Origin Story

It’s difficult to say what life is. What is the difference between non-living molecules interacting through the laws of physics and a biological, reproducing, living cell? At some point, these collections of molecules bubbling away in our planet’s primordial soup produced life. Mankind’s origin is geophysical, chemical and philosophical but I will be talking about the biological.

Earth is estimated to be around 4.5 billion years old with life existing in some form for most of that time. Our planet’s earliest evidence of life comes from mats of fossilised cyanobacteria called stromatolites, found in Australia. They are estimated at approximately 3.4 billion years old and still exist today. They are complex organisms with cell walls and protein-coding DNA, which is why scientists estimate life must have begun much earlier, around 3.8 billion years ago, with RNA…

Stromatolite
Fossilized stromatolite in Glacier National Park. The cross-sectioning of the layers can be seen because of erosion. Credit: David Ward / Montana State University

What is RNA?

Ribonucleic acid (RNA) is a biomolecule that codes genetic information. Structurally, RNA is a chain of nucleotides (shown below).

Nucleotide diagram

Chemically RNA is very similar to deoxyribonucleic acid (DNA) but differs in three ways:

  1. RNA is single-stranded
  2. RNA contains ribose sugar, which differs from deoxyribose in its hydroxyl group attached to the pentose ring
  3. Both are made up of four nucleotide bases, however RNA is made of an unmethylated form of thymine – uracil

RNA vs DNARNA World

The instructions for building any life form are contained in their genetic material. But how can something as complex as our hereditary machinery have arisen?

The “RNA World” hypothesis states that self-replicating RNA molecules were the precursor to life on Earth. It is thought that these initial biological molecules were formed through metal-based catalysis on the crystalline surface of minerals. However, living organisms require the catalysis of reactions that lead to more of the same molecule being produced.

This is where polynucleotides (chains of >1 nucleotide) come in. They act as a template, directly guiding the formation of exact copies of their own sequence via complementary base pairing of nucleotide subunits. However, this requires catalytic proteins such as RNA Polymerases which would not have existed initially. That is where the beauty of RNA lies – RNA molecules can act as catalysts! Therefore, it is RNA’s ability to carry information as well as catalyse reactions that forms the foundation of the RNA World theory.

There is one problem. Self-replicating RNA molecules haven’t been found in nature nor have they been constructed in the lab successfully. This is because:

  1. It is difficult for long RNA molecules to form by purely nonenzymatic means
  2. Ribonucleotides are difficult to form nonenzymatically
  3. RNA synthesis requires a complex sequence of reactions and competing reactions

Consequently, scientists suspected that life was born from molecules that resembled RNA but were chemically much simpler. This simpler form could have eventually acted as a template for complementary RNA molecules due to their similarity. This has been proven to work in the lab. Through natural selection, RNA would have replaced these molecules being more stable and efficient.

Evolution of DNA

In addition to complementary base pairing, other types of bonds form between nucleotides in the same chain, for example disulphide bridges and hydrogen bonds. These cause RNA to fold up in unique ways, as below. An enzyme’s specificity requires a surface with a unique shape and chemical properties. In a similar way, folded RNA chains can serve as enzymes, giving them a wider range of functions. So it is not difficult to imagine a biochemically sophisticated RNA World.

tRNA
A transfer-RNA (tRNA) molecule. Forms a complex with messenger-RNA (mRNA) and the ribosome to carry out protein synthesis.

This 3-D folded structure affects stability, function and ability to replicate. Therefore, some RNAs are more successful within the primordial soup than others. RNA World molecules would have been far less efficient and complex than today’s machinery. It is the same difference between DNA and RNA that allowed DNA to prevail.

“the primitive ribosome could have been made entirely of RNA”

Crick, 1968

The smoking gun for evidence of an RNA World is in the structure of a ribosome. Ribosomes aid with peptide-bond formation. Their active site lies deep within a core of RNA, whereas protein enzymes are more decorated.

Cell diagram - ribosomes
Ribosomes, found in our cells, are the primary site of translation (synthesis of proteins coded my messenger RNA).

Aliens!

The chemical conditions for initially producing RNA include Boron, Molybdenum and Oxygen. It is predicted that Mars had this ideal environment billions of years ago. If this is so, these life-suited molecules may have migrated from Mars’ environment to Earth via a process called panspermia.

Mars AttacksPanspermia is the idea that extremophilic life exists throughout the Universe, distributed by asteroids, comets, spacecraft and the like. Where these microorganisms meet suitable conditions, they can become active and evolution takes the wheel.  Many have long suspected that panspermia has helped to pave the way for the emergence of life.  Just a theory for now but I enjoy the idea of humans being somewhat cosmic.  Nothing to do with my extensive comic book collection…

Extremophiles are organisms that thrive in physically or chemically extreme conditions. For example, some bacteria in the Firmicute phylum produce endospores which protect it from harsh environments such as lack of nutrients, heat, freezing and radiation. When the environment becomes more favourable, the endospore can reactivate.

Yesterday (11th Aug 2014), the Rosetta Spacecraft completed a 55,000 kmph rendezvous with a comet. Scientists hope this might provide clues to the origin of life depending on what evidence the spacecraft finds. Of course, it doesn’t really answer where life came from, merely life on Earth.

Rosetta SpacecraftStill, details of the whole process remain obscure so there is a way to go yet before a complete understanding is reached. It is why nobody has yet been able to create life synthetically. Understanding the origin of life has implications for the search for life elsewhere in the Universe as well as breaking ground in synthetic biology on Earth.

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3 thoughts on “Origin Story

    1. Thank you, Martin!!

      My understanding of contemporary epigenetics is that it is the study of heritable changes in the function of genes that aren’t caused by merely the DNA sequence. For example, attached to DNA are methyl (CH3) groups which silence genes i.e. they stop certain genes from being activated. These methylation patterns vary between individuals (referred to as epigenomes) and are partly/mostly inherited from the previous generation. It’s these defined, molecular mechanisms that ‘epigenetics’ pertains to.

      In my opinion, the guy in that link talks a lot of pseudoscience. For example, he claims mutations are not random and uses the term ‘epigenetics’ very liberally in order to support his argument. A snake oil salesman.

      It’s true that certain experiences and stresses can trigger the release of certain chemicals in the body which can enter the nuclei of cells and interact with DNA. But I lose respect for his argument with all his nonsense about curing serious illness and evolution. There might be something to it though amidst the dense cloud of pseudoscience…

      Like

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