DNA. The Self-Replicating Molecule

DNA. The Self-Replicating Molecule

If given a time machine, every biologist on Earth would race to be sent back to an inconspicuous shallow rock pool, 4.28 billion years ago.

They’re there to witness the most important thing to ever happen on Earth. Abiogenesis, the origin of life.

The first life on Earth was not much more than just a chemical. It was a tiny, colourless molecule that would have been invisible to us, and it never directly left a trace. However, we carry its imprint within every cell of our bodies.

Abiogenesis happened when this molecule learned how to make a copy of itself.

Chemistry can pull off some pretty surprising things but this one was truly special. In this rock pool it crossed a threshold and became life, or at least took the most crucial step towards it.

Scientists dispute how exactly abiogenesis happened, making it one of the most fundamental and important mysteries in science, but we do have some ideas about the big picture.

First, a bit of context.

The Earth is unique among all the planets in our solar system as it is an oasis for complex chemistry.

Water engulfs the entire planet, stirring up its minerals and giving them a place to mix and combine, while the Sun heats them but without them getting so hot they break apart. There is an abundance of carbon dioxide, methane, and ammonia, chemicals that can combine into billions of variations.

One place where this complexity really comes together is in coastal rock pools, just like the ones we have today. They are an ‘interface’ between the gasses in the atmosphere, the minerals in solid rock, and liquid water and the substances dissolved within it. After millions of years of being stirred by heat from the Sun and being zapped by occasional bouts of lightning, an inundation of strange and wonderful molecules formed in these pools.

Scientists call this the ‘Primordial soup’.

Rock pools may be life’s original environment. Look at them now! Video by Mark Gee.

It’s worth noting that there is significant disagreement in science about the location of abiogenesis. The main competing theory to the rock pools places it in deep, geothermal vents at the bottom of the ocean.

In the soup, it is believed that our exceptionally interesting molecule formed.

It was some unique combination of smaller molecules, each of which floated freely and abundantly in this primeval soup. After millions of years, around 165 of them happened to join up, by chance, in just the right way.

By combining in this way the smaller molecules, (called ‘base pairs’) gave the molecule a special attribute.

The molecule could attract similar base pairs from the surrounding water, hold them in place, and join them together. It was like a template, or a mold, that easily and naturally combined base pairs.

In doing so it made rudimentary copies of itself out of the surrounding soup. In other words, it created a second generation.

We don’t know the exact chemical nature of this original ‘replicator’ like what exact base pairs it had, or how long it ‘lived’ for. After 4 billion years no fossils remain, and nowadays all base pairs that might appear in a rock pool are eaten by bacteria.

What we do know is the second generation of copies inherited the ability to replicate. They made their own copies, which made their own copies. Before long, the replicating molecules proliferated and spread throughout the sea wherever the environment allowed and base pairs were available.

The replicator molecules are called RNA, an ancestor to the DNA that we now carry in every cell in our bodies. This period in time is called the ‘RNA world’.

Eventually, later generations discovered how to build walls around themselves to protect the vulnerable RNA, creating a structure called a ‘cell’. Many also learned how to eat molecules from the surrounding water and use their energy to move around.

Generation after generation they became more complex, building elaborate structures around themselves that increased their chances of successfully replicating.

Some learned to capture energy from sunlight and became the ancestors of all plants. Others learned to consume other cells and became the ancestors of all animals and fungi.

Eventually, RNA was replaced by its descendant called DNA.

The life that we see around us today, from towering Redwood trees to human beings, are huge colonies of cells that work together to replicate – and every cell carries this descendant DNA inside of it, instructing it about what to make and how to replicate.

That first replicating molecule was a spark that set off an explosion of complexity and beauty greater than any other in the known universe, called the ‘tree of life’.

Over 4 billion years later, its descendants would invent smartphones, lunar modules, and study the universe to uncover the story of its own creation.

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