Astronomy 11-4

History of Life 11/4/99

http://darkwing.uoregon.edu/~bsl/astronomy/

How did chemisty and oceans produce this?

What are the major events in the history of life on earth, when did they happen, and how?
How has the planet helped shape the history of life and how has life helped shape the planet?
How do scientists go about studying these questions?

Some web sites to Explore
Udovic's list of important events in the evolution of life
How Old is the Earth?
How and why does radioative dating work?
How did life originate?
Origin of Photosynthesis
Origin of the Eukaryotic Cell
Some images from the history of life

Some web sites to explore:

Is there life on Europa? Or you could check out the NASA site on Europa.

How about levidence of life in the martian meteor discovered in Antarctica?

An excellent essay on the origin of life by Leslie Orgel.

Class Activity -- What would you consider to be some of the most important events in the history of life?

If we condensed earth's history into a 12-hour period, where would the major events appear on the clock?

Udovic's List of important "breakthrough" events in the history of life

Formation of Earth
Development of oceans; atmosphere
Origin of life
Photosynthesis using H₂0
Origin of Eukaryotic cell
Appearance of Multicellular organisms
Beatles visit the U. S.
Invasion of land by plants and animals
Evolution of consciousness
Invention of baseball
The evolution of sexual reproduction

I'll let you decide which are the serious ones.

What was the early atmosphere like and how did it form?

Condensation of water vapor is crucial
Main steps in evolution of the Earth's Atmosphere
On this planet it was too cold for water vapor to condense. Hence the atmosphere is all Carbon Dioxide
On this planet it was too hot for water vapor to condense. Hence the atmosphere is all Carbon Dioxide
On this planet it was just right. The carbon dioxide content of the earth's atmosphere is now all locked up in rocks.

There are hence two keys to the evolution of planetary atmospheres:

Fate of the water vapor (gaseous, liquid, solid)
Fate of the Carbon Dioxide (stays in atmosphere vs. dissolves in liquid water)

After condensation of water vapor produced the earth's oceans, thus sweeping out the carbon dioxide and locking it up into rocks, our atmosphere was mostly nitrogen.

No O₂!
No Ozone (O₃) layer! Hence no UV screen

Currently, our atmosphere is 72% nitrogen and 28% oxygen (everything else like H₂ and CO₂ exists only in trace amounts).

So where did the oxygen come from??

How did life originate?

Step 1 -- Origin of simple organic molecules (VIDEO EXCERPT)
- Primordial soup hypothesis
  - Miller's experiments -- produces simple organic molecules (e.g., amino acids) in simulated conditions.
  - UV probable source of energy for these chem. reactions
- "Extraterrestrial" hypothesis -- asteroids, cosmic dust
- Deep-sea vents?
- Liquid medium is important
  - Protects molecules from UV photon disruption
  - Ease of transport and interaction
Step 2 -- Formation of macromolecules (e.g., proteins or "peptide chains") from simple components
- Example formation of a peptide chain
- Problem -- water tends to destroy these compounds
- Need to concentrate monomers together to facilitate polymer formation.
- 4 billion years ago the moon was closer to the earth
  - Large lunar tides coupled with storms could have formed transient inland lakes.
  - When that water is evaporated over weeks/months, rich mixture of monomers would settle into the clays at the lake bottom.
- Clays --> Silicate Surfaces --> acts as a catalyst
- Peptide chains of around 100 amino acids can be found today
Step 3 -- Self-replicating system
- A "chicken and egg" problem?
- RNA can catalyze reactions AND self-replicate
- NOW natural selection can take over!!
Step 4 -- Evolution of membranes
- Membrane-like sheets form spontaneously when polypeptides and phospholipids heated together
Step 5 -- Metabolic Pathways
- May have evolved step by step as organisms began to produce the compounds that were becoming depleted in the 'primordial soup'
Take home message -- Our views of life's origins come from a combination of :
- Microfossils - dated at over 3.5 billion years
- Geochemical and geophysical studies that help us understand the physical conditions on the early Earth
- Simulation experiments that mimic these conditions in the laboratory
- Plausibility arguments

Origin of Photosynthesis

First organisms did not use solar energy
- energy source -- high-energy organic compunds in the primordial soup
  - OK until these compounds were depleted (like fossil fuels today?)
- chemo-autotrophs in the deep sea
Major breakthrough -- capturing solar energy directly
- First photosynthetic organisms used H₂S or H₂ as source of electrons
- H₂0 much more readily available
- Formation of stromatalites
Photosynthesis using H₂0 led to release of O₂ as a waste product
- Cyanobacteria-like fossil cells & filaments in 2.8 billion years old Australian rocks
- First major polluters (?), since 0₂ toxic to most of the organisms around at the time
- Accumulation of 0₂ in atmosphere didn't start for several hundred million years. Why??
- Paves way for aerobic respiration

Origin of Eukaryotic Cell

Complex cells appear about 2.1 billion years ago
- Compare a prokaryotic cell with a eukaryotic cell
Endosymbiotic theory
- Organelles (e.g., mitochondria and chloroplasts) in eukaryotic cells are descendents of bacteria
- Primitive eukaryotes such as Peloxyma lack mitochondria, but have symbiotic bacteria associated with them.
Breakthrough
- cells can adapt to new environments by altering size, shape, and number and kinds of organelles (see diversity of eukaryotic cell types)

Earth History condensed into a 12-hour period

Events from the class activity placed approximately where they belong based on current evidence. Each hour on the clock above represents approximately 380 million years. If midnight is the origin of Earth (4.6 billion years), then humans first appear around 11:59:30 am, or 30 seconds before present (noon).