Who Come First Chicken Or the Egg

Of all life’s unanswered questions, there are two that stand out as being the most significant and the most difficult to resolve - the meaning of life, and the origin of life.


The meaning of life? The answer to why the Universe exists? Well, I guess you could say we’re halfway there, because we already know the answer. We just have to figure out what that answer means. And the origin of life? Amazingly, we're actually finally making some progress on that one, because a new study has shown you can create the most simple building blocks of life using three things that would have been present in abundance on early Earth - hydrogen cyanide (HCN), hydrogen sulphide (H2S), and ultraviolet (UV) light.


But first, a little background. There are currently two main hypotheses for how life on Earth began. There’s the RNA world hypothesis, which states that RNA was the precursor for life because it can carry genetic information while also mimicking the actions of protein catalysts, which is crucial for spurring on vital reactions. Then there’s the Metabolism First hypothesis, which proposes that certain types of metal catalysts could have given rise to the organic molecules needed to trigger early life.


Both of these hypotheses have struggled to answer the ‘chicken and the egg’ conundrum of life’s beginnings though, as Robert F. Service explains at Science Mag:


"In order for life to have gotten started, there must have been a genetic molecule - something like DNA or RNA - capable of passing along blueprints for making proteins, the workhorse molecules of life. But modern cells can’t copy DNA and RNA without the help of proteins themselves. 
To make matters more vexing, none of these molecules can do their jobs without fatty lipids, which provide the membranes that cells need to hold their contents inside. And in yet another chicken-and-egg complication, protein-based enzymes (encoded by genetic molecules) are needed to synthesise lipids."


But a team led by chemist John Sutherland from the University of Cambridge in the UK has made a discovery that just might resolve this problem. Six years ago, they figured out that simple and very common carbon-rich molecules, acetylene and formaldehyde, can be put through a series of reactions to produce some of the precursors for RNA.

So perhaps billions of reactions between acetylene and formaldehyde over billions of years could have randomly given rise to the first RNA molecules. But, says Service, this doesn’t answer the question of where the acetylene and formaldehyde came from.


Sutherland and his team investigated, and came up with even simpler ingredients for RNA, and these ones we know were abundant when Earth was only newly formed - hydrogen cyanide, hydrogen sulphide, and ultraviolet light. Together, these three ingredients can not only produce ribonucleotides, which are the basic building blocks for RNA, but more importantly, they can also produce amino acids and lipids at the same time, which helps solve the conundrum outlined by Service above. The lipids are there to provide the materials for the cell membranes, and the amino acids are needed to form the proteins that help replace and pass on DNA and RNA.


"We show that precursors of ribonucleotides, amino acids and lipids can all be derived by the reductive homologation of hydrogen cyanide and some of its derivatives, and thus that all the cellular subsystems could have arisen simultaneously through common chemistry,” the team writes in Nature Chemistry.


So where did these chemicals come from? Meteorites could have converted hydrogen cyanide from some of the simplest molecules you can get - carbon, hydrogen, and nitrogen - right near early Earth. "Evidence suggests that life started during, or shortly after the abatement of, the Late Heavy Bombardment, and processes associated with meteorite impact have been implicated in the generation of hydrogen cyanide and phosphate on the Hadean [early] Earth,” the team writes. 
And hydrogen sulphide and ultraviolet light were already in the area, so it wouldn’t have taken much for the various molecules to eventually make contact with each other.


"This is a very important paper," molecular biologist Jack Szostak from the Massachusetts General Hospital in the US, who was not involved in the study told Service at Science Mag. "It proposes for the first time a scenario by which almost all of the essential building blocks for life could be assembled in one geological setting."

Is this a definitive answer to the question of where life came from? Not yet, it’s not exactly an easy thing to prove, but it sure does give scientists a lot to go on. The debate will rage on, but Sutherland's team might actually be on to something here.
Source: Science Mag

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