There are several spelling variations that together are attested by almost 1% of the MSS; 99% have Καιναν. Apparently only two omit, P75v and D, but no printed text follows their lead. So there is no reasonable doubt that Luke in fact wrote that Shelah was fathered by Cainan, not Arphaxad. This Cainan has been widely used to justify treating the genealogies in Genesis like accordions—if one name was demonstrably left out in the Genesis account, then who knows how many others were also left out. This Cainan is also used to deny the validity of constructing a strict chronology based on the time spans given in the genealogies.

But where did Luke get this information? The LXX contains Cainan in Genesis 11:12, but is so different from the Massoretic text here that it looks like fiction. Recall that the LXX we know is based on codices Vaticanus, Sinaiticus and Alexandrinus, produced centuries after Luke. It is more likely that our LXX is based on Luke than vice versa. Where then did Luke get it? I understand that Luke obtained the information about this Cainan from records existing in his day, and being correct information was led by the Holy Spirit to include it in his Gospel. Just like Jude, who quoted Enoch—Enoch's prophecy must have been in existence in Jude's day, but we have no copy in Hebrew today (though Jews are reported to have used one so recently as the 13th century ad); similarly we have no copy of Luke's source.*

This brief note was inspired by the discussion of the subject given by Dr. Floyd N. Jones in Chronology of the Old Testament† (which book comes close to solving all the alleged numerical discrepancies in the OT, at least as I see it). However, the explanation that follows is original with me (if anyone else has proposed it, I am unaware). Let's recall the exact wording of Genesis 11:12-13. “Arphaxad lived thirty-five years and begot Salah; after he begot Salah, Arphaxad lived four hundred and three years, and begot sons and daughters.”

The verb ‘begot’ requires that Salah be a blood descendant of Arphaxad, not adopted. He could be a grandson, the son of a son of Arphaxad, or even a great-grandson, etc., except that in this case the time frame only has room for one intervening generation. The plain meaning of the formula in the Text, ‘W lived X years and begot Y; after W begot Y he lived Z years,’ is that W was X years old when Y was born, is it not?‡ I take the clear meaning of the Hebrew Text to be that Arphaxad was 35 years old when Salah was born, whatever we may decide to do about ‘Cainan’.

Let's try to imagine the situation in the years immediately following the Flood. After the Flood the ‘name of the game’ was to replenish the earth. Indeed, the divine command was: “Be fruitful and multiply” (Gen. 9:1). So, whom could Noah's grandsons marry? Obviously their cousins, Noah's granddaughters. There would be an urgency to reproduce—thus, the girls would be married off at puberty, and the boys would not be wasting around either. The women would be giving birth as often as they possibly could. Really, the absolute top priority would be to increase the number of people.

Arphaxad was born two years after the flood, but his wife could have been born a year or two earlier. (The Sacred Text is clear to the effect that only eight souls entered the ark, but some of the women could have conceived during the Flood.) Thus, Arphaxad could have fathered “Cainan” when he was 17/18. Similarly, Cainan could have fathered Salah when he was 17/18. In this way Arphaxad could be said to have “begotten” Salah when he was 35. Cainan could have died early or been passed over in Genesis because the time span did not constitute a ‘generation’, or both. Or, as things got back to normal, culturally speaking, the haste with which Arphaxad and Cainan procreated might have been viewed as unseemly. The expedient of omitting Cainan would make the account more ‘normal’ while preserving precision as to the elapsed time.

But Luke would be correct in saying that Salah was “of” Cainan who was “of” Arphaxad. Salah was Arphaxad's grandson. In any case, the Messianic line was passed on by Salah. Without Luke's record I, for one, would never have stopped to consider what must have happened immediately following the Flood—the absolute priority must have been to increase the number of people.