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[Prog summaries - [Ice World] [The
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Once this relative stability had been established, the next step to the world we know today was the development of life. Recent discoveries show that the oldest zircon crystals contain a type of oxygen called oxygen 18. Oxygen 18 could only have been present if the zircon crystals had grown in the presence of large quantities of water.
The oxygen 18 reveals that water was present on the surface of Earth 200 million years after its formation, 800 million years earlier than previously thought.
It is thought that this water, a vital ingredient for life, came partly from steam from volcanoes, but also from outer space. Comets contain dust and frozen water vapour left over from the birth of the solar system, and according to the latest theories, it was these that delivered water to Earth. But a question mark remains because the proportions in Earth’s normal water, H2O, and heavy water, HDO, do not match those so far found in comets.
Even in this hostile environment, which would have been untenable for humans, it is thought bacteria developed, deriving their energy by converting hydrogen sulphide into sulphuric acid.
At the time, Earth continued to be pounded by space debris, and this is seen as a key factor in the birth of more complex life. The energy of the impact fused amino acids into peptides — halfway towards building a protein.
Microbial life began to flourish close to the surface and take advantage of the sun’s energy, evolving chlorophyll, which enabled the microbes to harness the sun’s energy through photosynthesis — and convert carbon dioxide into carbohydrates.
Around 3.46 billion years ago, stromatolites bacterium survived long enough to evolve into a variety of different bacterial colonies. They got their energy from absorbed sunlight and carbon dioxide from the atmosphere and produced a waste gas that would transform the entire planet: oxygen.
As well as enabling more complex organisms to breathe, oxygen also helped protect life from the sun’s lethal ultraviolet radiation as oxygen molecules broke apart in the upper atmosphere forming a layer of ozone.
It is thought that the first multi-celled life emerged relatively recently. For only the past 10% of Earth's history has the planet supported life visible to the naked eye.