Life from an RNA World — The Ancestor Within
- Michael Yarus
Without accidents, there would be no Darwinian evolution, as Michael Yarus points out in his fun new book on the RNA world. This is a short book with many chapters each of which is prefaced by a quotation from people as diverse as Elvis Costello and Walter Raleigh. Extant biology is even more diverse of course and, after paying obligatory homage to Darwin, Yarus shows how phylogenetic analysis leads back from this diversity down the tree of life to the Last Universal Common Ancestor (LUCA). LUCA is to origins of life researchers what 'Lucy' is to anthropologists, and to go back further is to enter a different, rather murky world — the RNA world.
This RNA-world scenario was first put into words by Walter Gilbert in 1986, but proposed two decades earlier by Carl Woese, Francis Crick and Leslie Orgel. Nowadays, the RNA world means different things to different people. To purists like Yarus, RNA in the RNA world bore the information, and did all the jobs, before acquiring the ability to make proteins by translation according to a primitive genetic code. Overwhelmed by the versatility of a nucleic acid of pronounced chemical fragility, Yarus is not happy with the notion that RNA could be a prebiotic product. Instead, he considers it to be the prebiotic invention of some earlier system. To chemists like myself, RNA seems complex, but I am aware that complexity is in the eye of the beholder, and if RNA can self-assemble from prebiotic feedstock molecules then it should be considered simple and prebiotically plausible. However, any systems chemistry that can make RNA in this way is probably going to make amino-acid derivatives, lipids and so on, so a pure RNA world seems unlikely.
Orgel and Jerry Joyce have for many years equated the dreams of molecular biologists with the nightmares of prebiotic chemists, but I am more positive. We have only scratched the surface of the chemistry, so to draw negative conclusions just yet seems premature. As an aside, it's worth considering how much more chemistry could be explored with just a fraction of the money NASA spends on smashing large lumps of metal into extraterrestrial bodies (rather than just scratching at the surface!).
After our chemistry class we then get to the chances of living systems arising. Yarus does a great job of getting around Fred Hoyle's analogy of tornados in a junkyard assembling a Boeing 747. This analogy has often been morphed into other comic forms and despite its original intentions has been leapt on by creationists as an argument against evolution. That it is a very poor analogy for evolution should rapidly become clear. After being reminded of the power of the inch-by-inch approach we next get a destruction of creationism, which paves the way for a molecular-biology primer leading to Crick's central dogma. Yarus is now in home territory and does a lovely job of describing RNA structure and function. Then we move on to test-tube evolution as pioneered by Sol Spiegelman, and subsequently developed as SELEX — Systematic Evolution of Ligands by Exponential Enrichment — by Joyce, Larry Gold and Jack Szostak. The fact that RNAs can be evolved to do lots of different things supports the purist view of the RNA world advocated by Yarus, but it doesn't prove it. How much better could all these jobs be done with ribonucleoproteins even if the protein components thereof were short, uncoded prebiotic products? Or, indeed, RNA in the presence of small inorganic or organic catalysts.
If we accept the purist view for a moment, we now have RNA-world organisms, or ribocytes, which, we are told, are jokingly referred to as ribosaurs by Peter Moore — good joke, great concept! Finally we move towards the mystery of the evolution of genetically coded protein synthesis. Yarus and his group have been significant contributors in showing experimentally that RNA can separately catalyse all of the reactions needed, and this provides a pretty good partial answer to the 'how' part of the big question if not the 'why' part. What a magnificent challenge it is for chemists to understand — and maybe even rerun — the molecular events that led to the evolutionary gold that Crick first caught a glimpse of!
So, should you buy this book and read it? Yes — if you have any interest in molecular evolution you will be enlivened, informed and amused. Will you get definitive proof of the RNA world? No — as Elvis Costello said, it's 'still too soon to know'!
On our bookshelf
Organotransition Metal Chemistry: From Bonding to Catalysis
- John Hartwig
This book provides a thorough coverage of the field of organotransition-metal chemistry. Beginning with the foundations of the topic in bonding and moving on to its application in catalysis, the book builds on the foundation established by the classic text in the field (by Collman, Hegedus, Norton and Finke) and includes the most important developments of the past 20 years.
The book contains a wealth of many clear diagrams that will make it suitable as a teaching source. The extensive referencing also allows the book to be used as a go-to reference — even for experienced practitioners in the field. It could find a place on the bookshelves both of the chemists who perform research directly in this field, and those who depend on its developments.