How? How do you know?
Natural
62
Don’t Know
13
Supernatural
73
Autonomous Don’t Know
External
31
36
87
Natural
Autonomous
Supernatural
External
The Origin of Life on Earth: Story Elements
• The Hadean Eon witnessed the accumulation
of water and chemicals from space (CH 2)
• Two hypothetical origins
– Space
• Earth was seeded with cells from
elsewhere (CH 3)
– Chemical evolution
• Life arose from non-life by the
combination of chemicals on the surface
of early Earth
The Origin of Life on Earth: Story Elements
• Chemical evolution
– Random inorganic interactions produced
molecules able to form similar molecules
– Information in simple molecules enabled the
synthesis of larger, stable, complex shapes
– Larger, stable, complex molecules could
participate in new kinds of chemical
reactions (CH 1)
The Origin of Life on Earth: Story Elements
• Chemical evolution
– Interacting systems of molecules became
enclosed in compartments
– Control was exerted over the entrance,
retention and exit of molecules and the
reactions that were taking place
– This doesn’t happen any more (CH 1)
The Origin of Life on Earth: Story Elements
• Chemical evolution
– Experimental support?
• Stanley Miller
–Strongly reducing atmosphere
»CH4, NH3, H2, H2O
–Electrical discharge
–Building blocks of life: amino acids
(CH 3)
The Origin of Life on Earth: Story Elements
• Chemical evolution
– The atmosphere was not strongly reducing
• CO2, H2S, SO2, N2
– Optically active isomers were selected from
racemic mixtures
– Building blocks had to be polymerized
• Clay minerals
• Deep ocean vents (CH 3)
The Origin of Life on Earth: Story Elements
• Chemical evolution
– Which came first - DNA or protein?
– Or RNA?
• RNA enzymes (ribozymes) can perform
some catalytic functions
• Peptide bonds are formed by the large
ribosomal subunit
• DNA is made from RNA by reverse
transcriptase (CH 3)
The Origin of Life on Earth: Story Elements
• Chemical evolution
– Doesn’t happen today - as far as we know
• The strongly oxidizing atmosphere favors
the reverse process
• Hungry heterotrophs would consume any
reduced compounds
• Comparable energy sources are not
present (CH 3)
The Origin of Life on Earth: Story Elements
• Life from chemicals
– Life requires compartmentation
• Lipids form bubbles in water
–Lipids + prebiotic chemistry + RNA
world = the bubble theory of life’s
origin (coacervates) (CH 4)
Video Introduction
• Remember your biochemistry – life is based on reduced carbon compounds
• oxidized carbon (CO2) is reduced by
NADPH in the Calvin Cycle
• today, the atmosphere is strongly
oxidizing (C as CO2).
Video Introduction
• “Life” implies reproduction of “like” offspring
- a hereditary mechanism
– amino acids and nucleotides can assemble in
any order with equal likelihood
• a 100 a.a. polypeptide can be assembled in
20100 ways (1.27 x 10130)
• a 300 a.a. polypeptide can be assembled in
20300 ways (2.04 x 10390)
–that’s raw probabilities
a (specific) hereditary mechanism
• “Life” implies reproduction of “like” offspring
- a hereditary mechanism
– amino acids can assemble in several ways
other than amino-carboxyl
• chemical synthesis of a.a. produces both
D and L stereoisomers; proteins have only
L
• non-protein amino acids occur in nature
• under ideal conditions, odds are small
• that’s chemistry
a (specific) hereditary mechanism
• solution?
– “an infinite number of monkeys typing on
an infinite number of keyboards will
eventually produce all of the great works of
literature”
– with infinite time, random combinations of
building blocks will assemble functional
proteins
…but
a (specific) hereditary mechanism
• earth has been habitable for ~4.0-4.5 billion
years (plenty of time for Darwinian evolution)
• life has been present for ~ 3.85 billion years
(apparently)
– life appeared in ~150-650 million years
• that’s really fast…
Video Introduction
• The Origin of Life: Researching the
Possibilities
– Contains excerpts from “The Great
Computer” a parable from The Hitchhiker’s
Guide to the Galaxy.
– Originally broadcast by the BBC under the
title,
Life is Impossible