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…
What is RNA?
Ribonucleic acid (RNA) is a biomolecule that codes genetic information. Structurally, RNA is a chain of nucleotides (shown below).
Chemically RNA is very similar to deoxyribonucleic acid (DNA) but differs in three ways:
- RNA is single-stranded
- RNA contains ribose sugar, which differs from deoxyribose in its hydroxyl group attached to the pentose ring
- Both are made up of four nucleotide bases, however RNA is made of an unmethylated form of thymine – uracil
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:
- It is difficult for long RNA molecules to form by purely nonenzymatic means
- Ribonucleotides are difficult to form nonenzymatically
- 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.
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”
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.
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.
Panspermia 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.
Still, 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.