Why did Earth need material from comets to assemble the raw materials for life? It's often speculated that matter from comets played a crucial role in the development of life on Earth. For example, Maps of Time says,


  Many of the ingredients of the early atmosphere (including much of its water), together with many of the organic chemicals that formed the first life forms, may have been brought to Earth by the comets that bombarded its surface for the first billion years of its life (~Kindle location 1922).


Why the presumption that these materials must be brought by comets? Did Earth even have the right atoms to make water and those organic chemicals (hydrogen, oxygen, carbon)? And what are comets doing with heavier elements in the first place? My understanding is that comets are (for most of their lives) very far from the run, and my impression was most of the elements other than hydrogen and helium and are concentrated close to the sun, with T Tauri Wind having push hydrogen and helium further away.
The earth in its early stages was far too hot for water to be in liquid form and the water vapor that would have been around at the time would have been blasted away via solar wind (http://en.wikipedia.org/wiki/Solar_wind). Anyone claiming to know exactly how water came to be on Earth is likely daft, but comets and asteroids seem to be the most common theory for the source. MinuteEarth, a YouTube creator has a great video about it but as a new user on this particular sub-site I cannot link multiple things at once.

https://www.youtube.com/watch?v=_LpgBvEPozk