There has been a lot of brouhaha lately over an article by John Farrell regarding the doctrine of original sin as understood by Catholics (and I suspect most conservative non-Catholic denominations) and its compatibility with modern theories of the origin of humans. Edward Feser has posted a response which documents the fracas and describes a very good Aristotelian answer to the whole ordeal. I'd suggest it as prerequisite reading for my own post here, as he lays out more fully the metaphysical presuppositions I'm working with.
In the combox of Feser's post, a commenter lays out the following dilemma. Granted what Farrell says about genetics, either
(1) Adam and Eve both mated with non-humans, and all of the organisms we now consider humans are descended from either Adam or Eve
(2) Some of the organisms who we now consider human are not in fact metaphysically human. They are merely biologically human.
I think the first option would be the way to go, though I wonder if the existence of certain of Dr. Feser's detractors provides evidence for the second suggestion. ;-) In seriousness though, it seems hard to accept the idea that some people are not metaphysically human. Agere sequitur esse--"action follows being"--and the fact is that, in the absence of any physical deformation, everyone we know of acts like a normal human being, or at least the vast majority of people do. There's not a significant part of the population which is truly brutish. Moreover, we might use the idea that Adam and Eve's descendents mated with non-humans to explain some supposed discrepancies in the Bible. For instance, in Genesis 4:17 Cain has children by his wife. But if Cain and Abel are the only listed descendents of Adam and Eve, where did the woman come from? Well, maybe from some of the hominids who had not been given rational souls. Since it is most consistent with everyday facts as well as theology, the first claim seems the more reasonable view.
However, the commenter brings up the interesting problem that this hypothesis could some day be falsified by genetics. First, we might simply bite this bullet, though as a matter of fact I'm not sure if it's entirely correct to say that we can test this genetically. (I honestly don't know, though I vaguely recall from a biology course this not being how modern genetics works.) But second, there's an interesting point I'd like to make which should make this a little less unappealing. I would say that our first parents could have been even earlier hominids and that this possibility should make our theory nice and unassailable again:
Consider Dr. Feser's conclusions. He shows that even if there were 10,000 members of the biological species homo sapiens, this wouldn't prove that they were all metaphysically human. But we can look at this another way as well:
For all we know, God infused the rational soul into an earlier biological species. Of course, it's open to analysis of the anatomy of earlier species whether it would have been possible to infuse a rational soul into them, since as Feser points out a certain amount of physical development is a necessary condition for having a rational soul. However, I'd suggest that the amount of physical development necessary may be less than one might suspect. Here's why: I think that for every biological species there is some particular form characteristic to that species; after all, the form is the principle of life for any living thing, and the natural ends of a given form determine how a particular organism will develop. Since homo sapiens members develop differently from, say, homo erectus members, it follows that in some sense they have different natural ends (at least with regards to physical development) and we should conclude that they have different forms. This isn't very controversial really. To quote Feser:
"In fact, some A-T philosophers would hold that the specific genetic and phenotypic traits typical of homo sapiens sapiens are not even essential to human beings considered as a metaphysical category: Anything that was both animal and rational would arguably be “human” in the relevant sense, even if it had a body plan radically different from ours. "
But what this means is that having the particular form that we have is not necessary for being a human being metaphysically speaking. The only things necessary and sufficient for being a human person is to be both rational and an animal.
This is an important point. Since having a homo sapiens form isn't strictly necessary for being a human being, this means that the amount of physical development necessary for exercising rational faculties will be different for each form. So having a smaller brain would be an impairment for a member of homo sapiens, since this is a defect relative to the homo sapiens form. But having a smaller brain may not be an impairment for a member of homo erectus. In fact, a brain the size of a very dysfunctional homo sapiens may very well be the brain size of a flourishing homo erectus. I think that this, in conjunction with the fact that the operations of a rational animal's intellect are primarily immaterial and suited by God particularly for that species' form, shows that there could have been fully functioning, rational animals in earlier hominid species. And we could have very well descended from a pair of these.
As an example to make my point clear, consider elephants (dolphins may be a similar case). These animals exhibit some of the most intelligent behavior in the entire animal kingdom. They'd seem to be ideal candidates in case God felt like endowing another species with intellect and will. Now, although elephants have much larger brains than we do (they weigh 11 lbs.!), God could endow them with an intellect which exercised the exact same functions we do, and metaphysically speaking we would all be on the same plane; we'd all be humans--homo sapiens, elephants, and all. In the same way, although we have larger brains, God could have endowed earlier hominid species with intellects particularly suited to their physical form which could have carried on rational functions just like ours do. Since they would have been both rational and animals, they would have counted as humans.
This is in many ways speculation. But in any case, I think the point still stands that we should be able to apply Feser's reasoning backwards. God could have infused rational souls into even earlier hominid species, and all human beings on this God's green earth could have descended from a pair of these hominids who were first endowed with that rational part of human nature. This line of reasoning might help to throw our theology back into the comfortable realm of unfalsifiability and consistency with modern genetics.