Transcript Planet Detection - University of Washington

Astronomy190 - Topics in Astronomy
Astronomy and Astrobiology
Lecture 12 : Origin of Life on Earth
Ty Robinson
Questions of the Day
• What is the evidence for the timing of life’s origin?
• What are some key components needed for the
origin of life?
• What are some possible settings for the origin of life?
• What is the RNA world, and what role would it have
had in the origin of life?
The CO2 Cycle as a Thermostat
Rainfall
Surface
temperature
Greenhouse
effect
(-)
Silicate
weathering
rate
Atmospheric
CO2
(Timescale ~ Millions of Years)
“We are the embodiment of a
cosmos grown to self awareness.
We have begun to contemplate our
origins. Star stuff pondering the
stars.”
- Carl Sagan (1934-1996)
?
Spontaneous Generation
“… of these instances of spontaneous
generation some come from putrefying earth or
vegetable matter, as is the case with a number
of insects, while others are spontaneously
generated in the inside of animals out of the
secretions of their several organs.”
- Aristotle (384-322 BC)
Darwin and common ancestry
Darwin and common ancestry
“…But if (and oh! what a big if!) we could conceive in
some warm little pond, with all sorts of ammonia and
phosphoric salts, light, heat, electricity, &c., present,
that a protein compound was chemically formed
ready to undergo still more complex changes, at the
present day such matter would be instantly
absorbed, which would not have been the case
before living creatures were found. “
- Charles Darwin (1809-1882)
Main questions about the origin of life
•
•
•
•
When? (Archean? Hadean?)
Where? (Ice? Vents? Warm little pond?)
How? (Chemical and biochemical evolution?)
What? (First replicating molecules? First cell? Last
?
Universal Common Ancestor?)
When?
Stromatolites: 3.5 billion years ago
Isotope fractionation (life is lazy):
3.85 billion years ago
Microfossils: 3.5 billion years ago
Where?
What did the Earth look like ~3.9-3.5 billion years ago?
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
• LHB pummeled Earth until ~3.8 Ga
Valley et al., 2002
Why is this a problem for the origin
of life?
• possibly very little continental mass
• lots of hydrothermal activity
• atmospheric composition still under
debate, no oxygen
Where?
What are the essential things a setting should have to be
ideal for the origin of life?
Carbon source
• organic molecules to make the building blocks of life
Energy source
• (to power many chemical reactions, we need to be able to
transfer electrons)
• --> a setting should have some molecules that can be electron
donors or electron acceptors
Concentration and catalysis
• we need to get a high enough concentration of ingredients to make
reactions happen
• reactions can happen more quickly with a catalyst
Questions?
Which of the following are not found/met in
the atmospheres of giant planets?
ingredients - carbon sources
energy source
concentration
energy source & concentration
Where?
Primordial soup
(open ocean or
“warm little pond”)
Pros:
• organic molecules from
ocean floor, delivery by
meteorites
• lightning produces electron
acceptors
Cons:
• too dilute
• no means of concentrating
materials
The Miller-Urey Experiment, 1950s
Where did the organic
molecules to make life come
from?
Miller and Urey simulated
the early ocean and
atmosphere with “lightning”
Gas cooled and “rained”
down into a flask: organic
compounds were created
Now: “atmosphere”
composition probably
incorrect
Where?
Sea ice
Pros:
• concentrates organic
molecules in brine pockets
• complex molecules are stable
in cold temperatures
Cons:
• source of organic molecules?
• energy source?
Photo credit: Marcela Ewert Sarmiento
• slow reaction rates
Where?
Hydrothermal
vents
Pros:
• good energy source
(molecules for giving and
receiving electrons)
• minerals can concentrate
molecules and catalyze
reactions
• production of organic
molecules
• last universal common
ancestor a thermophile?
Cons:
• some nutrients missing
• high temperatures destroy
some molecules
How (and what)?
“master copy”
DNA
transcription
RN
A
“working copy”
translation
pro
tein
How (and what)?
DNA
transcription
RN
A
translation
pro
tein
The RNA World
RNA as both information molecule and catalyst?
Natural selection in action: RNA molecules that were
better at making copies of themselves were more “fit”
The first “evolving” entity may have been an RNA molecule
The RNA World
How was
RNA first
made?
Clays may
have played
a role
How did it end?
Franchi & Ferris, 2002
DNA
How did it start?
Eventually, proteins
(somehow) took over as
the catalysts and DNA
(somehow) became the
main information
molecule.
transcription
RNA
translation
protein
Questions?
If clays did play an important role in the
origin of life, which of the following are
good sites for life’s origin?
hydrothermal vents
Earth’s early beaches
ice
warm pond
Encapsulation
Important because it kept beneficial molecules within
the cell without being released to the environment
(natural selection can act on the whole cell)
• Still unknown:
– When did it happen?
– How did it happen?
Lipid vesicles from a
meteorite
RNA in a lipid membrane (made in lab)
An (oversimplified) summary
“the molecular biologist’s dream”
LUCA: Our last universal common ancestor
LUCA
thermophilic biofilm
How do viruses fit into all this?
Three hypotheses for the origin
of viruses:
1.
Degenerative theory
(cells gone rogue)
2.
“Escaped” genetic
elements (genes gone rogue)
3.
Vestiges of the prebiotic
world (rogue since the beginning)
How do viruses fit into all this?
Are viruses remnants of the RNA world?
Forterre, 2005.
Did viruses “come up with” DNA first, and
then give it to cells?
Did life originate more than once?
• Would it be immediately be consumed by
pre-existing life?
• conditions for origin of life might not be right
for maintenance of life
• “Shadow biosphere?”
Panspermia
?
Questions of the Day
• What is the evidence for the timing of life’s origin?
• What are some key components needed for the
origin of life?
• What are some possible settings for the origin of life?
• What is the RNA world, and what role would it have
had in the origin of life?
Origin of eukaryotes?
“Endosymbiotic theory”
Lots of disagreement still goes on about this!
(When? How? In what order?)