The origin of multicellularity is considered a major transition in the evolution of life. It has independently evolved over 20 times in different pro- and eukaryote lineages. Underlying genetic mechanisms are still far from being understood. Moreover, it seems that the transition to multicellularity in cyanobacteria might have be correlated to the “Great Oxidation Event” (GOE), over 2.4 billion years ago. Here, I propose an ambitious multidisciplinary project, (i) to study the genetic mechanism that enabled the transition to multicellularity and (ii) to test the hypothesis that the accumulation of atmospheric oxygen is a consequence of the origin of multicellularity in cyanobacteria. Multicellularity might have been a key innovation during cyanobacterial history triggering adaptive radiation and abundance, consequently changing the biogeochemical cycles of Earth. To test this hypothesis I propose to resolve the timing and nature of cyanobacterial evolution, combining phylogenomic studies with palaeontological data. 30 cyanobacterial species have been chosen for next generation sequencing adding to the full genetic and morphological diversity of this phylum. The resultant data will be used for phylogenomic studies. Distinct gene sets associated with different multicellular lineages will be identified and their history reconstructed using Bayesian and maximum likelihood methods. Furthermore, cyanobacterial fossils from various ages will be analyzed applying different “state-of-the-art” analytical tools, such as Synchrotron X-ray tomography, and will be incorporated in an elaborate phylogenetic dating analyses using a Bayesian approach with different clock models. Results will be critically evaluated and compared to palaeoclimatic data, to reconstruct the origin and diversification of cyanobacteria and their significance to the GOE. This approach depicts the most sophisticated study of the coevolution of Earth and life over two billion years ago.