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Astronomy 141
Life in the Universe
Prof. Scott Gaudi

Lecture 3: Rise of Life on Earth


Key Ideas

Origin of Water on the Earth is not well understood

Early atmosphere likely had little oxygen

Origin of life on Earth very poorly understood.

Organic molecules are likely precursors to life, but where did they come from?
--Space?
--Pools of water?

RNA is a simpler molecule than DNA and can self-replicate

Early life was anaerobic, probably chemoautotrophic

Organisms evolved photosynthesis

Oxygen probably produced by cyanobacteria

Great step: Eukaryotes, Diversification, Colonization of Land

Rise of Homo Sapiens and the implications for extraterrestrial intelligence


Did Earth Form with a lot of Water?

Earth formed in a gaseous disk with condensates.
Materials are found as solids and liquids if they are below their condensation temperature at that point in the disk
What was the temperature of the disk where the Earth formed?
This it the Equilibrium Temperature
Brightness of the Sun decreases as distance squared .
A perfectly absorbing surface illuminated by light with a given energy per area per time will ultimately reach an equilibrium temperature such that:
Energy per area per time = constant times temperature to the fourth power.
The radiation from the sun heats the planet to its equilibrium temperature:
For the Sun, assuming a perfect absorber and that the energy is absorbed on one hemisphere but emitted over the entire surface, and assuming that the distance from the Sun (d) is equal to its average distance (the semimjaor axis a), the equilibrium temperature is:

The temperature of the disk at the place where the Earth was formed was ~270K.

Properties of Water
Water can have three phases
Solid
Liquid
Gas
Which phase it is in depends on:
Temperature
Pressure
At atmospheric pressure
Solid-Liquid at 0? Celsius
Liquid-Gas at 100? Celsius
At lower pressures
Solid-Liquid stays near 0? Celsius
Liquid-Gas at lower temperatures
At ~0 degrees Celsius and ~0.6% atmospheric pressure
Solid, Liquid, and Gas "Triple Point"
Below ~0.6% atmospheric pressure
Water is only a Solid or a Gas (solid to gas is called "sublimation")

The stuff that made the Earth was too hot for solid water.
Stuff that made Earth must have been dry
The early Earth was deficient in water and other volatiles


So where did the water come from?

Must've come from further out in the solar system
-Beyond ~2.7 AU.

Two possibilities:
--Comets
--Asteroids


Water from Comets

Comets hail from outer solar system (tens of AU)
Measurements of the abundance of HDO from three comets.
All the same, and double that of ocean water!
Caveat: Maybe not representative of outer solar system comets?


Asteroids

Outer asteroid belt rich with water (comets in the asteroid belt!)
Measurements of the abundance of HD from chondrites -- same as oceans
Elemental abundance of chondrites unlike Earth


Asteroids

One theory proposes that:
--Jupiter stirred up the asteroid belt
--Sent water-rich asteroids to the Earth
--Cause of late bombardment and water

Jupiter needed for water?

Origin of water unclear, but has important implications


Volatiles released by outgassing

Volcanoes produce:
--water, carbon dioxide, silicon dioxide, carbon monoxide
--no Oxygen!
Water condenses to form oceans
What little oxygen was produced was removed by oxidation
Evidence from geological record, e.g. red beds


Early Earth atmosphere had little oxygen


Genesis of Life is very poorly understood


Origin of Complex Organics

Oxygen-rich atmosphere prevents complex organic molecules from forming outside of cells

Various proposals
--Spontaneous origin in shallow pools?
--Organic molecules from space?
--Chemical reactions near deep-sea vents


Miller-Urey Experiment

Attempted to simulate chemical conditions on early Earth

Resulted in brown 'goo' - amino acids!

It is not clear if early atmosphere had enough Hydrogen


Complex Organics (or Life) from Space?

Comets also contain organic chemicals
Earth, Mars, etc., are constantly trading stuff
Impacts dislodge material which travels to Earth
We have found meteorites from Mars (only a few dozen!)
Such 'trading' was more common in the past.


Hydrothermal Vents?

Utilizes chemical energy (rather than sunlight)
Has some support from the tree of life


From Organics to Replication

RNA is DNA's simpler cousin
--One strand rather than two
--Easier to replicate

DNA cannot replicate itself
--Requires a catalyst

RNA can be its own catalyst


RNA World

How did self-replicating RNA come about?
Must have a way of concentrating the organic molecules
--Clays
--Vesicles (pre-cells made out of lipids)


Pre-Cells

Cells concentrate organics - speed up reactions.
Create 'individuals' and so enabled natural selection
Form spontaneously under certain conditions


From Molecules to Prokaryotes

We can come up with a plausible story for how this happened.


Nature of Early Life

Anaerobic

Probably chemoautotrophic
--Energy from chemical reactions
--Carbon from carbon dioxide dissolved in the ocean

Likely evolved very quickly.

Eventually organisms evolved photosynthesis
--Useful for harvesting energy of the sun


Photosynthesis and Oxygen

Photosynthesis
carbon dioxide + water + sunlight = organic compounds + oxygen

Produced by blue-green algae (cyanobacteria)


History of Oxygen on the Earth

~2.4 billion years ago
--Cyanobacteria begin making O2

2.4-0.4 billion years ago
-Oxygen levels slowly rise as sinks are saturated.


Timeline of Complexity

1.2 billion years ago
--Multicellular organisms

550 million years ago
--Cambrian Explosion
--Large diversification of life
--Causes? Oxygen? Genetic complexity? Climate change? Predators?

475 million years
--Colonization of land
--Life on land is harder
--Build-up of ozone


Rise of Homo Sapiens

appeared ~100,000 years ago


It took less than a few hundred million years for life to develop.

It took ~50 times longer for intelligent life to develop.



See A Note about Graphics to learn why the graphics shown in the lectures are generally not reproduced with these notes.

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