4.0 Introduction

1. Earth Formed 4.5 Billion Years Ago
1. Important "Ages"
   1. Oldest rocks 4.3 billion years
   2. Oldest microfossils are 5 billion years old

2. Certain Correct Events Gave Rise to Life on Earth fig 4.1

4.1 All living things share key characteristics

1. Early Earth Was Inhospitable
1. Formed as Hot Molten Mass
1. Atmospheric vapor formed liquid water as earth cooled
2. Oceans were rich in chemical compounds

2. Life Emerged From Complex Molecules

2. What Is Life?
1. Must Define Life to Determine Whether or Not It Exists
1. Potential characteristics: Value as a definition
1. Movement: Not descriptive of only life fig 4.2
2. Sensitivity: Some life not apparently responsive fig 4.3
3. Death: Meaningless concept
4. Complexity: Describes nonlife also

2. Definition of life must not only be necessary, possessed by all life but sufficient, possessed by only life

2. Fundamental Properties of Life
1. Cellular organization fig 4.4
2. Sensitivity
3. Growth
1. Metabolism: Assimilation of energy resulting in growth
2. Creation of carbon-carbon covalent bonds

4. Development
5. Reproduction
1. Pass traits on from one generation to next
2. All organisms ultimately die

6. Regulation
7. Homeostasis

3. The Key Role of Heredity
1. Bubbles and microspheres fulfil these properties but are not alive fig 4.5
1. Spontaneously form, enclose small amount of water
2. Can enclose energy-processing molecules
3. May grow and subdivide

2. Heredity is a necessary criterion for life
1. A genetic system based on replication of DNA
2. Allows for adaptation and evolution over time

3. Coacervates exist, but have no predictable past or future
1. Wholly dependent on changing environment, not alive
2. Change is not evolution unless passed on to future generation

4. Evolution is the very essence of life

4.2 There are many ideas about the origin of life

1. Ideas about the Origin of Life
1. Possible Origins of Life on Earth
1. Special creation
1. Life created by supernatural or divine forces
2. Oldest hypothesis, most widely accepted
3. Forms basis of unscientific "scientific creationism"

2. Extraterrestrial origin
1. Hypothesis of panspermia
2. Cosmic dust or meteors carried life here from distant planet
3. Discovery of possible fossils in rocks from Mars
4. Cannot be disproved, still unscientific

3. Spontaneous origin
1. Life originated from inanimate matter
2. Force leading to life was selection, culminating in evolution of cells

2. Taking a Scientific Viewpoint
1. Focus on third possibility without discounting others
1. Content of biological examination
2. Only scientific origin permitting testable hypotheses via fossils

2. Examine origination of organic molecules first fig 4.6
3. Consider organization of molecules into living cells next

2. What the Early Earth Was Like
1. A Reducing Atmosphere
1. Primarily carbon dioxide, nitrogen gas, water
1. Secondarily hydrogen sulfide, ammonia, methane
2. Debatable whether free hydrogen gas was present

2. Referred to as a reducing atmosphere
1. Requires less energy to form carbon molecules
2. Free oxygen gas absent

2. High Temperatures
1. Surface of earth remained hot for a billion years
1. Fueled by constant bombardment of rubble from solar system
2. Life evolved in extreme hot temperatures

2. Conditions greatly different from what exists now
1. Presently shielded from UV radiation by ozone layer
2. Significant geothermal energy available fig 4.7
3. Prompted chemical reactions of atmospheric materials

3. Testing the Spontaneous Origin Hypothesis
1. Repeat Processes of Origin
1. Use same atmosphere as early earth
2. Exclude oxygen gas
3. Place atmosphere over liquid water
4. Maintain temperature at 100ºC
5. Simulate lightening with sparks

2. The Miller-Urey Experiment fig 4.8
1. Methane formed carbon compounds fig 4.9
1. Formaldehyde, hydrogen cyanide
2. Further combined into formic acid, urea

2. Later experiments produced carbon compounds
1. Amino acids: Glycine, alanine, valine, proline, glutamic, aspartic acids
2. Adenine produced, one of the bases found in DNA and RNA

