This chapter is a brief summary of the evolution of life on Earth through time.
Historical geology is the science that examines concepts of evolution and geologic time as preserved in the fossil record. Historical geology is relevant to all other sciences that involve studies of the physical environment!. This chapter is a very brief summary of the history of life and discussions about some major geologic events shaping planet Earth. Figure 2.1 highlights many of the key geological and biological events that occurred, impacting life, leading to the present.
Earth formed from the accumulation of dust, gases, asteroids, and small planetesimal in the stellar nebula (as discussed in Chapter 1). During this early period in Earth history conditions on the surface of the planet were probably too hot for oceans to exist. However, over time the surface cooled enough for oceans to form and persist. However, the oceans and atmosphere were chemically very different than what exists today. The Early Earth had no significant free oxygen in the air or oceans, and the oceans were rich in organic compounds, essential for the development of evolution and life. The oldest sedimentary rocks on Earth preserve evidence of biological activity, but only on a primitive microbial level. Early evolution was taking place on the molecular, intercellular, and microbial scales for the first 3 billion years of Earth's history. Eventually primitive life forms began to use photosynthesis as a source of energy, and gradually (over a billion years) the atmosphere and oceans became an oxygen-rich environment allowing more complex life forms to evolve.
Figure 2.1. Geologic Time Scale with highlights in evolution and events in Earth history.