In Darwin’s original version of evolutionary theory, every transition happened smoothly. (Natura non facit saltus, he quoted from Linnaeus.) But this dictum has always stretched credibility, as there are so many gaps between diverse organisms in nature. How do you go “gradually” from asexual to sexual reproduction, for example?
For years, Lynn Margulis has told us her answer, and sometimes, some of us are listening. Her favorite word is endosymbiont. A small, parasitic bacterium can take up residence in a host cell. In the short term, its interest is served by killing the host cell and reproducing as fast as it can. But over time, a better strategy is to leave the host alive, so its resources can be tapped over and over. Better yet, why not actively promote the health of the host cell, and moderate virulence to conserve the host’s resources for the long haul. Parasitism becomes symbiosis. The bacterium becomes an organelle, essential for the cell’s metabolism. This is the origin of mitochondria and chloroplasts.
The biggest jump of all occurred billions of years back when primitive life forms based on RNA alone became cells with nuclei, protecting their genetic treasure in the form of DNA. Now along comes Patrick Forterre of the University of Paris-Sud with a deliciously radical theory about how that transition happened: DNA was invented by viruses.
* The first step was that viruses invented DNA in order to stabilize their genomes and resist attacks by the host cell.
* Next, the virus took up residence in a cell, protecting its DNA in a shell.
* The virus and cell learn to live with each other, and evolve a co-dependence over time.
* Finally, the cell begins to realize that the DNA is a superior way to store its own genetic information. All the mechanisms for replicating DNA and translating individual genes into RNA are already there, provided by the virus.
* One by one, the cell’s RNA genes are copied into DNA, where they are stored more safely.
* The cell learns to depend on the DNA and its transcription. RNA becomes a temporary messenger molecule.
Forterre claims he can trace this ancestry by studying modern viral DNA replication, in all its forms, some of which is similar to the chemistry used by higher life, and some of which is unique to viruses. Two other virtues of this theory:
* That viruses have an immediate fitness benefit from substituting DNA for RNA (a way to escape the host’s defenses), while for cells, the switch to DNA offers only long-term benefits, so it evolves with more difficulty.
* That viruses evolve more nimbly than other organisms, because their reproductive cycle is so rapid and their survival ratio so low.
(Although this theory is new and yet controversial, it is already well-accepted that viruses play a crucial role in later evolution. They are constantly borrowing genes from one organism, inserting them into another, sometimes of an entirely different species. This lateral gene transfer adds diversity, and again makes leaps possible.)