The Bibles that we use today are based on a Hebrew Refined Master Text, usually the Biblica Hebraica. A comparison of the genealogies in Genesis 5:1-32
and 11:10-26 of what is in our Bibles, which translated from the Hebrew, with the Greek Septuagint version, reveals that the Greek version has more years between the time of Adam’s creation and Abraham, because (1) The man’s age when his child is born is longer, usually by 100 years, and (2) Cainan is included in the Greek, but not in the Hebrew.
The following three articles by Henry B. Smith, which are available on the internet, clearly document the case for using the Septuagint genealogies in Genesis 5 and 11, in preference to the standard Hebrew text that all of our Bible translations have:
(1) MT, SP, or LXX? Deciphering a Chronological and Textual conundrum in Genesis 5
(2) Methuselah’s Begetting Age in Genesis 5:25 and the Primeval chronology of the Septuagint: A Closer Look at the Textual and Historical Evidence
(3) From Adam to Abraham: An Update on the Genesis 5 and 11 Research Project: Dec 16, 2017
The fact that Luke’s genealogy includes “Cainan” (Luke 3:36), gives much credence to the case that the Septuagint genealogies in Genesis 5 and 11 are more accurate than the Hebrew text’s. This has the net effect of adding at least 1,250 years to the time from Adam’s creation to Abraham. Expressed slightly differently, it would mean that Adam was created at least 1,250 years earlier, meaning that humans have been on earth for at least 7,250 years.
However, as strong as the case may be for the Septuagint’s genealogies, it is wise not to be dogmatic. Why? The very fact that the inclusion of “Cainan” in Luke 3:36 differs from the standard Hebrew text of Genesis 11, should also give us pause. If God allowed his Word, the Bible to have such a difference indicates that using genealogies to create a chronological timetable, or timeline, may not be what God wants us to do! This is one area where there is enough doubt as to which genealogical table is correct, the Hebrew or the Greek, that we can conclude it is not really important.
See the upcoming article: Are Genealogies Important for Christians?