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

(-)Surface

temperature

Rainfall

Silicateweatheringrate

AtmosphericCO2

Greenhouseeffect

(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

Microfossils: 3.5 billion years ago

Isotope fractionation (life is lazy): 3.85 billion years ago

Where?What did the Earth look like ~3.9-3.5 billion years

ago?

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

Valley et al., 2002

• LHB pummeled Earth until ~3.8 Ga

• possibly very little continental mass

• lots of hydrothermal activity

• atmospheric composition still under debate, no oxygen

Why is this a problem for the origin of life?

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

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

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

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?

Photo credit: Marcela Ewert Sarmiento

Sea ice

Pros:

• concentrates organic molecules in brine pockets

• complex molecules are stable in cold temperatures

Cons:

• source of organic molecules?

• energy source?

• slow reaction rates

Where?Hydrothermal

ventsPros:

• 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)?

DNA

RNA

protein

transcription

translation

“master copy”

“working copy”

How (and what)?

DNA

RNA

protein

transcription

translation

The RNA World

Natural selection in action: RNA molecules that were better at making copies of themselves were more “fit”

RNA as both information molecule and catalyst?

The first “evolving” entity may have been an RNA molecule

The RNA World

Franchi & Ferris, 2002

How was RNA first made?

Clays may have played a role

Eventually, proteins (somehow) took over as the catalysts and DNA (somehow) became the main information molecule.

DNA

RNA

protein

transcription

translation

How did it start?

How did it end?

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

thermophilic biofilm

LUCA

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?

Did viruses “come up with” DNA first, and then give it to cells?

Forterre, 2005.

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?

Lots of disagreement still goes on about this! (When? How? In what order?)

“Endosymbiotic theory”