A fundamental biochemical advance in life development is the appearance of molecules which allow the gaseous exchange between the exterior and the interior of cells.

Although there is no solid proof, at present day we can find different molecules that are probably evolution of a primitive ones.

Examples of this evolution are chlorophyll, which are the base in the photosynthesis in bacterias dated 3.500 million years ago and the base of current green plants as well, and hemoglobin, which are the base of breathing in some invertebrates and all vertebrates.

They both have a common structure formed by 4 pyrrolic nucleus joined (attached) to a Magnesium atom (Mg2+) in chlorophyll and a Ferric atom (Fe) in hemoglobin.

Chlorocruorines, green colored annelids, and hemerythrins, violet colored that regulate breathing in some kind of worms, also contain iron.

Finally, hemocyanin, blue colored and found in arthropods and mollusks, contains copper.

                    chlorophyll                                                Hemoglobin

Essentially, all these pigments are the actual representatives of different biochemical tests that allowed the evolution of breathing organs, cutaneous or internal, of vegetables and plants.

In order to understand this evolution, it may be useful to remark on Bohr effect (hemoglobin's property), described for the first time in 1904 by the Danish physiologist Christian Bohr (father of the physicist Niels Bohr), who established that a lower pH (more acidic) hemoglobin will join the oxygen with less affinity.

Consequently, an increase in carbonic dioxide concentration in the atmosphere, which is directly related to protons concentration in blood, entails a pH decreasing that leads to a decrease in the affinity of hemoglobin for oxygen.

That affinity difference indicates that this different breathing pigments evolved in different conditions according to the composition of the earth's atmosphere.