Hemoglobin is an ancient molecule that has been found in organisms representing all kingdoms of life. In bacteria, archaeons, and fungi, similar forms of hemoglobin evolved as a terminal electron acceptor in cellular respiration and may have also served a role in scavenging O2 to avoid damage due to oxidative stress.
Plant hemoglobins are known that bind and transport O2 within the cell. In soybeans, hemoglobin binds O2 in root nodule cells to keep concentrations low enough to avoid inhibiting nitrogenase, the enzyme used by symbiotic rhizobial bacteria in the root nodules for nitrogen fixation (Chapter 29).
Invertebrate hemoglobins found in roundworms, annelid worms, and arthropods bind O2 in the circulatory fluid, as discussed above. They share considerable gene and amino acid sequence similarity with vertebrate hemoglobins, indicating that a common ancestor to invertebrate and vertebrate animals evolved a shared form of hemoglobin more than 670 million years ago.