3. The Path of Chemical Evolution
1. Debate regarding origin of first organic molecules
2. RNA first, heredity required for consistent production of biomolecules
1. Supported by discovery of ribozymes
2. RNA in ribosomes also has an enzymatic function

3. Proteins first since nothing can be replicated without enzymes
1. Nucleic acid units too complex to form spontaneously
2. Have created synthetic nucleotides that replicate and "mutate"

1. Theories About the Origin of Cells
1. Cell Origins: The Importance of Bubbles
1. Certain molecules spontaneously form hollow spherical bubbles in water
2. Structure shields hydrophobic regions from contacting water
3. May have been similar to froth formed along sea coasts

2. Oparin's Bubble Theory
1. Theory of primary abiogenesis
2. Called first cell-like structures protobionts
3. Led to Urey-Miller experiments

3. A Host of Bubble Theories
1. Synonyms: Microspheres, protocells, protobionts, micells, liposomes, coacervates
2. Name may be dependent on contents of bubble structure
3. Example: Coacervate lipid bubble
1. Form two layer outer boundary resembling lipid bi-layer
2. Accumulate lipid subunits, grow and pinch off projections
3. May contain amino acids that facilitate acid-base reactions

4. A Bubble Scenario
1. Life's origin was likely proceeded by bubble-mediated chemical evolution fig 4.10
2. Microdrops contained different kinds of chemicals
1. Ones containing amino acids last longer
2. Survive since they carry out metabolic reactions and actively grow

3. Microdrops that incorporated molecules and energy persisted
4. Ones that grew and divided were favored also
5. Life began when reliable heredity began

5. Current Thinking
1. Potential for heredity and bubbles
1. No mechanism for lipid coacervates
2. Mechanism imaginable for protein microspheres
3. Protein microspheres form in dry conditions, not in water

2. First components may have been RNA molecules
1. RNA acts as template to assemble new RNA
2. Initial steps led to more complex and stable RNA molecules
3. Stability improved by envelopment by lipid

2. The Earliest Cells
1. Microfossils fig 4.11
1. Fossils found in ancient rocks
2. Microfossils closely resemble present day bacteria fig 4.12
1. Single-celled , 1 to 2 microns in diameter
2. No external appendages
3. Little evidence of internal structures

3. Simple organisms like these called prokaryotes
1. Name means "before nucleus"
2. Eukaryotes with nuclei evolved later

4. Only bacteria existed for first two billion years

2. Ancient Bacteria: Archaebacteria
1. Unusual organisms found in uncommon environments
1. Different from present day bacteria in form and metabolism
2. Little evolution of forms living in unchanging habitats
3. Are living relics of early life

2. Methane-producing bacteria
3. Grow only in oxygen-free environment
1. Anaerobic, poisoned by oxygen
2. Convert CO₂ and H₂ into CH₄ (methane)

4. Resemble other bacteria only superficially

3. Unusual Cell Structures
1. Structure of membrane and cell wall significantly different
1. Absence of peptidoglycan in cell walls
2. Unusual lipids in cell membranes

2. Fundamental differences in metabolic processes

4. Earth's First Organisms
1. Other archaebacteria live in extreme environments
1. Extreme halophiles: Very salty environments
2. Extreme thermophiles: Very hot environments
1. Environment similar earth's early conditions
2. May be relics of the first organisms

2. Comparison of extreme thermophiles and other organisms

5. Eubacteria
1. Strong cell walls, simpler gene architecture
2. Some capture light energy, are photosynthetic
1. Transform it into chemical bond energy
2. Utilize a variety of pigments

3. Cyanobacteria (blue-green algae) are an important group fig 4.13
1. Possess chlorophyll pigment
2. Decisive role in increasing oxygen in earth's atmosphere
3. Increased ozone, protection from ultraviolet radiation
4. Some caused accumulation of limestone deposits

3. Do Bacteria Exist on Other Worlds?
1. Nature of The Earth as a Planet Reflects Its Life Forms
1. Farther from sun
1. Colder temperature, water in the form of a solid
2. Chemical reactions slower
3. Carbon compounds brittle

