Photosynthesis.
Nothing in life survives without it.
Photosynthesis is a very complex process where plants use energy from sunlight to turn water and carbon dioxide into sugar (carbohydrates) and free oxygen.
The sugar helps the plants survive, and we need to breath in the oxygen to survive, as well.
HOWEVER, photosynthesis couldn't have evolved, because every step of the process has to already be in place for it to even happen.
(It would have had to appear all at once!)
Unless it appeared all at once, there could be no life on earth.
Source: Chemistry Concepts and Applications, Glencoe McGraw-Hill, 2002, page 734; and a high school biology/chemistry teacher who majored in both biology and chemistry
