Christians believe that reason has always existed because God has always existed. We believe that God endowed mankind with the ability to reason because he made man in his own image.

For anyone to say that we are the only ones which God chose to give reasoning abilities is unscriptural. The scriptures speak of Angels. They too have reasoning abilities.

Also, we do not know whether God has created other reasoning human, or non-human material creatures on other planets. So making a claim that no other reasoning creatures exist is conjecture.

That reminds me about a article I have read. Researchers has come to the conclusion that also apes, who don't have as complex language as human beings, has the ability to calculate. They really know what calculating is about.
Propably,- some other animals has the ability to calculate as well.
Article:

http://www.columbia.edu/cu/news/00/01/monkeys.html

So why is apes able to calculate? I think calculating isn't only something that need to be learned. Beings seems to just understand some of it, even if they don't have developed a complex language yet.
(Some scientists thinks that language is necessary to reflect over things,- especially to remember earlier memories).
But I think it's somehow possible to remember earlier memories even if complex language isn't developed.

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The Heavenly Afterlife exist.