2. Closer to sun
1. Warmer temperature
2. Chemical bonds and carbon compounds less stable

3. Evolution of carbon-based life
1. Limited by temperature, dependent on distance to sun
2. Affected by size of earth and gravitational pull

2. Ancient Bacteria on Mars?
1. Discovered potential evidence of biological activity in Mars meteorite
2. Suggested presence of ancient bacteria
1. Biotic contamination of minerals
1. Found magnetite often associated with earth bacteria
2. Deposits formed at temperatures too hot for life

2. Organic residues
1. Polycyclic aromatic hydrocarbons found on interior
2. Not like usual earth or meteorite PAHs
3. Like ones bacteria dissolve into during rock forming processes

3. Bacteria-like structures
1. Carbonate spheres exhibit potential microfossils fig 4.14
2. Iron oxide and iron sulfide deposits lie within spheres
3. Cross-sections may reveal cell wall structure fig 4.15

3. Contamination of samples not a likely problem
4. Evidence is indirect and compelling, need more research

3. Bacteria in Alien Earth Environments
1. Potential that life originated in deep-sea hydrothermal vents
1. Vents are chemically-rich, environment comparatively calm
2. Thermophilic archaebacteria are most ancient group of organisms

2. Possibility of life evolving on moon of Jupiter fig 4.16

4. Life in Other Solar Systems
1. Billions of stars resembling sun
1. 10% with planetary systems
2. Chance for proper size and distance allows 1015 earth-like planets

2. Evolution of Different Life Forms
1. Could evolve life from other chemicals
2. Silicon chemistry similar to carbon chemistry

1. Early Fossils Are Small, Simple Cells
1. All Fossils Older Than 1.5 Billion Years Are Structurally Similar
2. Simple Cells Evolved into First Eukaryotes fig 4.17

2. The First Eukaryotic Cells
1. Visually Different Microfossils Appear After 1.5 Billion Years fig 4.18
1. Much larger size, as much as 60 microns
2. Have internal membranes, some contain membrane-bound structures
3. Possess thicker walls, branched filaments or spines
4. New cells called eukaryotes
1. Name means "true nucleus"
2. Includes all organisms other than bacteria

5. Rapidly evolved to produce diverse life forms fig 4.19
6. Pelomyxa, a model early eukaryote
1. Has nucleus, but lacking microtubules, divides like a prokaryote
2. Lacks mitochondria, but has bacteria that perform same function

2. Origin of Mitochondria and Chloroplasts
1. Margulis' endosymbiotic theory of endosymbiotic bacteria
2. Evolution of eukaryotes involved symbiosis with prokaryotes
1. Examples: Mitochondria, chloroplasts, flagella, centrioles
2. Provided cells with energy, photosynthesis, movement
3. Possess own DNA similar to that of bacteria

3. Sexual Reproduction
1. Promotes genetic recombination, raw material for evolution
2. Evolved process of meiosis

4. Multicellularity
1. Single cell organisms formed colonies
2. Division of labor established within a colony

1. The Kingdoms of Life
1. Kingdom Classification
1. First organization attributed to Linneaus: Plants v.s. animals (2 kingdoms)
2. Haeckel added Protista which included bacteria, protists and sponges (3)
3. Copeland separated bacteria into Monera (4)
4. Whittaker put fungi into separate kingdom (5)
5. Margulis moved motile fungi into Protista (5)
6. Molecular studies split archaebacteria into own kingdom (6)

2. The Six Kingdoms
1. Kingdom Archaebacteria: Prokaryotic, archaebacteria
2. Kingdom Eubacteria: Prokaryotic, eubacteria
3. Kingdom Protista: Eukaryotic, unicellular heterotrophs or photosynthesizers
4. Kingdom Fungi: Eukaryotic, multicellular, non-motile heterotrophs
5. Kingdom Plantae: Eukaryotic, multicellular, terrestrial photosynthesizers
6. Kingdom Animalia: Eukaryotic, multicellular, motile heterotrophs