The problem. Here’s the reality: Darwin upended special creation, leaving basically just one plausible chess move to anyone who accepts evolution and posits a literal Adam and Eve: the miraculous insertion of a soul into an already evolved species of animal (perhaps Homo heidelbergensis).
Without the special creation of our biological species, one needs to posit the special creation of souls–souls that enter the world via an already evolved species. This means at least two miraculous births–from two separate animal mothers.
Put another way, there must be a soulless hominid Mary and a soulless hominid Elizabeth of the Stone Age, and one has to give birth to Adam, the other to Eve–creating the first couple with actual human souls (miraculously placed there, presumably, by God).
Who will Cain and Abel marry? Once you’ve got Adam and Eve and their offspring going, there’s still another problem. You’ve got the non-souled, native population to deal with. They either have to be displaced or assimilated by interbreeding.
Yes, that means bestiality.
If you’re going to trace all of humanity today, and original sin, to a first couple, then Adam and Eve’s soul mutation had to spread somehow through an already existing population of non-souled hominids (exactly the way a biological mutation spreads through a population–it starts with one and works its way out to the whole group).
But now things are getting quite complicated.
Drop evolution and return to biblical literalism? By contrast with the messiness of evolution, special creation is simple. It starts everything with two (two lions, two people, etc.). It doesn’t cloud up the boundaries between species, and doesn’t set ancestry along a continuum (as evolution does). God snaps his fingers, shapes dirt, or says abracadabra, and there things are. Easy. Magic. No mixing. No hybridity. No fuss.
God rests on the seventh day, everything is in its place, and it’s all good.
But with evolution, all is restlessness, mixing, competing goods, and ongoing negotiation–akin to democracy. No rest for the wicked. Life starts with the first cell that divided three billion years ago–and, like Lot’s wife, it never really looks back. Lineages from that point forward are along a continuum (species divisions along the same lineage are for our convenience).
And with messy evolution, species populations almost never bottleneck at two, then recover again. Ours certainly never did (if contemporary geneticists are to be believed).
A sharp break with the past. So for both evolution and the Adam and Eve thesis to go together, the soul mutation has to be treated in a way that no single biological mutation is treated: as the defining event separating one species from another.
Again, this sort of dramatic break between the parents of Adam and Eve, and Adam and Eve themselves, is necessary only because Darwin threw a wrench into the special creation thesis. In nature, a single mutation spreads through a population, but geneticists don’t, from that single mutation, identify a new species from it.
Geneticists tend to look for the build-up of a lot of mutations in a population as a whole before saying, “We’ve got a new species here.” Because evolutionary lineages are, in truth, along a continuum, these scientific declarations are judgment calls; designation for our convenience, not objective demarcations in nature.
So, according to geneticists, the truth is this: there was no first human–or first human couple. (See the video at the end of this post for an explanation of this.) There was, instead, a lot of spreading around of mutations among a diverse breeding population–never just two–and we, the people living today, are the most recent iteration of that large group process.
Put another way, even if each mutation started with only one individual, each species itself–including the human species–is the product of a pool of mutations derived from many individuals, not just one (or an Adam and Eve couple).
So if a person is going to define Adam and Eve–because of a single mutation, the soul mutation–as a different species from their hominid parents, and that mutation is going to spread, then we’re talking about displacement or assimilation of soulless hominids by interbreeding with the souled. Which, again, means bestiality.
Breaking the spell. If you don’t regard Adam and Eve’s single and miraculous soul mutation as sufficient to mark a new species, then you’re talking about Adam and Eve as just another single point variation within the same species, and this won’t do for biblical literalists and traditionalists.
So Darwin and contemporary geneticists have got those committed to a literal Adam and Eve boxed into taking positions that are difficult to defend.
But how about just breaking the spell of literalism altogether, denying that Adam and Eve ever existed? You can do this by simply treating the creation stories in Genesis as campfire etiological narratives. This is what most naturally fits the evidence.
But going with what most naturally fits the evidence also means giving up on original sin and Jesus as the second Adam. This, arguably, constitutes the end of traditional Christianity itself. So allegorical interpretation takes time to get used to. Galileo also had a simpler theory–in his case, surrounding the heavens—but had to wait, while under house arrest, on his church to play catch-up. “Your idea works in practice, but what about in theory?” Rome wasn’t built in a day–nor fundamentalist forms of religion dismantled.
Prometheus Unbound 
You are kicking a horse that has been dead sixteen hundred years. The educated, interested in reason have been long aware of St. Augustine of Hippo and his: “The Literal Interpretation of Genesis” (Espousing a literal interpretation will make you look foolish.) There was never a need to wait for Darwin to recognize that fallacy.
Alan,
Augustine was of two minds on the matter. At one point he insisted on a 6,000 year old earth, and I don’t think Augustine is a good example of a man dedicated to reason. Here’s a link to someone grappling with whether Augustine was, in fact, a young Earth creationist:
http://www.patheos.com/blogs/hallq/2013/01/was-augustine-a-young-earth-creationist/
As for beating a dead horse, both Evangelicals who accept an old Earth and Catholics who accept an old Earth have been grappling for a couple of years now with whether one can safely bid adieu to a literal Adam and Eve and still retain the doctrine of original sin and Jesus as the second Adam. Geneticists insist that the human lineage has never, at any time, bottlenecked down to two individuals.
Now I am paraphrasing from a translation, but Augustine’s position was that you should reevaluate your interpretaions (ie: grapple with) when new facts were uncovered. The body of evidence challenging a young earth was collected over a thousand years after his death. So your first point is a bit ridiculous – Augustine similarly did not accept Einstein’s theory of relativity, nor could he read or write English but so what?
Theologians’ have been ‘grappling’ with Adam and Eve for thousands of years for whatever reason. They’ve been grappling with the implications of evolution for a century and a half. Grappling with ideas is foremost in their job descriptions. That is largely why they are theologians in the first place.