When did organisms start depending on iron?

Iron is used by almost every living organism on Earth. The only known exception is the bacteria that cause Lyme disease.  This means that it was likely to be used by some of the very first organisms to appear on the planet. There was excess iron around and little oxygen in the atmosphere. Iron is an extremely abundant element & has chemical properties that make it potentially useful in a variety of chemical processes.  So it makes sense that it would be one the building block of any living thing.  It would have been present in bacteria in the early oceans.  So, we're already dating back 2.7 billion years or more.
One of the evolution theory (man evolved from reptiles) in brief is as shown below.
Fish>Amphibian>Human

You need to look at the evolution of hemoglobin to get an answer to this