Essential Understandings Life's Evolution on Earth:

• Life has been on our planet for billions of years. The evidence for this length of time is primarily found from two sources; 1) fossils and 2) the age of certain rocks. The age of our planet is 4.54 billion years old. The oldest rocks that geologist have found are in northwestern Canada and Australia.

• There are 1,000 million years in 1 billion years. 1,000 million years = 1 billion years. The oldest fossils that have been found are 3.5 billion years old. These fossils come from a type of bacteria, called cyanobacteria, and were found in western Australia. They are among the easiest fossils to recognize.

• In addition to fossil and rock evidence other areas of study provide evidence for evolution. These areas include comparative anatomy, development of embryos, DNA comparisons, species distribution, evolution observed, the predictive power of evolution, nested hierarchies of traits, and artificial selection. Artificial selection is the identification by humans of desirable traits in plants and animals, such as dogs, cats, fish and many crops. 

• Most of the time our planet's life has been very small and found only in the oceans. From the fossil record we know that starting around 530 million years ago, life on our planet started to become more diverse and larger. This event was called the Cambrian Explosion by paleontologists.

• Charles Darwin is famous because he discovered how life changes over time, his theory of natural selection. Natural selection is the process through which populations of living organisms adapt and change. Individuals in a population are naturally variable. This variation means that some individuals have traits better suited to the environment than others. Individuals with adaptive traits—traits that give them some advantage—are more likely to survive and reproduce. These individuals then pass the adaptive traits on to their offspring.