A Timescale for the Radiation of Photosynthetic Eukaryotes

Oxygenic photosynthesis is considered the most important evolutionary innovation in the history of Earth. It depends on two photosystems, responsible for the photolysis of water and the reduction of carbon dioxide. Oxygen and carbohydrates are released at the end of the reaction. Extraordinary, the oxygen released created the stratospheric ozone layer, and transformed the ocean chemistry, whereas the carbohydrates are the primary source of energy for complex cells. Several lines of evidence indicate the photosynthesis arose in the ancestors of cyanobacteria. It was spread over some eukaryotes by the acquisition of a free-living cyanobacterium, which evolved into photosynthetic plastid, the chloroplast. The timing of the chloroplast emergence is still controversy. Estimated ages range from 600 to 2100 million years ago (Mya) in accordance to previous studies. The aim of this study is to clarify several aspects of the origin and diversification of photosynthetic eukaryotes. For this purpose, we utilized a data set based on 27 protein-coding genes from genomes of cyanobacteria and photosynthetic eukaryotes, more genes than other papers that also utilized plastid genes, and performed the Bayesian analysis method to estimate the divergence times of the photosynthetic eukaryotes. Results showed photosynthetic eukaryotes emerged Late Mesoproterozoic about 1342 Mya. The Early Proterozoic oceans did not have adequate conditions for eukaryotes, because chemical elements such as zinc and molybdenum were at reduced concentrations, and they are essential to the formation of eukaryotic proteins.