The only creatures that existed in the RNA World that still exist today, are viruses.
They are the only creatures that still use RNA to store their genome– they never upgraded to V2.0, DNA.
In fact the creatures that might have written V2.0, retroviruses, are still around today too. They carry their genomes as RNA, but convert it to DNA via reverse transcription in order to take advantage of their hosts.
Ive written a bit about reverse transcription before… its kind of a mess to describe… however every retrovirus, from simple retroviruses like Avian Leukosis Virus (only gag, pol, and env) to wildly complex HIV-1 (gag, pol, env, vif, vpu, vpr, tat, rev, nef) to ERVs, they all do reverse transcription the ‘same’ way: Happens in the cytoplasm, tRNA primer starts reverse transcription, RT hops around a few times, badda-bing, badda-boom, dsDNA moves from the cytoplasm into the nucleus for integration.
Hepatitis B, a DNA virus, has evolved a different way to do things— instead of using a tRNA primer to start reverse transcription, it uses the same RNA structure it uses to package its genome into virions to initiate reverse transcription. This might be a new thing, or it might be a very old, leftover thing. But its totally different from the way retroviruses do it.
And then we have non-LTR retrotransposons like LINEs and SINEs. They make up most of that “half your genome is retroviral” thing you hear me say all the time. Chunks of DNA in your genome that proliferate like bunnies. I have LINEs you dont have. You have SINEs I dont have. Aaaand sometimes that leads to trouble (genetic diseases, yay…).
Even though LINEs code for reverse transcriptase proteins (which SINEs steal), ‘just like’ retroviruses, their reveres transcription process is nothing like that of the retroviruses and ERVs we know and love today. Since its not ‘normal’ it gets its own name– Instead of ‘reverse transcription’, their process is called ‘target primed reverse transcription‘.
LINEs code for an mRNA, which is taken to the cytoplasm to be translated into a protein. Halfway through (after the RT protein is made), the rest of the RNA curls up (not translated), and the RT&leftover RNA is carried back into the nucleus. There, it finds/makes damaged DNA– a nick where the leftover RNA can sliiiiide in, and match up with the exposed DNA. A leftover -OH group on that unlucky bit of DNA is used as a primer for reverse transcription. All of the action happens in the nucleus.
The point is, though LINEs/SINEs are retrotransposons, they dont look like anything alive today. What happened to the retroviruses that behaved like LINEs/SINEs? They are extinct, or, at least we havent found one yet. So ironically, the only survivors of their species are ‘mistakes’ that incorporated into germ-line cells. Nestled in the warm confines of our DNA, being passed on vertically from parent–>offspring for millions and millions and millions of years…
I wonder what they used to look like.