Elemens of Creations?

Creation! the Genesis Story: was the bible originally written to read "In a beginning" and not "In the beginning"? Further, human were told to "..multiply and replenish the earth..." Was there a catastrophe that eradicated mankind and living things so that the earth needed to be replenished? Also, when the bible speaks of "one day", is a 24 hour day, a 1,000 years day or, another time period used in Mesopotamia?