Structural evolution occurred simultaneously with the nutritional evolution of premonera. It is believed that nucleic acid evolved at first within the cell and then escaped away from the cell into the open ocean. Nucleic acid escaped from one cell, existing free for a time in an inactive state. The nucleic acids in each cell again condensed into thread like filaments involving protein as integral part of it, thus formed chromosomes. Furthermore a membrane came to be present around all the chromosomes in a cell leading to the evolution of nucleus, thus evolved eukaryota.