9781422283349

PHOTOSYNTHESIS A step forward was taken by life when a way evolved to use the energy of the sun to power living processes. Photosynthesis involves the energy of sunlight being stored as chemical energy that can be used by the organism. Carbon dioxide gas from the atmosphere is combined with water to form glucose, a simple sugar. As part of this process, oxygen is produced. Oxygen was a deadly poison to the earliest organisms. It reacts very strongly with other substances and would have disrupted the formation of complex molecules that life depends on. Fortunately there was a solution to the problem. Three billion years ago the Earth’s oceans contained a great deal of dissolved iron. The oxygen produced by photosynthesis was combined with this iron to form a now familiar substance—rust. Bound up with the iron, oxygen was now harmless. The non-soluble iron oxide sank down and settled on the ocean floor. Over millions of years it formed rocks known as banded ironstone formations, or BIFs, evidence today of the changes that occurred long ago. The oxygen-producing life forms were restricted to places where there was a supply of iron to combine with their oxygen. Some organisms retreated to places where there was no oxygen, such as the sediments on the seafloor. Bacteria that do not require oxygen to survive can still be found in such places today. As the levels of oxygen increased, organisms appeared that were able to tolerate it. Next, organisms began

to put the oxygen to use in extracting energy more efficiently from their food.

Trilobites, such as this fossil, were among the first multi-cellular animals to appear on earth more than 520 million years ago. These marine creatures lived in relatively shallow waters as well as in deep seas.

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