderful mind, we humans are equipped with a couple marvelous hands. To be sure, so are some other creatures. But, the combination of our mind and our bodies to include our hands enables us to act by which I mean to purposely do things that make the future probability of things different. OK, ignore probability, let’s just say make the future different. For example, we can fashion a tool. With a tool, we can become more efficient in our work. Imagining that possibility, we forego immediate satisfaction and make ourselves better off in the future. There you have it. We are neither ruled by instinct nor by immediate gratification. Hence, we are moral creatures with the power to choose between right and wrong. Ayn Rand argues that right and wrong are not choices, or social mores, or pronouncements from on high, but are scientific facts and that acting based on morality is the most selfish thing you can do.
There is a certain emotional hollowness to the argument. You are honest, productive, prudent, courageous, etc., because you are selfish. Kind of turns everything upside down, doesn’t it?