In this paper a new hypothesis about the connection of the development or multicelluar animals in the "cambrian explosion" and the enrichment of molecular oxygen in the athmosphere is presented. Recent work in molecular epigenetics has shown that the reversible methylation of DNA and lysine residues in histone proteins is essential for cellular differentiation. However, the removal of methyl groups from lysines and DNA is chemically difficult and it requires oxygen. It is proposed that because of this reason, multicellular animals appeared only after enrichment of atmospheric oxygen. In this model, oxygen is not needed to burn food and thereby extract energy, but to "burn" methyl groups and allow reversible epigenetic signalling.