The Septuagint (along with the Samaritan Pentateuch) and Flavius Josephus in his Jewish Antiquities allow for about 6 to 7 hundred more years from our days back to the Flood (which accordingly would date back to the 31st century b.c. - there are some variants).
The Masoretic textual tradition sets about the 24th century as date of the flood cataclysm. That makes it hard for any early chronology of mankind to appear historical for anyone inclined towards the biblical account. Most translations regard the Masoretic numbers as the most reliable ones. However, they appear to be the latest, since Flavius Josephus (a prominent Pharisee and historian of the first century) does not know the Masoretic numbers.
What motivated the Rabbis/Masoretes to change the numbers and shorten the early chronology of mankind?
An English translation of the relevant chapter (11) can be seen here: http://ccat.sas.upenn.edu/nets/edition/ (corrected in acc. with Flav.Jos. Ant.Jud., Theophilus, Africanus, Eusebius, who used an older version of Septuagint, 1st to 3rd cent. and (oldest, 2nd cent.) Papyrus Bodmer´s Luke 3:36, which do not have that second, later inserted Kainan)
10 And these are the generations of Sem: Sem was a son of one hundred years when he became the father of Arphaxad, in the second year after the flood. ... 12 And Arphaxad lived one hundred thirty-five (MT: thirty-five) years and became the father of Sala.
... 14 And Sala lived one hundred thirty (MT: thirty) years and became the father of Eber. ... 16 And Eber lived one hundred thirty-four (MT: thirty-four) years and became the father of Phalek. ... 18 And Phalek lived one hundred thirty (MT: thirty) years and became the father of Ragau. ... 20 And Ragau lived one hundred thirty-two (MT: thirty-two) years and became the father of Serouch. ... 22 And Serouch lived one hundred thirty (MT: thirty) years and became the father of Nachor. ... 24 And Nachor lived seventy-nine (MT: twenty-nine) years and became the father of Thara. ... 26 And Thara lived seventy years (130 years -> Abram) and became the father of Abram and Nachor and Harran.
Flavius Josephus writes in his 6th book:
- I will now treat of the Hebrews. The son of Phaleg, whose father Was Heber, was Ragau; whose son was Serug, to whom was born Nahor; his son was Terah, who was the father of Abraham, who accordingly was the tenth from Noah, and was born in the two hundred and ninety-second year after the deluge; for Terah begat Abram in his seventieth year. Nahor begat Haran when he was one hundred and twenty years old; Nahor was born to Serug in his hundred and thirty-second year; Ragau had Serug at one hundred and thirty; at the same age also Phaleg had Ragau; Heber begat Phaleg in his hundred and thirty-fourth year; he himself being begotten by Sala when he was a hundred and thirty years old, whom Arphaxad had for his son at the hundred and thirty-fifth year of his age. Arphaxad was the son of Shem, and born twelve years after the deluge.
Is the Septuagint (and Flavius Josephus´) chronology inflated, as almost every translation of the bible seems to maintain, or is - on the contrary - the Masoretic Text deflated for some reason?
For instance, this Wikipedia article shows the differences for some of the genealogies in Genesis: