A statement I recently read: 'Evolution is debunked and gives no basis for morality. Natural Selection throws away info, it does not add. You need DNA to make RNA, and RNA to make DNA, so they had to come into existence at the same time. Created.'. Who can shed a light?
There have been two answers that refer to the RNA World Hypothesis — which I do think is relevant — but both contain aspects I, personally, find unsatisfactory (as I commented originally). As this question has come up again from the Community, let me answer the question in a way that I, at least, find satisfactory.
I am only concerned with the biological part of the question:
“You need DNA to make RNA, and RNA to make DNA, so they had to come into existence at the same time.”
The question deals with something we do not know — the times at which DNA and RNA came into existence — so all we can deal with is the logic of the argument, i.e. is the conclusion of contemporaneous appearance necessarily true?
To refute that all that is needed is an alternative hypothesis for a sequential appearance that is consistent with biology and chemistry.
- Contemporary biological systems exist in which DNA is not need to make RNA — namely RNA viruses. Although these depend on organisms (bacteria or eukaryotes) that have DNA genomes, their replication does not directly/mechanistically involve DNA. This, by itself, invalidates the first assertion (“you need DNA to make RNA”).
- Turning to the origin of cellular life, which currently has exclusively DNA genomes, there is therefore no biochemical/mechanistic problem in envisaging that the ancestor of modern organism had an RNA genome at a time when pathways for synthesizing the precursors of DNA did not exist. Note that this does not require the other aspects of an RNA world (RNA, rather than protein, enzymes). These may be likely, but are not part of this argument.
- So all that is required is a biochemically plausible hypothesis for the emergence of pathways for the synthesis of deoxyribose and thymidine, of enzymes to polymerize them, the switch to a (more stable and less error-prone) DNA genome, and the switch to this new genome encoding and transcribing the RNAs that are still necessary for protein synthesis and the like. I maintain that there is no reason in principle that this is not possible.
Whether or not the sequence of events was as in the scenario above, I argue that it demonstrates how RNA and DNA could have arisen at different times but given rise to the inter-dependent situation we see today, and demonstrates the basic flaw in the argument in the question.
According to RNA world hypothesis at some point the organisms had RNA genome only and only later DNA and transcription machinery was built to protect the genome(DNA sits in the nucleus and does not interact much with others). RNA got both informations storage capability as well as the catalytic activity. Thus it can be concluded that at some point, RNA was the primary genome and RNA alone could let an organism survive due to its catalytic properties.
So to answer your question, it is a probability only but still, RNA appeared before DNA
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I'm reading your questions as:
Since DNA and RNA co-depend on each other, which one existed first in the course of evolution
The honest answer to this is: we don't really know.
The RNA world hypothesis is a pretty good and also much studied and developed theory, but it's nonetheless a theory still. Even with solid experimental prove that a pure 'RNA world' can over time evolve to make DNA, won't let us know if it actually happened like this in the past or not.
A bit more background on why an intial "RNA world" from which DNA evolves at some point is theoretically possible:
In modern life forms genetic information is stored in DNA, then translated in RNA and a lot of functional elements are the resulting proteins. This is already a pretty complicated system so it was hypothesized that RNA might have one of the first molecules used for all of these steps. RNA can obviously store information (though not quite as long as DNA, since get degraded more quickly) and it can also have catalytic function, which is obvious from e.g. the ribosome, which still uses catalytic RNA units to build up all proteins.