1. Do Now… In your notebook: Write a couple sentences or bullet points on the following:
What components does something need to have to be a cell?
Think about the origin of life on Earth…
What’s your idea of the conditions on Earth when it first formed?
What is your understanding about how life on earth began?
What steps would have needed to happen for life to arise on Earth?
What questions do you have about how this might have happened?
Now turn to a partner and discuss for 2 min.

2. AP Bio: 10.6.11 - History of Life on Earth
Homework:
Study for tomorrow’s test
PS 6 due next Thursday (today is the only day we will discuss it)
Today’s Agenda:
Period 1: Lecture/Notes: How did life begin?
Period 2: Test Review, and History of Life Timelines
Goals for Today:
Define cells
Evaluate the evidence for the origins of life on Earth
Explain the RNA world hypothesis, and evaluate the evidence for it
Identify 10 key events in the history of life on Earth, and explain their importance

3. AP Biology Unit 3: Life is Cellular
monosaccharides
polysaccharides
fatty acids
phospholipids
phosphates
LIFE ??
amino acids
enzymes & other proteins
nucleotides
ATP
nucleic acids (DNA & RNA)

4. What is LIFE?
Life is made of cells that have a metabolism and can self-replicate (reproduce)
What makes a cell?
Membrane (phospholipid bilayer)
Cytosol (fluid inside cell) – a watery solution of nutrients, enzymes, etc. (cytoplasm includes cytosol + organelles outside nucleus)
DNA (hereditary info)
Proteins made from the same 20 amino acids – used as enzymes, structural components, etc.
Ribosomes (translate genetic info into proteins)
All cells contain all these components at some point in their “lives”

5. How did life begin?
How did carbon-based monomers (sugars, amino acids, nucleotides...) form in the first place? How did monomers assemble into polymers without enzymes to help? How did genetic material become self-replicating? How did the first cells form?

6. Early Earth: HOT and BOMBARDED

7. Chemistry of Early (Pre-Biotic) Earth
Atmosphere of CO2 and N2
Also present: CH4, H2S, H2, NH2
What’s missing?
No oxygen gas – and since oxygen often helps break things down (think rust, cell respiration, combustion), the lack of O2 makes the environment more conducive to synthesis (more reducing)
Lots of lightning and volcanic activity
Warm ocean water
For life to form…
Spontaneous formation of biochemical building blocks (amino acids, nucleotides, etc.)
Assemble monomers into polymers without enzymes
Self-replicating genetic material
Enclose it all in a membrane

8. Step 1: Spontaneous monomer synthesis
Miller & Urey’s experiment
Results:
a variety of amino acids
oily hydrocarbons
some nucleosides (pre-cursors to nucleotides)
simple sugars

9. But…
New evidence suggests that the early atmosphere was not as reducing as Miller and Urey’s experiment required
Another possibility – CO2 and H2 reacting at alkaline thermal vents

10. Step 2: Abiotic Polymerization
No biological catalysts (no enzymes)
Experiments have shown that some clay substrates can act as inorganic catalysts – on the clay-y shores of the ocean or a freshwater pond?

11. Which came first, DNA or proteins?
Step 3: Origin of Self-Replicating Genetic Molecules
Which came first, DNA or proteins?

12. Our Current Genetic Situation:
DNA RNA protein
catalyst
protein catalyst
rRNA catalyst
protein

13. the chicken-or-the-egg conundrum
DNA requires proteins (enzymes) to replicate and to make RNA
Making proteins requires DNA and RNA

14. Ribozymes and the RNA world

15. Evidence for the RNA World
Ribosomes…
Made of rRNA wrapped around proteins
Experiments on ribosomes show:
de-activating ribosomal proteins have no effect on their catalytic ability
De-activating rRNA elimates their catalytic ability
Thus: rRNA acts as (the most important) biological catalyst – it is a ribozyme
Small self-replicating RNA molecules have been synthesized in the lab – they can copy themselves indefinitely without enzymes, just a continuous supply of nucleotides

16. Step 4: Package it into a membrane

17. Phylogenetic Tree for all living things
Time >present
Nodes indicate most recent common ancestor of two branches

19. AP Bio: 10.13.11 - History of Life on Earth
Homework:
Finish PS 6
Goals for Today:
Identify 10 key events in the history of life on Earth, and explain their importance
Explain the how, why, and significance of changes in Earth’s atmosphere over time
Explain how prokaryotic structures and functions have allowed bacteria to conquer the world
Today’s Agenda:
Period 1: Earth History Timeline and Discussion
Period 2: Group work on PS 7 - Prokaryotes

20. Timeline Activity: Top 10 Events in the History of Life on Earth
Count off by 5’s to form groups of 3-4 people
Each person – bring your notes from PS 6 #1
As a group: Choose top 10 events in life’s history
On chart paper:
Create a timeline that is roughly to scale – but don’t spend more than 5 minutes figuring out the scale
If necessary, you can create a “zoom in” timeline for specific portions of your overall timeline
Plot each of your top 10 events on the timeline
Beneath each event, explain why it is so important – how did it affect life on earth?
If you have time, illustrate your timeline so it looks pretty
Be ready to explain/present your timeline to the class

22. Where did the oxygen come from??
Original atmosphere contained no O2
Atmosphere of CO2 and N2
Also present: CH4, H2S, H2, NH2
2.7 bya: Atmospheric oxygen begins to increase… why?
Photosynthetic prokaryotes! Cyanobacteria.
CO2 + H2O  glucose + O2
How did this affect other life on earth?
Obligate anaerobes die off
Evolution of aerobes – use O2 for highly efficient breakdown of glucose to yield high amounts of ATP (aerobic cellular respiration)

23. The little guys that changed the world…