According to Genesis and also according to the book of Jasher, people
lived long life spans before Noah's Flood and after the flood life spans
decreased rapidly. For example Methuselah lived 969 years, Adam 930 years,
Jerod 962 years, Noah 950 years. What is the explanation of this longevity?
And what is the explanation of the rapid decrease in longevity after Noah's
Flood?

  Possibilities:

1. That's a bunchuv bovine scatology. The Bible and the book of Jasher are
both full of &^%%#!

2. They were primitive people who didn't have any calendars and didn't know
the date of their birth.

3. Those guys were actually several guys, each with the same name.  For
example Methuselah the First, Methuselah the Second, Methu
selah the Third,
and so on.

4. Maybe this is for real. How much did you say you want for that bridge you
have for sale?

5. Some other explanation.

  If we assume number 4 (that it's for real), then the question is why were
life spans live so long before the flood and why did life spans decrease
after the flood?

  Wild, weird-ass theory:
Before the Flood, the soil was very fertile; after the Flood, the nutrients
in the soil were washed into the ocean.  That's why we need to add salt to
our diet.  And why ocean food is more nutritious than land food.  Before the
Flood, the soil was abundant in sodium and chlorine and iodine and magnesium
and a bunchuv other stuff.  The ocean is abundant in nutrients that are
soluble; the land might still be abundant in nutrients that are not soluble.

   Noah started building the Ark at age 595 and finished in 5 years at age
600, according
 to Jasher. And he lived to 950 years, 350 years after the
Flood. His life span does not seem to have significantly decreased. This
suggests that supernutrition affected genes somehow.