Tal­mu­dic Ques­tion #38

I have no idea if this is the cur­rent think­ing, but here's my thought. As­sume that the "RNA world" ex­isted, with self repli­cat­ing ri­bozymes as the ini­tia­tors, and pre­sum­ably there was also some par­al­lel (pos­si­bly ear­lier or later) de­vel­op­ment of com­part­ments (mem­branes) which I'll ig­nore :). A sub­se­quent event would seem to me to be the es­tab­lish­ment of a template/effector sep­a­ra­tion (i.e. the ef­fec­tor func­tion in one mol­e­cule, the tem­plate for mak­ing it in an­other). This could well have been a dsRNA at first, but it seems to me that evo­lu­tion would fa­vor the for­ma­tion of a more dis­tinct tem­plate (DNA) on the grounds that this would make the repli­ca­tor more ro­bust. How? I think pos­si­bly by fur­ther seg­re­gat­ing ef­fec­tor func­tion (ri­bozyme ac­tiv­ity) from tem­plate struc­ture.
Just as an ex­am­ple, a ri­bozyme that has RNAse ac­tiv­ity might be more likely to cleave it's own dsRNA tem­plate, but not it's ds­DNA tem­plate. Thus, at a pop­u­la­tion level, the pro­to­cells with the ds­DNA tem­plate might be more ro­bust, be­cause this sort of mis­take would be less likely.
An even cooler idea! maybe ds­DNA evolved as a de­fense mech­a­nism against RNA di­gest­ing ri­bozymes! I like that...
Fun stuff, as al­ways!

There is a the­ory that Patrick Forterre has put forth re­gard­ing ex­actly this ques­tion.
He pro­poses that dur­ing the RNA World, cer­tain viruses evolved DNA in or­der to evade host de­fense. Sub­se­quent par­a­sita­tion of DNA viruses into RNA hosts could po­ten­tially have al­lowed the uti­liza­tion of DNA by cel­lu­lar en­ti­ties. Es­pe­cially when we look at the con­sis­tancy of re­com­bi­na­tional events swap­ping ge­netic ma­te­r­ial be­tween virus and host, and vice versa.
Sci­ence had a news re­view on this the­ory: Sci­ence 12 May 2006: V312, no5775, pp870-872
http://www.sciencemag.org/cgi/content/summary/312/5775/870
🙂