Cain, as in Cain and Abel, means smith. So the second man on earth was Mr Smith.
It rather changes the feel of the thing, when the murderer has a common-or-garden name like that. The Mark of Smith, doesn't sound quite as harrowing. Raising Smith seems quite a civilised upbringing.
Incidentally, Cain had a yellow beard, symbolic of his villainy. It's mentioned in The Merry Wives of Windsor:
MISTRESS QUICKLY Does he not wear a great round beard, like a glover's paring-knife?
SIMPLE No, forsooth: he hath but a little wee face, with a little yellow beard, a Cain-coloured beard.
It should be noted that the Hebrew q-y-n could mean Smith, and could also mean he who was created. I am following Sir James Frazer here, which I rarely do.
As Jack Charlton asked when told to play in goal for England: "Am I my brother's keeper?"
An all-too-common scene