Doctors in a medical emergency always have the same priority: to protect the body's vital life-support systems.
If a patient's internal transport system – the blood – cannot carry oxygen from the lungs and nutrients from the stomach to the cells in the body, then the victim quickly starves. And if waste products such as carbon dioxide and toxic acids are left to fester because they cannot be removed, it is almost as quickly poisoned.
During this period of Earth's history (from about 3.7 billion to 1 billion years ago) a global life-support mechanism emerged that works in a strikingly similar way to the human respiratory system. Without this mechanism microscopic bacteria from 2 billion years ago could never have evolved into plants, animals and people.
The first and most simple part of the Earth's life-support mechanism is very well known. It is rain. As the sun beats down on the planet's surface, the seas get warmer, and some of the water evaporates into steam. Once in the air, the steam cools to form clouds, which get blown about by the wind, eventually to fall elsewhere as rain. Without this automatic fresh water supply, most living things on land and sea would almost certainly perish. No pipes, no pumps, no need for power stations – it just happens, every day, the most precious of all free gifts.
Beneath the surface of what seems a very simple process, an important partnership between the Earth and her living things developed sometime between 3.7 billion and 2 billion years ago. For rain to fall, clouds need to form. Steam molecules can condense back into water only if there is some kind of surface or "seed" around which they can cluster. Luckily, waste gases produced by early bacteria provided perfect surfaces around which steam could turn back into water to form rain. In this way bacteria help nature operate one of her most important life-support systems by seeding clouds. Cloud cover also creates a reflective blanket that sends many of the Sun's scorching-hot rays shooting back into space. And so they help to cool the planet, improving conditions for life on Earth.
This is just one of a number of partnerships between the Earth and living things that help control the climate and prevent excessively hot temperatures from harming life. Another of the Earth's life-support processes helped reduce the levels of salt in the sea preventing the poisoning of early life. It is called "plate tectonics". As you read this, you are sitting on a crust of the Earth which is floating like a giant raft on an underground sea of boiling-hot lava. The Earth's crust is split up into a number of floating plates that are in constant motion. Each plate is either drifting apart from, or bashing into, another one. When they collide they form mountain ranges that soar high into the sky. When they drift apart, huge ocean ridges form in their wake.
So much pressure builds up in the rocks of the Earth that the movement of the Earth's plates causes massive earthquakes and volcanoes, hot geysers and tsunamis. This process has helped secure life on Earth by removing excessive amounts of poisonous salt from the seas. Because the Earth's crust has split into separate plates that move around, like giant pieces of a puzzle, evaporated sea salt gets safely stored deep beneath mountain ranges. As long as the plates continue to move, mountains of salt will always be safely buried under the rocks, leaving levels of salt in the sea low enough for life to continue to thrive.
For billions of years this tectonic cycle has been churning up the surface of the Earth in ultra-slow motion, drastically changing weather patterns, making and destroying super-continents and crumpling crusts as if they were thin pieces of tin foil. Such are the Earth's life-support processes that seem to have kept everything – from the composition of atmosphere, global temperatures and the saltiness of the sea – sufficient for life to thrive. Without these systems, the evolution of complex life as we know it would have been impossible.