I received a very interesting question via email from Patrick Chan:
According to the theory of evolution, why couldn't future man be materially different from present day or modern man, such that he is no longer distinguishable from modern man (by "materially," I include genetic and biochemical differences which may or may not manifest themselves physically)? As far as I can tell, it's possible according to evolution.
And perhaps as a result of such differences, why couldn't future man differ markedly from modern man in other ways? Maybe future man will have a different psychological makeup and emotional life, for instance, and thus be subject to and experience different temptations, sufferings, etc. than what modern man experiences.
What I'm getting at is that it's possible Christ himself might not share with future man what he shares with modern man. It's possible Christ would no longer be "one of us" in the sense that he would no longer be able to share in future man's "humanity," assuming future man can at least still be considered part of the mammalian species homo sapien. (Of course, if future man is so different that he can no longer be classified as a homo sapien, then that raises other questions.) This would undercut Scripture (e.g. Heb. 2:14-18; 4:15-16).
In other words, if it's possible for man to evolve into something different than he is today -- whether it's only a slight difference or whether it's as jarringly dissimilar as depicted in a movie like 2001: A Space Odyssey (in which man is a different species) -- then what would that make Christ in his incarnation as a man? On the evolutionary tree of life, modern man, and therefore Christ himself since he came as a modern man, could very well be to future man what an ape-man might be to us. Evolutionarily speaking, Christ in his incarnation would be a different being than future man. I'll not mince words: as far as I can tell, it's possible that the evolutionary equivalent of an ape-man might have died for your sins.
My response (addressed to him, since I first wrote it in an email response to him):
That's an interesting argument, and I think the worry is legitimate. I'm not sure if it's necessarily a problem, however. For one thing, unless you hold to open theism, God knows the future and has a providential plan of salvation, so God could easily work things out to avoid any real problem from this sort of thing.
Another issue to keep in mind is that we don't want to say that Christ experienced every possible temptation in exactly the way every human being might. That would mean he'd have to be a woman tempted toward heterosexual lust for men as well as being gay and being tempted toward homosexual lust for men (never mind homosexual lust for women). As far as we know, he didn't experience temptation related to severe physical deformities. He had the use of his legs, since he walked, and he could see. So he never had the temptation to blame God for making him deformed in those particular ways.
The key with this issue is that he must have had the possibility of being tempted with enough kinds of temptation that he knew what it is like to experience those kinds of temptation, enough that with extra knowledge of how people are tempted he can use a perfected sort of imagination to put himself in our place.
But I think the real issue with his sacrifice for sin is that it deals with the sin of those who have moral responsibility, are fallen, and are offered the opportunity to repent. This excludes fallen angels, but it would include anyone who descends from human beings who might be thought to be something other than human. I don't think it's our humanity per se, as a biological species, that matters. It's our descent from Adam, our sin nature, and our being offered forgiveness. The first Adam and the second Adam are both relevant for any descendant of the first Adam, even if the biological features might change, as long as the moral properties are of the right sort.
So I do think a theistic evolutionist has the resources to deal with this problem. It is an interesting issue, though, so I appreciate your bringing it up. I hadn't thought about it at all.