Chapter 15 - Chapter15LectureOutline Introduction

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Chapter 15 Lecture Outline Introduction How Ancient Bacteria Changed the World A. The evolution of life has had a profound effect on the Earth. 1. Photosynthetic prokaryotes (cyanobacteria) evolved very early in the history of life and left unique fossilized communities called stromatolites. 2. Modern-day cyanobacteria of this type are found in ponds, lakes, and shallow oceans and are virtually indistinguishable from the early forms found in stromatolites. Cyanobacteria that form thick mates or mounds are today found only in inhospitable environments. 3. In addition to being the ancestors of today’s cyanobacteria, these first photosynthetic cyanobacteria produced Earth’s first oxygen-rich atmosphere. 4. Photosynthetic prokaryotes were dominant for about 2 billion years, from nearly 3 billion years ago (bya) to about 1 bya. 5. This chapter begins a survey of all of Earth’s life forms in an evolutionary context, beginning with the evolution of life itself. B. The two goals of the chapters in this unit are as follows: 1. To examine the roles that various organisms have had on the history of life on Earth. 2. To introduce the reader to the diversity of life on earth. I. Early Earth and the Origin of Life Module 16.1 Life began on a young Earth. A. The age of the universe is estimated to be between 10 and 20 billion years old, while Earth coalesced from gathering interstellar matter about 4.6 bya. B. The first atmosphere was likely to have been dominated by hot hydrogen gas. However, the Earth’s gravity was not strong enough to hold onto the light H 2 . C. Studies of modern volcanoes suggest that Earth’s second early atmosphere was composed of water vapor, carbon dioxide, nitrogen, hydrogen sulfide (H 2 S), and possibly some methane (CH 4 ) and ammonia (NH 3 ). D. Earth’s crust cooled and solidified, condensing water vapor into early seas. Early Earth was also subject to intense lightning, volcanic activity, and ultraviolet radiation (Figure 16.1A). NOTE: It is ironic that life arose under conditions that included bombardment by UV radiation, and now a major environmental concern is the depletion of the ozone layer that protects the planet from this radiation (Modules 7.14 and 38.4). E. Fossil evidence shows that photosynthetic prokaryotes existed by 3.5 bya (Figures 16.1B and C). NOTE: The immensity of geological time and the very early events discussed can be made more meaningful by putting them in perspective. Borrowing an idea used by many, use a geologic time scale divided into a “life-on-Earth year.” On such a scale, prokaryotic life evolves in mid-March, eukaryotes first appeared around September 1, dinosaurs flourished around
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Christmas, and the typical human life span of 70 years is represented by the last half-second on December 31. F.
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Chapter 15 - Chapter15LectureOutline Introduction

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