Life makes rocks
If the Earth were completely lifeless, the rocks that make it up would be quite different from those we know. Although the Earth has produced life, life has also produced the Earth we see around us. The importance of life in making rocks is most apparent in the case of limestone, the various types of which are all derived from one or other form of marine life. But limestone is not unique in having its origin in living matter.
In the geological record, the first definite signs of life are found in the Precambrian. While the creatures of that era were simple and less diverse than later species, they did build impressive structures. The best-known are stromatolites, which in Australia and elsewhere have left fossil beds many metres thick. They are generally accepted to be the remains of thick mats of material produced by algae in shallow seawater. Their descendants are still with us but have declined in importance with the arrival of true plants and animals.
At the same time, virtually all the economically important minerals we use are derived from living creatures. The most obvious are the hydrocarbons – oil, coal and gas – which are derived from rotted plants. The whole global warming issue really arises because they were laid down over many millions of years and we are releasing the carbon they contain in just a few hundred, while being amazed that this has any effect on the Earth.
Despite the introduction of fertilizers derived from atmospheric nitrogen, phosphate fertilizer is still responsible for encouraging most of the world's food production. Most of the big phosphate deposits are derived from animal bones and shells. Not all sources of phosphorus are ancient, however – there is a thriving industry in mining guano, recent bird-droppings, for use as fertilizer.