A very average dead rock, by planetary standards. Then, about a billion years later, something quite miraculous happened- the first glimpses of life were seen, and they came from what seemed to be nothing.
Since abiogenesis, Earth, and life on it, have been through a lot- from single celled creatures to underwater plants to animals that roamed land, our planet was soon teeming with an unimaginable variety of species. Now, as we celebrate our semicentennial Earth Day and worry about Earth’s future, let’s probe a bit deeper into this wonder we are trying to conserve, and understand just how precious it is.
Despite scientists searching tirelessly for signs of alien life, Earth remains the only planet known to host life. Life, being something we are surrounded by, is something most people take for granted. Our planet was blessed with life- but how and why? Think about it — sentience arising from simple chemicals? Just what made Earth special?
Where did life come from?
Life is composed of interactions between four organic chemicals: amino acids, lipids, carbohydrates and nucleic acids. Search for the origin of life leads to the search for the origin of amino acids.
The Geothermal Pool
In a letter to his friend Joseph Hooker, Charles Darwin wondered if “we could conceive in some warm little pond with all sorts of ammonia and phosphoric salts, light, heat, electricity etcetera present, […]” .
Though a simple musing, his words were not far off from what has become one of the most significant theories made in response to the question of life on Earth. Researchers believe life emerged in a geothermal (volcanic) pool.
The chemical composition of modern cytoplasm is strikingly similar to that of volcanic pools. It is theorised that primitive cells had simple and leaky cell membranes, allowing ions of high concentrations to enter the cell. With time the cell adapted to these ions and came to depend upon them, which is how we can predict the surroundings of the first cells. The idea that these chemical traits are preserved over time has been called the Chemistry Conservation Principle .
The Miller-Urey Experiment
But how did those primitive cells even come to be?
Scientists Alexander Oparin and J.B.S. Haldane theorised that conditions on primitive Earth were ideal for the formation of ‘life.’ In 1952, Stanley Miller decided to give their theory experimental backing. Along with his mentor Harold Urey, he designed a setup consisting of water, methane, ammonia and hydrogen. When exposed to electricity, they found that five amino acids had been formed .
Perhaps this was the origin of life: electricity, in the form of lightning, struck a primordial pool with the perfect reactants, and catalysed the formation of amino acids. As time went on, the amino acids formed proteins and proteins formed more complex life forms .
Another theory rejects the idea of life starting on Earth altogether. Earth is where life thrives, but it was brought to our planet from outer space. This theory is called panspermia.
While a bacterium riding a meteor onto Earth sounds like something out of a science fiction novel, there are quite a few compelling pieces of evidence suggesting this could have very well happened.
Bacterial spores can survive for millenia without nutrients. They are also known to survive extreme temperatures, radiation levels and pressures. Comets and meteorites provide them with protection also.
Interestingly, the Late Heavy Bombardment — a theorised event in which the Earth was heavily bombarded by leftover space material- is said to have occured about 4.1 and 3.8 billion years ago, the same time frame as some of the earliest evidence of life on Earth .
It is worth noting that panspermia, though viable, is largely speculative and does not answer the question of how life emerged in the first place in the universe.
Goldilocks and The Few Requirements
Wherever it came from, life couldn’t have survived just anywhere. The Rare Earth Hypothesis, commonly called the Goldilocks Hypothesis, attempts to explain why it only occurred on Earth. It lists certain features that must be ‘just right’ for life to evolve and grow the way it has .
Distance From The Sun
Earth is 149,597,870 km away from the Sun. This distance is crucial, and ideal, as Earth’s temperature is ideal for liquid water to exist. If it were further away water would turn icy, and if it were closer water would evaporate.
The distance also allows the Sun to be a good energy source for life on the planet.
Why does liquid water matter? It was the solvent in the primordial chemical soup that possibly triggered life, and remains an ideal medium for metabolic reactions. Its solvent properties sustain vital life processes in the body.
Its other unique properties help make the planet hospitable in a plethora of other ways. Water has a higher density in liquid form than in solid- a very strange property. Yet floating ice helps insulate the water below it, allowing life to exist in cold conditions. Its exceptionally high specific heat capacity sustains weather systems and creates hospitable climates. For a more specific example, take a look at plants- water’s adhesive and cohesive properties form the backbone of their transport system.
Earth is also the only known planet with an atmosphere compatible to life. For example, it is the perfect thickness. Too thick like on Venus, and sunlight would never reach the surface; too thin and the planet would not retain enough heat.
There are other properties, too. Earth’s magnetism, for one, keeps us safe from solar flares.
Life was here. What next?
Bacteria are hardly what come to mind when we talk about life on Earth. When we say ‘life,’ we usually mean organisms that survive and evolve the way everything around (and including) us has.
Life may have been born from a chance interaction, but intelligence is the result of a chain of incredibly improbable complex adaptations. As far as we know, even throughout the history of our planet, complex adaptations took place very rarely. Critical and defining features of creatures like us were not frequent occurrences, but evolved only a handful of times. They were one-offs that got lucky and spread across species. From the formation of backbones to multifunctional eukaryotic cells, these events are necessary predecessors to large organ systems and communication networks that allow us to ponder these questions today. These events depended upon each other, and all of them upon the greatest improbability of them all- life. If life was an outlier, intelligence was nothing short of a miracle .
It is said that there are more stars in the universe than grains of sand in all the beaches on Earth. Those stars have planetary systems around them- one estimate for the number of planets in the observable universe is 1x 1024, or one septillion. And still- scientists around the world search these planets and still there is nothing concrete, proving intelligent life may exist outside of our world. Even if it did, there are plenty of possibilities where intelligence isn’t developed enough to communicate with other planets, or where it was wiped out completely .
For all practical purposes, and certainly for the period of time Earth is going to be able to sustain us, we alone are the universe’s shot at intelligent beings. We are incredibly lucky to have a planet like ours. One that was given the gift of life, and one that is adapted to host it. After billions of years, the one thing it doesn’t seem to be able to host is reckless human activity. Our actions are permanently destroying carefully balanced systems that have taken millenia to form, at a pace so quick we can barely keep up with ourselves. If we don’t act fast, Earth will lose all the life it has gained. Earth will go back to being a dead rock- and this time, it may not be lucky enough to have a young scientist working away on its surface billions of years later, wondering where it all went.
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