the old testament is in the most part stories adapted from older cultures than the Hebrews with the characters names changed to reflect their own cultural heritage
we still do this thesedays. Look at the two american versions of War of the Worlds where the martians run roughshod over US locations which in the original book was set in surrey, England.
there is more than adequate proof for instance that the five books of the original pentateuch were all adapted from mesopotamian stories
flood story Gilgamesh written down 2500bce
Noah flood story written down 600bce
if you examine the details its very clear where the hebrews got all the details about that story from, and the Akkadian story of Gilgamesh was written at a time when there were no Hebrews in existence so the claim for oral history like the Ark of Noah doesn't hold water.

as someone pointed out earlier there are lists of Sumerian kings who reigned for impossible lengths of time. Some of these kings are attested in the archaeological record but lived perfectly normal life spans so the mistake is clearly one of accounting
the mistake came about when theses king lists were copied from Sumerian into Akkadian
The sumerians used a sexagesimal system of recording numbers and the Akkadians used base ten the same as us. To get the length of reigns in our numerical system you simply divide the stated reign by 60
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Jushur of Kish: 1200 years / 60 = 20 years
Kullassina-bel of Kish: 960 years / 60 = 16 years
http://en.wikipedia.org/wiki/Sumerian_king_list#The_listso basically along with the old stories adapted to their own needs they inherited the belief that the people they had got the stories from had leaders in ancient times who had lived a very long time. They simply adapted this along with the stories not realising the error because at that point in time the prevalent belief was not in science but in religion
if someone influential said something was true then you had to accept it and pass it on or else your own life might suddenly get incredibly shorter
someone asked this same question at another forum and without mentioning which forum I think its within the rules to post his answer to this question because he explained it much better than I just did
The Answer. That's Right. The Answer
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This really has been a mind-expanding thread, full of ingenious answers to the OP question. Some of them were fascinating to read. Others were hilarious.
Now here's the correct answer.
1. Between the rivers
Originally posted by Byrd
In the Sumerian king lists, some are shown to be living 900,000 years or so.
Sumeria was in Mesopotamia, the land between the Euphrates and Tigris rivers. Mesopotamia was also the place where the ancestors of the Jewish people arose; the Bible tells us Abraham came from 'Ur of the Chaldees', which was in Mesopotamia. Those long-lived Biblical patriarchs you good folk have been discussing were all Mesopotamian. Even Adam was; according to Biblical report, the Garden of Eden, where he was inspired with life, was in Mesopotamia.
2. The numbers game
Now the Sumerians, to whose 'king list' Byrd refers, had been civilized longer than just about anybody -- around nine thousand years, longer even than the ancient Egyptians. This was the place where writing was invented and where Hammurabi, the granddaddy of all bookkeepers, kept his records. Over the millennia, different city-states and empires arose in Mesopotamia. Power and ideas were passed from declining and dying states to emerging ones. Often these ideas mutated or diversified in the process of transfer.
Arithmetic, a fairly early invention, was one of the ideas that got mutated slightly in transfer. The way it mutated holds the answer to the OP question.
3. All our bases are belong to you
The common-or-garden arithmetic we use every day makes use of a 'base' of 10. What this means is that we use a set of ten digits (1 to 9 plus 0), and when we want to write a number that is more than nine, we use the same digits to refer to multiples of ten -- tens, hundreds, thousands, and so on. When we want to write the number of miles in a marathon, we write
26
where the 2 stands for 'two times ten' and the 6 stands for one times six. Whenever we see the figure 26, we know exactly how much it stands for: the number of miles that are in a marathon, or the age at which Keats died of tuberculosis.
But now, suppose we change our 'base' from 10 to, say, 6. We then have only six digits (1 to 5 plus 0) with which to write down any quantity we can think of. To write down the number of miles in a marathon, we would write, not '26' but
42
where '4' means 'four times six' (twenty-four) and '2' just means two (twenty-four plus two equals twenty-six). The actual number stays the same (the number of miles in a marathon) but the figure is different. Confused? No need to be. It's simple arithmetic, no big trick to it.
It becomes a little harder to deal with (for us decimal habituees) when you start using bases higher than ten. You have to have invent new digits. Perhaps you decide to use A for 11, B for 12 and so on, but whatever you decide, it's messy. I wouldn't even bring it up, except that this is exactly what some ancient Mesopotamians did, some of the time: they used base 60 arithmetic. They had a basic set of sixty digits. At other periods of history, ancient Mesopotamian cultures used ordinary base ten arithmetic, the same as we do.
4. Swapping bases
Knowing that ancient Mesopotamian societies alternated between base 10 and base 60 arithmetic, we can see quite clearly how those amazing age tallies in the early Old Testament and the Sumerian King Lists were generated.
It goes like this.
Somewhere in Sumeria, a king dies. A scribe records the length of his reign on a clay tablet: 'King Ashur reigned for 13 years'. He's using base 10 arithmetic.
A few generations later, in another part of Mesopotamia where they use base 60 arithmetic, another scribe copies out what was written by the first on another tablet: 'King Ashur reigned for 13 years.' But in this society, '13' doesn't mean thirteen; it means sixty-three, and that's what he writes down in the script of his era, not knowing he's made a mistake by not converting the base from 10 to 60 at the same time. So now King Ashur appears to have ruled not for thirteen but for sixty-three years.
A couple more generations, a couple more base swaps and he's lived for 3,603 years. Methuselah eat your heart out.
And there you have it. It's nowhere near as exciting as variable planetary spin, the Finger of God and some of the other theories posted here, but this, I fear, is the real explanation for the apparently extraordinary longevity of Methuselah and his fellow-patriarchs. They were bookkeeping errors, the lot of them.