Abstract

For almost half the history of life on Earth, the complexity of all organisms was limited to that of simple prokaryotic cells such as contemporary bacteria. The process by which genes are activated, which is at the root of the functioning of all living beings, was entirely regulated by proteins. This set up a limit on cellular complexity, as finding even larger proteins became computationally unfeasible. The eukaryotic cell—characterized by membrane-bound nucleus and organelles—emerged as a compromise between a conserved process of gene growth and a change in genetic regulation, which incorporated noncoding sequences. This increase in cellular complexity, which occurred continuously but in an abrupt manner at a critical point, unlocked the path toward multicellular organisms.