As complicated and fascinating a question as this might be to anybody else, it seems to me that for a Christian, the answer is pretty cut and dry: an unequivocal and resounding "No."
In the second volume of C.S. Lewis' Space Trilogy Perelandra, he explains that the inhabitants of Venus (unlike those of Mars encountered by the protagonist Ransom in Out of the Silent Planet) are human because they were created after the Incarnation.
Lewis understands that it is by virtue of our reason that we share in God's Divine Nature in such a special way. We are animals through and through. But for the faculty of reason we would be nothing more. It is our reason by which we are made in the image of God.
Sometime around the Year of Our Lord 1, Jesus the Christ was born to a pure Virgin in a stable in Bethlehem. God became Man. Henceforth humanity itself has been intimately and personally bound up with the Trinitarian Godhead. The Second Person of the Trinity, the Word, the Logos, Reason himself, is human; therefore, all temporal, rational beings must be human.
I would go even further than Lewis, though. When Christ ascended into heaven 40 days after being raised from the dead, he re-entered the eternal sphere, taking his body and human nature with him. As a result, even though the Incarnation occurred in time, it is an eternal reality, and in fact the second person of the Trinity is now, has always been, and will always be the God-Man Jesus of Nazareth, body, Mother, and all.
This is, for instance, why many scriptural theologians believe Jesus appeared several times in the Old Testament. These appearances are called "Christophanies," which word stems from the Greek Christos and phanein, "to show." This is, of course, related to our words "phantasm" and "epiphany," which means "to show forth." I don't happen to agree with this hypothesis. Nevertheless, I admit that it is possible.
At any rate, the Incarnation and the Ascension make it impossible for intelligent life on other planets to exist. While some might find this disappointing, it really makes sense. If there were intelligent alien species we would have to consider whether or not they, too, were fallen, and, if so, how, seeing as the Incarnation applies to humanity only, how it would be God would redeem them. It's a whole thing.
That being said, there's no reason that we shouldn't find all manner of animal life scattered throughout the universe. It's only fitting as God seems to adore diversity. It gets even more interesting when you consider that they may not be fallen as the creatures here on earth are. (See my Ugly Creatures series).
