At Huffington Post, Suzan Mazur, author of The Origin of Life Circus, interviews origin of life researcher Ricardo Flores, of the Institute for Cellular and Molecular Plant Biology, Valencia, Spain, who argues that a viroid-like entity is a prime candidate for the first replicon on Earth. Viroids are subviral world parasites, non-protein coding RNAs.
This from the interview:
Ricardo Flores: We know now that there is a subviral world, which was not realized for the first 70 years of the 20th century. . . . Viroids represent the smallest organisms in terms of size on the biological scale. Compared to the genome of the tobacco mosaic virus, the viroid genome is 20 times smaller. . . . The simplest is the oldest in evolutionary terms and viroids fit very well with this notion.
I consider a viroid-like entity actually a very good candidate for the first replicon that may have populated Earth at the beginning of life. This enthralling hypothesis was first proposed by Theodor Diener, the discoverer of viroids.
But when initial experiments on viroids were done in the 1970s, it was difficult to convince the scientific community something of that size with such peculiar properties would be able to replicate.
A viroid RNA, for instance, doesn’t code for any protein. It doesn’t need to go to ribosomes and instruct them to generate proteins. What a viroid needs is to go to a cell nucleus or chloroplast — if it’s a plant — and parasitize an RNA polymerase. . . . A viroid needs to find a way to be replicated and that’s it. Nothing else.
Suzan Mazur: What is life to you? How do you think about it?
Ricardo Flores: For me the key point of life is the ability to get copies in an appropriate medium, copies essentially identical to the original one.
If you consider bacteria growing in a Petri dish, you only need to add a very simple medium — some sugars and mineral salts — that’s it. Bacteria will grow and reproduce. If you go one step down to a very simple mycoplasma. . . you need a much more complex medium. And if you go one step further down from that to a virus or viroid, you need an even more complex medium. A virus or viroid cannot replicate except within a living cell.
With bacteria, mycoplasma and virus or viroid, when the complexity of one of the components of the system increases — the other decreases. When the bacteria is very complex, the medium can be simple. When the mycoplasma is less complex, the medium must be more complex. And when the virus or viroid is even less complex, then you need a very complex medium — a cell.
The complexity of the whole system must be in some way preserved. If you simplify one of the components, you need to increase the complexity of the other components of the system. For life you need complexity of the whole system. More.
Hey, wait a minute. If viroids are that simple, how would they have survived without a complex environment (they now survive only inside complex environments, plants)? Aren’t they just as likely to be stripped down remnants of something that was once more complex? Devolution?
Whence the complex environment on early Earth? Especially if we want to insist, as above, that “The simplest is the oldest in evolutionary terms and viroids fit very well with this notion.”*
See also: Why origin of life is an ongoing puzzle.