Life on the Earth

Abstract

Change in life forms over the long span of the Earth’s history, and the theory of evolution are discussed in chapter 7. Along with the tenets of geologic time (chapters 3 and 4) and plate tectonics (chapters 5 and 6), evolution encompasses another foundational idea in geology. This chapter examines the history of evolutionary thought and theory, starting with Charles Darwin and his work on natural selection. The historic “Bone Wars” that occurred with the discovery of the dinosaur fossils is an example of how fossils are used and sometimes misused to unravel the evolution of a significant branch in the Earth’s history of life. So too, the story of horses and their ancestors is portrayed in the Cenozoic era, as early equine ancestor species responded in their body size and tooth and foot structure to changes in climate and the opening of grasslands. The number and variety of life forms waxes and wanes over geologic time, through evolution and sometimes extinction events, only to re-emerge over eons, eras, periods, and epochs, leading to pulses of biodiversity in the fossil record. The theory of evolution was forged after the work by Darwin and others by later developments in molecular biology and DNA research which support modern evolutionary theory.