CHAPTER FIVE

THE ORIGIN OF LIFE

The primitive Earth

The age of meteorites tells us that the solar system – and therefore the Earth – was born roughly 4.6 billion years ago. The oldest terrestrial rocks are zircone crystals (zirconium silicates) which are 4.2 million years old, but these stones do not, age apart, tell us much, because they are igneous, or magmatic, rocks whose melting processes have erased any trace of history. Much more interesting are the sedimentary rocks, because these were formed by materials that sank to the bottom of ancient seas, and may still contain remnants of the past.
The oldest sediments have been found at Isua, in Greenland, and are 3.8 billion years old, which means that, at that time, there were immense streches of water on our planet, and that the first oceans had originated many million years earlier.
But what was there, in addition to water, on the primitive Earth? The four external planets of the solar system (Jupiter, Saturn, Uranus and Neptune) are still made up mainly of hydrogen, helium, methane, ammonia and water, and it is likely that those same chemicals were abundant everywhere else in the solar system, and therefore even in its four inner planets (Mercury, Venus, Earth and Mars). These were too small to trap light chemicals, such as hydrogen and helium, but the Earth had a large enough mass to keep all the others. It is likely therefore that the Earth’s first atmosphere had great amounts of methane (CH4), ammonia (NH3) and water, and was, as a result, heavy and reducing, like Jupiter’s.