Kick-starting life on Earth

Life was once thought to have slowly started in a hot primordial soup. But modern thinking is life began with a bang, maybe many times over and probably in temperatures more like today.

Infant Earth was certainly no place for delicate human beings, the toxic gases would have killed us almost instantly, but life had to begin somewhere. In a private letter to a friend, Charles Darwin wrote that life may have started in a ‘warm little pond’. Perhaps thinking such a place may have offered the right protected conditions for life to delicately evolve over many eons. Now, 150 years later, the turbulent oceans are considered to be that ‘pond’ and life is thought to be far more keen and robust.

A tumultuous beginning

Fossil records show life on Earth started around 3.5 billion years ago; about a billion years after our world came into existence. In its infancy our planet was a broiling, bubbling cauldron of molten rock, pummelled by asteroids that scorched the skies. Gradually this ocean of magma cooled enough to form the first hard crust across the globe; the oldest rocks found are in the Jack Hills in Western Australia and date back 4.4 billion years.

The comets that bombarded our early world for hundreds of millions of years are thought to have brought a significant amount of the water we have today. The rest coming from volcanic activity as steam escaped from spewing magma. Meteorites may also have brought organic material with them that helped form life’s first building blocks, although interplanetary dust could also have played a big role in this. Today these tiny cosmic particles still fall down onto the planet and their collective mass far outweighs that of comets.

Life’s first baby steps

Once the Earth’s crust formed, condensing steam rained back down onto the planet’s surface and began collecting in low-lying areas. Thunder storms lasting hundreds or thousands of years would likely have wracked the globe to create our oceans. And the lightning from storms and volcanoes may have given life the electric kick-start it needed.

In 1953, Stanley Miller conducted experiments to recreate conditions similar to early Earth. He put gases and water into a flask, heated them and introduced electric sparks to mimic lightning. The result was a sludge containing five amino acids. He was the first human to create these organic building blocks of life – amino acids that make up DNA and 75% of our dry body weight. Another of his experiments used a type of volcanic apparatus and managed to create 22 amino acids.

From this and other experiments (such as the creation of rapidly self-replicating RNA – thought to be the precursor to DNA) the evolution of life is thought to be far more ready to get underway when it can and less rare an occurrence. Despite today’s organisms all coming from just one tree of life that can be traced back to “LUCA” (the last universal common ancestor), some think life may have arisen from many different streams in the very beginning – although no evidence has yet been found. If this did happen, then perhaps life was later limited through competition where our LUCA-line dominated all others, the simple unsuitability of other streams for long-term survival or the first mass extinction on Earth.

Primordial soup was more like gazpacho

For many years life was believed to have started from organic material in a hot ocean environment – a sort of primordial soup. Samples of rock indicated ocean temperatures were a barmy 55 to 85 ⁰C – the average temperature at the ocean surface today is about 17⁰C. However, researchers now believe the temperature of the atmosphere and ocean was more similar to modern day.

The Barberton Greenstone Belt in South Africa contains mountains that may now tower over the landscape but were once part of the ancient ocean floor. They are made from rocks that formed at the same time as Earth’s early life, 3.5 billion years ago. Previous tests on the rocks showed temperatures when they formed to be high, but these samples are now thought to have been close to a hot hydrothermal vent. More recent tests show the Barberton rocks to have formed in far cooler temperatures, meaning Earth would have been far more agreeable to life far sooner than thought.

Surely we are not alone

Excitingly we still have more to learn about the beginning of life on Earth, questions still to be answered and discoveries still to be made. But one thing is certain, our world is a wonderful Goldilocks planet with just the right conditions for life, but we are not unique and more are increasingly being found throughout the cosmos orbiting other stars. In its early days our planet was no picnic, yet life still evolved and flourished, and the universe is ripe with the building blocks to create RNA and DNA, and maybe other forms. So if life can evolve here, what’s really out there?

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