Good question; it gives me a chance to clarify what I was trying (perhaps unsuccessfully) to say.
Part of the problem is what “contagion” means. I don’t envision the spread of a “disease,” but rather the cultural and empirical spread of behavior and attitudes through communal interaction among Homo sapiens.
What I was trying (speculatively) to describe is very different from Augustine’s notion of “original sin” and is much closer to Eastern Orthodoxy. I am a Wesleyan in theological orientation, and John Wesley was significantly influenced by the Eastern Fathers on a number of points.
Part of the problem with Augustine’s notion of original sin (which assumes that everyone is born with a “sin nature”) is that he misunderstood Paul’s point in Romans 5:12.
Augustine rightly saw that “sin came into the world through one man, and death came through sin”; fair enough—this fits Genesis an it is compatible with my notion of the cultural and communal spread of sin through the human community.
But Augustine used an Old Latin translation of the New Testament (which shows up later in the Latin Vulgate); this translation goes on to say “and so death spread to all, in whom all have sinned” (where “in whom” means in Adam).
Augustine thought that because we (somehow mysteriously) sinned “in Adam” we genetically inherit a sinful nature from our parents. And many Western Christians since Augustine have followed his lead in thinking that everyone born after Adam is automatically a sinner, which often leads to very pessimistic view of human nature.
This is not my view; nor is it Paul’s.
The NRSV better represents the original Greek of Paul’s formulation when it translates the contested phrase as “because all have sinned.” (Rom 5:12).
The point is not that all humanity mysteriously sinned in Adam, but simply that we have all sinned, which is an empirical fact.
What I tried to show in my blog post was that instead of an immediate change in human nature, the narrative of Genesis portrays a process by which humans come more and more under the sway of sin.
And I tried to think of what that might have been like among early Homo sapiens.