1. The Origin and Evolution of Life on Earth
Biology Chapter 12 Section 3
With information from
www3.geosc.psu.edu/~ruk15/teaching/Exobiology.ppt
windward.hawaii.edu/facstaff/miliefsky-m
lakesidehs.dekalb.k12.ga.us
and
daphne.palomar.edu/mlane/ASTR210/lecturenotes/

2. Objectives
Give the sequence of events that may have led to complex organic molecules on Earth
Give the sequence of events that may have led to prokaryotic organisms on Earth
Discuss the events leading to the evolution of autotrophs and indicate the importance of autotrophs on Earth

3. When did life form? Age of the Earth: 4.6 billion years
Oldest rocks: 3.8 – 4.0 billion years
Life not possible during period of heavy bombardment ~ 4.0 billion years ago

4. Early Earth Atmosphere
Suns energy stripped away 1st atmosphere (mostly H2 and He)
2nd atmosphere formed from volcanic outgassing

5. Atmosphere
The gases that likely made up the ‘new’ atmosphere are those that were expelled by volcanoes.
Water vapor (H2O)
Carbon dioxide (CO2)
Sulfur dioxide (SO2)
Carbon monoxide (CO)
Hydrogen sulfide (H2S)
Hydrogen cyanide (HCN)
Nitrogen (N2)
Hydrogen (H2)

6. When did life form?
Oceans began to form BYA as Earth cooled (water condensed from atmosphere and impacted from comets)
Near the end of the heavy bombardment
Signatures of life: 12C/13C suggests photosynthetic life existed ~ 4.0 BYA

7. When did life begin? Microfossils dating to 3.5 billion years ago
Difficult to distinguish from mineral structures
Analysis shows that some structures contain organic carbon
-found in at least 3 sites

8. Early Life Stromatolites (3.5 BYa)
Rocks with distinctive layer structure
Look identical to living mats of microbes
Layers of microbes and sediment
Top layer uses photosynthesis
Lower layers use top layer’s byproducts

9. When did life begin?
Evidence in metamorphic rocks that life existed billions years ago
Low C12/C13 fraction in rock layers suggests life
Biological processes prefer C12 to C13
Find lower fraction of C13
Non-biological processes have no preference

10. Where did life begin? Land is unlikely Shallow ponds
No O2, no ozone: UV destroys molecular bonds
Shallow ponds
Once favored, full of organic material
When evaporated, organic chemical concentration increases making it easier to combine complex molecules leading to life
Current experiments indicate lack of chemical energy sufficient to support life
Deep-sea vents/hot springs
DNA evidence suggests that early organisms survived in conditions similar to deep-sea vents
Plenty of chemical energy available

11. Origins: Modern Ideas
Simple organic molecule formation
The primordial soup hypothesis was an early hypothesis about the origin of life.
Organic molecules could have been synthesized from simple reactions.
UV light from the Sun and electric discharge in lightning might have been the primary energy sources.

12. Stanley Miller and Harold Urey were the first to show that simple organic molecules could be made from inorganic compounds.
Later, scientists found that hydrogen cyanide could be formed from even simpler molecules in simulated early Earth environments.

13. Variations of Miller-Urey Experiment
Different mixes of gases to represent atmosphere
Different energy sources, like UV (sunlight)
Results: ALL PRODUCE AMINO ACIDS AND COMPLEX ORGANIC MOLECULES
Not as much as original experiment
MUST be more sources of organic material

14. Other possible Sources of Organic Molecules
Chemical reactions in atmosphere
Lab experiments show this is likely
Organic material brought by impacts
Chemical analysis of comets and carbonaceous chondrites show that they have organic molecules
Chemical reactions near deep-sea vents
Heat from undersea volcano can fuel chemical reactions between water and minerals

15. Genetic Code
Some RNA sequences appear to have changed very little through time.
Many biologists consider RNA to have been life’s first coding system.
Other researchers have proposed that clay crystals could have provided an initial template for RNA replication.

16. Early Cell-like Structures
Advantages to enclosing enzymes with RNA molecules
Close proximity increases rate of reactions between them
Isolate contents from outside world

17. Nonliving Pre-Cells have Lifelike Behavior
Grow in size until unstable then split to form a ‘daughter’ cell
Selectively allow other types of molecules to pass in/out of membrane
Store energy in the form of electric voltage (amounts of ions inside/outside cell)

18. Quick Summary

19. Panspermia?
Panspermia = life originated elsewhere and migrated to Earth
Life began in rock, then kicked off the planet by an impact
Support: organic material is everywhere, and some bacteria can withstand large amounts of radiation and go dormant under low atmospheric conditions
Problem: entire solar system was under heavy bombardment at the same time
Problem: rock to be ejected out of its own system, then fall into ours and hit the tiny planet of Earth

20. Panspermia Martian meteorites
Both have possible fossil evidence of life on Mars

21. Living cyanobacteria
Microfossils in carbonaceous chondrites

22. Early Life Photosynthesizing Prokaryotes Archaea are autotrophic.
They do not obtain their energy from the Sun.
Archaea also do not need or produce oxygen.

23. Photosynthesis Most important new metabolic process evolved gradually
Organisms that lived close to ocean surface probably developed means of absorbing sunlight (UV in particular)
Once absorbed, developed method of turning it into energy
Modern organisms: purple sulfur bacteria and green sulfur bacteria use H2S instead of H2O for photosynthesis

24. Photosynthesis
Using water for photosynthesis developed later, perhaps 3.5 billion years ago
First appearing in cyanobacteria (blue-green algae)
By product of O2, released into atmosphere
Changed the world (Oxygen revolution)!

25. Rise of O2 O2 is highly reactive
All initial O2 would react with rock and minerals in water
O2 could not accumulate in atmosphere until surface water was saturated
Rocks 2-3 BYA called banded iron formations, show atmosphere had <1% of current amount of O2
Rock evidence suggests that O2 amounts in atmosphere began to rise about 2.0 BYA
Clear evidence of O2 near current levels appears only 200 million YA
Find charcoal (fossil fuel)
Indicates enough O2 in atmosphere for fires to burn

26. Rise of O2 Rise of O2 would have created a crisis for life
O2 reacts with bonds of organic materials
Surviving species avoided effects of O2 because they lived or migrated to underground locations
Many anaerobic microbes found in such locales today

27. Early Eukaryotes Fossil evidence for eukaryotes dates to 2.1 BYA
O2 increasing in atmosphere
DNA evidence suggests that prokaryotes and eukaryotes separated from common ancestor much earlier
O2 played a key role in eukaryote evolution
Cells can produce energy more efficiently using aerobic metabolism than anaerobic metabolism
Adaptations of aerobic organisms that required more energy could develop

28. The Endosymbiont Theory
The ancestors of eukaryotic cells lived in association with prokaryotic cells.
The relationship between the cells became mutually beneficial, and the prokaryotic symbionts became organelles in eukaryotic cells.
This theory explains the origin of chloroplasts and mitochondria.

29. Endosymbiont Theory