The fossil record shows that the stromatolites built by cyanobacteria 2 and 3 billion years ago are virtually identical to those built by their modern descendants, which is just a part of much evidence revealing that bacteria have barely changed in billions of years. They appeared very early in the history of life and have conserved their complexity ever since. The eukaryotes, however, did the opposite. They repeatedly increased the complexity of their cells and eventually broke the cellular barrier and (...) gave origin to all living creatures that we see around us. This gives us one of the major problems in evolution: why have the prokaryotes maintained the same complexity throughout the history of life while the eukaryotes have become increasingly more complex? Here it is shown that a solution does exist, but it is based on experimental data that so far have largely been ignored. It is based on the discovery that, in addition to the genetic code, many other organic codes exist in living systems. The potential to generate organic codes was already present in the common ancestor but was not transmitted indefinitely to all its descendants. After the genetic code and the signal transduction codes that gave origin to the first cells, the prokaryotes evolved no other organic code, whereas the ancestors of the eukaryotes continued to explore the coding space and gave origin to splicing codes, histone code, cytoskeleton codes, tubulin code, compartment codes, and sequence codes. This experimental fact suggests that the prokaryotes did not increase their complexity because they did not evolve new organic codes, whereas the eukaryotes became increasingly more complex because they maintained the potential to bring new codes into existence. (shrink)