1. Life’s Origin and Early Evolution
Chapter 18
Biology Concepts and Applications, Eight Edition, by Starr, Evers, Starr. Brooks/Cole, Cengage Learning 2011.

2. 18.2 Beginnings: The Big Bang
Earth formed approximately 4.6 billion years ago

3. Big Bang Theory Big Bang Theory
Model describing the formation of the universe as a nearly instant distribution of matter through space
The universe began billion years ago (13.7BYA)
The Event:
all existing matter and energy appeared and exploded outward from a single point

4. Big Bang Theory The Event:
Single elements of hydrogen and helium formed
Over millions of years, gravity drew the gases together and formed giant stars and eventually galaxies
Asteroids collided and merged together
The heavier the asteroid (pre-planetary rock)  the more gravitational pull they exerted and the more material they gathered
5 billion years ago  the sun was formed
4.6 billion years ago  Earth and planets in our solar system formed

5. Conditions on Early Earth
First Air Contained  water vapor, carbon dioxide, and gaseous hydrogen and nitrogen
LITTLE OR NO OXYGEN
Later, life began in salty runoff pooled in early seas

6. 18.3 Stanley Miller’s Experiment
Proposed that reactions in Earth’s early atmosphere could have produced building blocks for the first life – Chemical Hypothesis
Experiment
Procedure: Placed water and gases (methane, ammonia, and hydrogen gas) into a reaction chamber
Observation: As the mix circulated, sparks from electrodes simulated lightning
Result: Within weeks, a variety of amino acids and other small molecules formed

7. 18.3 Stanley Miller’s Experiment

8. Reactions at Hydrothermal Vents
Hydrothermal vent – variation on the chemical hypothesis
Rocky, underwater opening where mineral-rich water heated by geothermal energy steams out
Reactions at vents can produce organic building blocks
Experiment:
Hot water + carbon monoxide + potassium cyanide + metal ions (like those near vents) 
Result: amino acid formation

9. Extraterrestrial Hypothesis
Organic compounds were brought to earth by comets, meteorites and other materials from space
Evidence: (from the video Origins)
Amino acids found in meteorites that was found in Australia
Capsule experiment (amino acids can become peptides under conditions of high temp and pressure
Space dust and comets have amino acids

10. Key Concepts: ABIOTIC SYNTHESIS OF ORGANIC COMPOUNDS
When Earth first formed about 4 billion years ago, conditions were too harsh to support life
Over time, its crust cooled, seas formed, and organic compounds of the sort now found in living cells may have formed spontaneously or arrived in meteorites

11. 18.4 From Polymers to Cells All cells have a
plasma membrane,
genome of DNA that is transcribed into RNA and protein
All cells replicated and pass on copies of genetic material to descendants
Self-replicating genetic systems require proteins (including enzymes) and nucleic acids

12. Origin of Metabolism
Proteins and nucleic acids may self-assemble when certain conditions are met
Clay-template hypothesis
Clay (- charge) attach + charge molecules
Low tide, evaporation concentrated the subunits
Concentration and energy from the sun caused the molecules to bond together as polymers
Hydrothermal vent hypothesis
High pressure, high temperature environment
Iron sulfide in rocks donated electrons to dissolved carbon monoxide  start reactions that lead to the formation of larger organic compounds

13. Protocells: Origins of the Plasma Membrane?
Laboratory produced protocell
Lipid bilayer encloses RNA
Cell grows by adding fatty acids and nucleotides
Mechanical force causes protocell division

14. Origin of the Plasma Membrane
Protocell
Membranous sac that contains lipid-enclosed collections of interaction molecules
It is able to take up materials and replicate itself
Hypothesized to have formed prior to the earliest life forms

15. Proposed sequence for the evolution of cells
living cells
membrane-bound proto-cells
Self-replicating system enclosed in a
selectively permeable, protective lipid sphere
enzymes and
other proteins
DNA
RNA
Proposed sequence for the evolution of cells
formation of
protein-RNA systems,
evolution of DNA
formation of
lipid spheres
spontaneous formation of lipids,
carbohydrates, amino acids, proteins,
nucleotides under abiotic conditions
Fig. 18.7, p.295

16. Origins of Self-Replicating Genetic Systems
Hypothesis: RNA world
RNA stores genetic information, but breaks apart easily and mutates often
Ribozymes: Catalytic RNAs
Switch from RNA to DNA
Makes the genome more stable
Defense against viruses that attack RNA-based cells

17. 18.5 Life’s Early Evolution
3.8 billion years ago, oxygen levels in atmosphere and seas were low
Early prokaryotic cells probably were anaerobic
Stromatolites
Dome-shaped structures composed of layers of bacterial cells and sediment
Cyanobacteria and photosynthetic bacteria grow here

18. Stromatolites

19. The Oxygen Atmosphere
Cyanobacteria evolved photosynthesis and released waste gas oxygen
Changed Earth’s atmosphere
Free oxygen levels increased and carbon dioxide levels decreased
Increased oxygen favored aerobic respiration
Key innovation in evolution of eukaryotic cells

20. The Oxygen Environment
Consequences of Life
Oxygen interferes with self-assembly of complex organic compounds. Life no longer could arise from nonliving materials
Oxygen put organisms that thrived in aerobic conditions at an advantage. Aerobic respiration allowed the evolution of multicelled eukaryotes
Some oxygen molecules broke apart and then recombined as ozone (O3). The ozone layer reduced the amount of solar UV radiation.
Without the ozone life could not have moved onto land

21. Key Concepts: ORIGIN AND EARLY EVOLUTION OF CELLS
Laboratory experiments and advanced computer simulations support the hypothesis that forerunners of living cells arose through known physical and chemical processes, such as tendency of lipids to assemble into membrane-like structures when mixed with water

22. Key Concepts: ORIGIN AND EARLY EVOLUTION OF CELLS (cont.)
First cells probably were anaerobic prokaryotes
Some gave rise to bacteria, others to archaeans and to ancestors of eukaryotic cells
Photosynthetic bacteria started releasing free oxygen into the atmosphere
Oxygen accumulated over time and became a global selection pressure

23. 18.4 Where Did Organelles Come From?
Eukaryotic internal membranes may have evolved through infoldings of cell membrane

24. infolding of plasma membrane
DNA
infolding of plasma membrane
Fig a, p.298

25. Fig b, p.298

26. Endosymbiosis
Endosymbiosis  One cell lives and reproduces inside another
Host and guest cells come to depend upon one another for essential metabolic processes
Mitochondria and chloroplasts may have evolved by endosymbiosis

27. Key Concepts: HOW THE FIRST EUKARYOTIC CELLS EVOLVED
A nucleus, ER, and other membrane-enclosed organelles are among the defining features of eukaryotic cells
Some organelles may have evolved from infoldings of the plasma membrane
Mitochondria and chloroplasts probably are descendants of bacterial cells that became modified after taking up residence in host cells

28. Key Concepts: VISUAL PREVIEW OF THE HISTORY OF LIFE
Key events in life’s origin and early evolution can be correlated with the geologic time scale
A time line for milestones in the history of life offers insight into shared connections among all organisms