No life form could reduce oxidised iron unless it had available a far greedier metal than iron.

Aluminum is greedier than Iron as are magnesium, calcium, sodium, potassium, etc. Even the H+ Ion is greedier than Iron. Our body is littered with H+ Ions. We take H+ Ion inhibitors when we get ulcers.

And just what the heck does the Iron in Hemoglobin do?
Just sit there?

Hemoglobin absorbs and releases O2 -isn't that because of, or enhanced by, oxidation and reduction reactions involving Iron?

The Iron in hemoglobin comes from Iron in our food and where does IT come from? Plants, right? Where do plants get it?

While one single organism may not be able to do the trick, a community of organisms tailored to the task may well suffice.