...back when the bible was written, a year (as interpreted by man) was shorter than today (seeing as different cultures had different ways of measuring time, this is quite a credible answer)...
the bible is wrong, taking into consideration, especially, the fact that the average life expectancy is longer today than it ever has been.
There's no evidence of the lifespans of people "dropping off" progressively through the Genesis book and there's certainly no Biblical explanation for the way human beings are now living longer lives than ever.
Its very very clear in Genesis the life span was steadily declining. For example...
“Kenan lived on for seventy years. Then he became father to
Mahalalel. And after his fathering Mahalale,l Kenan continued to live
eight hundred and forty years. Meanwhile he became father to sons and
daughters. So all the days of Kenan amounted to nine hundred and ten
years and he died. And Mahalalel lived on for sixty-five years. Then
he became father to Jared.” (Gen. 5:12-15)
Using passages like this, its easy to create a list of lifespans...
Adam 930
Seth 912
Enosh 905
Kenan 910
Mahalalel 895
Jared 962
Methuselah 969
Lamech 777
Noah 950
Shem 600
Arpachshad 438
Shelah 433
Eber 464
Peleg 239
Reu 239
Serug 230
Nahor 148
Terah 205
Abraham 175
The idea that they confused months with years, or Lunar Years with solar years is just plain wrong. Otherwise there will be 6 year old fathers running around in the Old Testament!
The idea for eternal life is very simple genetics. It has been proven by experiments using fruit flys.
The basic premise for eternal life is the fact of how cells split. Cells split by making a copy of the DNA strand, then halving the cell body, hence replicating the cell. But, this happens Billions of times, and the same thing happens when u make a photocopy of a photocopy, of a photocopy... It eventually degenerates. This is how we age. Cells arent perfect anymore, and lead to imperfect copies which then lead to breakdown in the cells.
The experiments in fruit flys has lead to an interesting discovery. By breeding fruit flys, by only allowing the ones with the highest life span to breed, led to the discovery that by this selective breeding allowed the flys to live from a life span of 3-5 days, to 2 weeks!! An increase of more than 150% If this was done to humans, the average span will be 175 years!!
This allowed scientists to find the cause of this degeneration in the human cells. In the human DNA strand, there is a kind of "fuse" similar to that of a bomb. Every time the cell replicates, this fuse shortens, untill eventually, the fuse runs out, and allows the DNA strand to breakdown. The longer the fuse, the longer we live. Scientists are in the process of figuring out how to stop this fuse from shortening. Find this out, and we will have eternal life.
Its funny tho, on another point. People who believe in the bible select which portions to take literally and those to take as "just a story". Taking the bible as a literal story leads to some serious contradictions. Taking it as a story, leads to the problem of that it is all fantasy...