9781422283349

natural selection at work that pushed things along a particular path and acted to preserve those changes that gave an advantage. Those molecules that could make many accurate copies of themselves would get the biggest share of the raw materials and would have the best chance of surviving. One thing we can be certain about is that the very earliest forms of life got the energy they needed from the chemicals around them. As millions of years went by this source must have become increasingly scarce. A life form that could use another source of energy would have an advantage over the others. In the 1950s, geologists discovered microscopic structures in ancient rocks over 3 billion years old. These structures closely resembled life forms alive on Earth today. They were fossils of bacteria, including a type of bacteria called blue- green bacteria or cyanobacteria. What was important about them was that if they were like the present day

cyanobacteria they would have been able to get the energy they needed by photosynthesis .

Fossil evidence indicates that the first green plants on land were very similar to this modern liverwort moss. The development of tiny plants that got their energy from photosynthesis more than two billion years ago released large amounts of oxygen into the Earth’s atmosphere, causing radical changes. Over millions of years the amount of oxygen in the atmosphere increased as more was produced by photosynthesizing organisms. In the upper atmosphere oxygen was converted into ozone by the ultraviolet radiation. This ozone shield absorbed much of the UV radiation, preventing it from reaching the surface where it would have harmed the complex life forms that were evolving.

1 1 .