1. Chapter 25: The Origin of Life on Earth CHAPTER 25 The Origin of Life on Earth

2. Chapter 25: The Origin of Life on Earth How Can We Study a Unique Event that Happened Several Billion Years Ago? How Can We Study a Unique Event that Happened Several Billion Years Ago? Necessary Conditions for the Origin of Life Necessary Conditions for the Origin of Life Protobionts: Enclosing Prebiotic Systems Protobionts: Enclosing Prebiotic Systems

3. Chapter 25: The Origin of Life on Earth Photosynthesis Is the Source of Atmospheric O 2 Photosynthesis Is the Source of Atmospheric O 2 Is Life Evolving from Nonlife Today? Is Life Evolving from Nonlife Today? Does Life Exist Elsewhere in the Universe? Does Life Exist Elsewhere in the Universe?

4. Chapter 25: The Origin of Life on Earth How Can We Study a Unique Event that Happened Several Billion Years Ago? Life originated from nonliving matter nearly 4 billion years ago.Life originated from nonliving matter nearly 4 billion years ago. Life’s origins can be studied scientifically by following the principle of continuity, the signature principle, and the “no free lunch” principle.Life’s origins can be studied scientifically by following the principle of continuity, the signature principle, and the “no free lunch” principle.4

5. Chapter 25: The Origin of Life on Earth Necessary Conditions for the Origin of Life Earth at the time of life’s origin had a reducing atmosphere.Earth at the time of life’s origin had a reducing atmosphere. Under conditions that resemble Earth’s early atmosphere, small molecules essential to living systems form and polymerize.Under conditions that resemble Earth’s early atmosphere, small molecules essential to living systems form and polymerize. Review Figure 25.2 25.2 5

6. Chapter 25: The Origin of Life on Earth Figure 25.2 figure 25-02.jpg

7. Chapter 25: The Origin of Life on Earth Necessary Conditions for the Origin of Life Before life appeared, polymerization reactions generated the carbohydrates, lipids, amino acids, and nucleic acids of which organisms are composed.Before life appeared, polymerization reactions generated the carbohydrates, lipids, amino acids, and nucleic acids of which organisms are composed. These molecules accumulated in the oceans.These molecules accumulated in the oceans.7

8. Chapter 25: The Origin of Life on Earth Protobionts: Enclosing Prebiotic Systems The earliest protobionts probably had lipid- based membranes.The earliest protobionts probably had lipid- based membranes. Review Figure 25.3 25.3 8

9. Chapter 25: The Origin of Life on Earth Figure 25.3 figure 25-03.jpg

10. Chapter 25: The Origin of Life on Earth Protobionts: Enclosing Prebiotic Systems The first genetic material may have been RNA that had a catalytic function and an information transfer function.The first genetic material may have been RNA that had a catalytic function and an information transfer function. Some RNA’s—called ribozymes—have catalytic functions today.Some RNA’s—called ribozymes—have catalytic functions today. Review Figure 25.4 25.4 10

11. Chapter 25: The Origin of Life on Earth Figure 25.4 figure 25-04.jpg

12. Chapter 25: The Origin of Life on Earth Protobionts: Enclosing Prebiotic Systems DNA probably evolved after RNA-based life became surrounded by membranes that provided an environment in which DNA was stable.DNA probably evolved after RNA-based life became surrounded by membranes that provided an environment in which DNA was stable.12

13. Chapter 25: The Origin of Life on Earth Photosynthesis Is the Source of Atmospheric O 2 Cyanobacteria, which evolved the ability to split water into hydrogen ions and O 2, created atmospheric O 2. Accumulation of free O 2 in the atmosphere made possible the evolution of aerobic metabolism.Cyanobacteria, which evolved the ability to split water into hydrogen ions and O 2, created atmospheric O 2. Accumulation of free O 2 in the atmosphere made possible the evolution of aerobic metabolism.13

14. Chapter 25: The Origin of Life on Earth Is Life Evolving from Nonlife Today? Because most of the chemical reactions that gave rise to life occur readily under the conditions that prevailed on early Earth, life’s evolution was probably nearly inevitable.Because most of the chemical reactions that gave rise to life occur readily under the conditions that prevailed on early Earth, life’s evolution was probably nearly inevitable.14

15. Chapter 25: The Origin of Life on Earth Is Life Evolving from Nonlife Today? Experiments by Louis Pasteur and others convinced scientists that life does not come from nonlife on Earth today.Experiments by Louis Pasteur and others convinced scientists that life does not come from nonlife on Earth today. Review Figure 25.6 25.6 15

16. Chapter 25: The Origin of Life on Earth Figure 25.6 figure 25-06.jpg

17. Chapter 25: The Origin of Life on Earth Is Life Evolving from Nonlife Today? New life is no longer being assembled from nonliving matter because simple biological molecules that form in today’s environment are oxidized or consumed by existing life.New life is no longer being assembled from nonliving matter because simple biological molecules that form in today’s environment are oxidized or consumed by existing life.17

18. Chapter 25: The Origin of Life on Earth Does Life Exist Elsewhere in the Universe? Conditions that permit the evolution and maintenance of simple prokaryotic life may be widespread in the universeConditions that permit the evolution and maintenance of simple prokaryotic life may be widespread in the universe Multicellular life has more stringent requirements, including a planet with a relatively circular orbit, a rapid rate of spin, nearby planets that intercept impacts, and a large moon that stabilizes the planet’s orbit.Multicellular life has more stringent requirements, including a planet with a relatively circular orbit, a rapid rate of spin, nearby planets that intercept impacts, and a large moon that stabilizes the planet’s orbit. Such conditions may be very rare.Such conditions may be very rare.18

19. Chapter 25: The Origin of Life on Earth Does Life Exist Elsewhere in the Universe? Although conditions on Earth have fluctuated greatly, they have been suitable for multicellular organisms for nearly a billion years.Although conditions on Earth have fluctuated greatly, they have been suitable for multicellular organisms for nearly a billion years.19