I never really contribute here but this is an area that interests me and I wanted to share my thoughts. Continuing off of the 'descendants of Cain' idea, there are fringe hemerneutic theorists who believe some of Cain's descendants actually survived the flood, and that they were all in some way or another cursed to bear his mark. It was never stated anywhere what the mark actually was, but I find this passage from Demian by Hermann Hesse interesting and relevant. It represents the mark of Cain as intelligence, perspective, a non-standard way of thinking and the perpetual isolation it results in.
"We who bore the mark [of Cain] might well be considered by the rest of the world as strange, even as insane and dangerous. We had awoken, or were awakening, and we were striving for an ever perfect state of wakefulness, whereas the ambition and quest for happiness of the others consisted of linking their opinions, ideals, and duties, their life and happiness, ever more closely with those of the herd. They, too, strove; they, too showed signs of strength and greatness. But as we saw it, whereas we marked men represented Nature’s determination to create something new, individual, and forward-looking, the others lived in the determination to stay the same. For them, mankind was a fully formed entity that had to be preserved and protected. For us mankind was a distant future toward which we were all journeying, whose aspect no one knew, whose laws weren’t written down anywhere."
I'm not inclined to believe Adam read this book or anything, but perhaps he'd have found it interesting and may have seen himself to bear the mark in a way.