Cofty - The atmosphere of the earth 4 billion years ago contained no oxygen therefore the composition of the oceans was also quite different. Modern vents are also teeming with existing lifeforms.Life will not likely arise a second time on earth. However each of the steps to life is being investigated in labs all over the world.
So, in essence (and please bare with me because I'm a layman in these issues) the supposed early pre-conditions for life to appear are now impossible to occur again in nature because life itself has produced new conditions - such as the abundance of oxygen, and therefore, water - that preclude the arise of life again - at least life as we know it?
Eden