Noah was 500 when he became the father of Japheth. Genesis 7 tells us that Noah was 600 when the Flood came. This would make the date of the Flood according to the various texttypes:
- 1656 AM, Masoretic
- 2242 AM, Septuagint
- 1307 AM, Samaritan
The corrupt texts have been deliberately changed. This is obvious because in Genesis 5 the age of fathering the son, the remaining years and the total age is given. As the first 2 figures sum to the 3rd an error in one of the figures would lead to an incorrect sum yet in all texttypes all figures add up. Josephus gives different figures again though they are similar to the Septuagint. I have not seen figures for the Dead Sea Scrolls, I do not know whether there are any manuscripts of the early chapters of Genesis found. It would be interesting to know as some Dead Sea Scrolls preserve in Hebrew a more Septuagint texttype.
One thing that points away from the Septuagint is that by its chronology Methuselah outlives the Flood which is not possible. If the meaning of Methselah is "when he dies it shall be sent," then this points to the accuracy of the Masoretic and the Samaritan which both have Methuselah's death in the year of the Deluge.
A further possible argument against the Septuagint is that is was translated perhaps 250 BC. Many cultures claimed antiquity for themselves. There may have been a desire to lengthen Hebrew history, either to make claims for priority, or to allow time to accommodate the claims of other cultures; it would not do to have Yahweh creating the earth many years after Egypt was founded. Egyptian history is not as old as is sometimes claimed, it postdates the Flood which leaves even less time for it to develop, but this is a possible argument for the translators changing the figures in Genesis 5 and 11. Interestingly the age at fatherhood for the Septuagint is exactly 100 years greater than the Masoretic for most men. Setterfield suggests a mark for 100 has been omitted in the switch from paleo-Hebrew glyphs to the square Hebrew (Setterfield favours the Septuagint as being original as did many church fathers). I am not convinced this is an adequate explanation as there is a loss of 100 years for the years they lived after fathering the relevant descendant: a deliberate change in whichever texttype is errant.
This does raise an interesting point though, how old was Jared when Enoch was born? If the Masoretic is original and the Septugint routinely added 100 years (except for Noah for other reasons) why not make Jared 262? Is the Samaritan correct here? Was 262 seen as just too old? Noah was 500. Does the Samaritan decrease the age in line with the surrounding patriarchs? But why would a culture want to minimise its ancestry? And if we decide to follow the Samaritan then both Methuselah and Lamech die in the year of the Flood. Possible but it does seem a little convenient.
The fact that the first 2 figures add up to the third in all versions (corrupt and original) is evidence that the men changing the ages in translation understood the chronology to be airtight, there are no gaps.
Though I think the Septuagint is underrated in current English translations, I tend towards the Masoretic figures in Genesis 5. I have no desire to make the world any older or shorter than it is. Claims of cultural antiquity no longer bother me, all cultures must post date the Flood and even using the Septuagint leaves one at odds with secular dating for many post-Flood artefacts. And good arguments can be made for shortening many chronologies.
There is a place for looking for common themes amongst the varying figures. The problem is that the corrupt figures are not accidental, they are deliberate, and deducing the original is that much harder. It is not the case that a misread letter explains variant readings. The most similarities can do is fix ages for specific men. The only agreement for the antediluvian patriarchs across all 3 texttypes is that of Noah.
It would be wonderful to find a manuscript in paleo-Hebrew. It may help point us toward the original.