How did it all get started? What have we still to learn about the oldest molecules in our bodies?
Question time.
What came first? The chicken or the egg? The real answer to this, I think at least, is the egg: the first chicken was born out of an egg laid by something that, biologically speaking, was not quite a chicken. There must, after all, be a dividing line somewhere in the evolutionary timeline between what is and isn’t a chicken. Great. Next question.
How did life on earth begin? Ok. That’s tough. Through the history of the world, there have been many theories on this. Most notably, the theory of divine creation, whereby a deity or deities of some nature create life out of sheer will and cosmic force. There currently isn’t any evidence against this theory, and given its supernatural nature, it’s likely there never will be, so I won’t form any arguments against that. Physical evidence can’t contradict beliefs which are not physical in nature. One thing we have disproven though is abiogenesis, right? Abiogenesis, the theory that life just pops up spontaneously in certain situations. Louis Pasteur proved that growths in rotting meat came from microbes in the air, not spontaneously arising lifeforms. So, said Pasteur, life must come from life. So, life has existed…forever?
Here comes the original chicken and egg story, one which has yet to be resolved by any scientists in a really powerful way. The question is this: what came first, RNA or protein? RNA is a simpler, less structurally sound version of DNA with a single strand to compare to DNA’s famous double stranded helix. It’s used in lots of cellular processes in all cells containing DNA, and is still the main genetic code for some arguably living things such as viruses. We need both nucleic acids and proteins for life as we know it: nucleic acids (such as DNA or RNA) store information. Proteins do the work, based on that information.
Protein is the product of RNA. So that settles it. Protein comes from RNA. RNA is the egg from which the chicken hatches. The theory that RNA was the original ‘biomolecule’ is called the RNA world hypothesis.
Now we just have to figure out where RNA comes from. RNA replicates itself! Using…proteins, specifically proteins known as RNA polymerases. OK. Now to introduce a rival theory. This theory states that natural processes using proteins to break down and use energy, known as metabolism, came first, and RNA caught on later and became a way of organising the whole shindig. This is called the metabolism first theory. How do we solve this? New research might give us a clue. But first, a word on abiogenesis.
Four key names from the beginning of life
The names are Oparin and Haldane, and Miller and Urey. In that order, specifically. Oparin and Haldane were two scientists who hypothesised that life, in the form of organic molecules, arose from basic inorganic compounds rattling around and hitting each other at the right velocities. After all, from a biochemical perspective, there’s nothing spooky about life. It’s just the right sets of chemical reactions happening with the right sets of chemicals. That’s a particularly unromantic way of looking at it, but it makes it more approachable from a scientific point of view.
Miller and Urey provided the evidence that this was possible. In 1952, in the famous Miller-Urey experiment (famous to biologists and nerds, don’t feel too left out if you hadn’t heard of it), they simulated what they believed to be conditions from the primordial soup from which life is believed to have arisen, four billion years ago. This included an atmosphere full of carbon dioxide, hydrogen sulfide, nitrogen, and sulfur dioxide (lots of volcanic eruptions back then), along with greatly increased temperatures. Simply adding a spark, said Miller, yielded 11 of the 20 amino acids.* So life, perhaps, did originate with a strike of lightning, after all.
Back to chickens and eggs
So what has modern science got to say on this debate? Honestly, an awful lot: I don’t have the time to go into everything in a short article like this, and you probably don’t have the time to read it. So let’s fast forward to fascinating work being done right now in the Simon Fraser University. This research has developed an RNA ‘ribozyme’ which can act as an RNA polymerase. A ribozyme is a piece of RNA which can cause specific reactions to occur. So now, having the RNA egg, we are faced with the possibility of also having an RNA chicken. Our chicken and egg story may have just become a story of eggs and eggs. RNA floating around may be able to create living systems all by itself, without the need for proteins, and that is something truly revolutionary in the quest for the beginning of life. Currently, these researchers are trying to create a full living and evolving system entirely composed of RNA, which would be seen as strong evidence that an RNA world was possible.
That’s all for today.
I hope you learned something new!
Read more:
https://science.sciencemag.org/content/371/6535/1225 - article from the new and exciting work in Simon Fraser University
https://biologydirect.biomedcentral.com/articles/10.1186/1745-6150-7-23 - a great and comprehensive article on the arguments for and against (OK, mainly against) the RNA world hypothesis, but also reasons why it’s our best hypothesis. It gets a little technical in parts but is mostly very understandable and illuminating
*Amino acids, it should be noted, are the building blocks of proteins.
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