Crises and Innovation in Early Life

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Transcript Crises and Innovation in Early Life

The Emergence of Complex Life
It is an error to imagine that evolution signifies a
constant tendency to increased perfection. That
process undoubtedly involves a constant
remodeling of the organism in adaptation to new
conditions; but it depends on the nature of those
conditions whether the direction of the
modifications effected shall be upward or
downward
Thomas Henry Huxley (1823-1913)
The Emergence of Complex Life
We wish to learn
• What evolutionary advances have taken place at the
level of the cell?
• What are the major events in the history of life?
• What causes extinctions, and how are extinctions
related to opportunities for new evolutionary
advances?
• Are rates of extinction and rates of evolution
uniform or variable?
The Emergence of Complex Life
“Every individual alive
today, the highest as
well as the lowest, is
derived in an unbroken
line from the first and
lowest forms”
- August Weismann
Life’s Origins – A Chronology
4.6 bya
3.8 bya
3.5 bya
3 bya
2.5 – 2 bya
2 – 1.5 bya
1.5 bya
0.6 bya
formation of primitive earth and atmosphere
first chemical evidence of life
first fossils of procaryotic cells
advent of photosynthesis
communities of procaryotes (stromatolites)
oxygen accumulation
eukaryotic algae
Cambrian explosion
Evolution of Earth’s Atmosphere
Life’s Origins – The Questions
• Where did the raw material for life
come from?
• How did monomers develop?
• How did polymers develop?
• How did an isolated cell form?
• How did reproduction begin?
Life’s Origins – Best Answers
• Where did the raw material for life come from?
– Early earth’s atmosphere
• How did monomers develop?
– Miller-Urey experiment
• How did polymers develop?
– Polymerization on clays, evaporation
• How did an isolated cell form?
– Enclosed membrane of lipid cells
• How did reproduction begin?
– RNA has the ability both to self-replicate and catayze
reactions
Oparin’s Hypothesis
In the atmosphere of the early Earth, energy in the form
of ultraviolet light from the sun or lightning
discharges could have created complex organic
molecules from gasses such as CH4, NH3, and H2.
These complex molecules might have been similar to
the building blocks of life – the amino acids which,
when strung together in long chains, from proteins.
Once formed, the complex organic molecules could
have somehow clumped together in larger units,
eventually taking on the characteristics of primitive
cells.
The gradual synthesis would have taken place in the
early ocean, which he described as a “soup” of
organic molecules.
Miller’s Experiment
• Miller’s classic experiment produced the
organic building blocks of life from a
simulated “primitive atmosphere” of
methane, ammonia, and hydrogen.
• Using a high-energy electrical spark to
simulate natural lightning, amino acids were
formed.
• More recent experiments indicate that the
ammonia and methane (though to be
uncommon in the primordial atmosphere)
can be replaced by carbon dioxide, which
was abundant in the early atmosphere.
• Recent experiments also show that the
electrical discharge mechanism can be
replaced by using energy from ultraviolet
light.
Findings Since Miller
• Amino acids can be replaced from other,
more common pre-cursors, and using UV
light instead of an electric spark
• Molecules able to catalyze chemical
reactions have been formed
• RNA has been shown to have catalytic as
well as self-replicating capability
• How did polymers develop?
– Polymerization on clays, evaporation
• How did an isolated cell form?
– Enclosed membrane of lipid cells
aggregation of
macromolecules
Lipids in an aqueous
solution form coacervates
Molecular Clues
Molecules of living organisms are rich in carbon
compounds containing hydrogen
suggests little or no free oxygen on primitive earth
Only 20 amino acids of the left-handed variety are used by
living things in proteins
suggests a single origin of life
DNA and RNA are the universal basis of all life forms
suggests great advantage of this molecular machinery for
reproduction and growth
ATP is the universal energy currency of all living things
suggests a common origin for metabolism
In all cells, the first steps of carbohydrate metabolism
involve fermentation, and the last steps in aerobic
organisms use oxygen in respiration
suggests that aerobic respiration evolved from anaerobic
Which came first?
Life’s origin requires a molecule that can both store
information and catalyze the synthesis of other
molecules. RNA can catalyze simple reactions and
can help as a template for protein synthesis and for
more RNA synthesis. This suggests that RNA was
probably the first genetic molecule to start life. Later
we suspect that DNA evolved to be a more stable
molecule, and proteins evolved to be more efficient
enzymes. RNA with catalytic activity is referred to as
ribozyme.
A. DNA
B. RNA
C. Protein
D. Carbohydrates
E. Lipids
DNA
(Genetic

RNA

(Information
and catalytic)
(Catalytic and Information)
Structural)
PROTEIN
Crises and Innovation in Early Life
Heterotrophy (consuming organic compounds) almost certainly
evolved before autotrophy (producing organic compounds from
inorganic materials)
Innovation: autotrophy. The earliest autotrophs likely derived
their H from H2 or H2S (akin to chemosynthesis by bacteria of
deep sea vents)
Crisis: the H source became exhausted
Innovation: Photosynthesis (using energy of sunlight to cleave
H from H20)
Crisis: the resulting O2 poisoned the atmosphere (after more
than one billion years of earth ‘rusting”)
Innovation: aerobic respiration
Advent of the Eukaryotic Cell
• Prokaryotic cell
- lacks internal membranes
- little internal organization
- bacteria, blue-green algae
• Eukaryotic cell
- nucleus (internal membrane)
- sub-cellular organelles
-chromosomes
-mitochondria
-chloroplasts
- plants, animals, protozoans, fungi
Eukaryotic and Prokaryotic Cells
The Probable Origin of Mitochondria and
Chloroplasts in Eukaryotic Cells
Endosymbiont origin
-accidental?
-Benefit was
efficiency?
-Benefit was ability
to become larger
(to escape being
engulfed)?
Symbiosis Within a Modern Cell
• The ancestors of the chloroplasts in today's plant cells may
have resembled Chlorella, the green, photosynthetic, singlecelled algae living symbiotically within the cytoplasm of the
Paramecium pictured here.
Multicellularity
• Size limits on how large a single cell can grow and
still function efficiently
• One solution: form colonies (filamentous green
algae, sponges, etc.) Functions are not
coordinated.
• Advanced multicellular organisms show cell
differentiation. Specialized cells form tissues,
different tissues to act collectively as organs, and
different organs coordinate within the organism
• Evolved more than one billion years ago
The four eons of earth history.
Bya = billion years ago, mya = million years ago
The three Eras of
the Phaneozoic,
further divided into
periods.
Major events and
mass extinctions
are noted
Major Events in the
History of Life
• The history of life involves enormous change
• On occasion many species went extinct in a short
time – mass extinctions
• Over time, life has become more diverse and more
complex
• Extinction is commonplace – average species lasts
2 - 10 million years; on average, 1 –2 species go
extinct per year.
• The Earth’s geological and biological histories are
intertwined.
New Arrivals, Diversification, and Decline
Punctuated Equilibrium
• Two views of evolutionary change
- gradual and steady, verses:
- long periods of stasis interrupted by episodes of
rapid change?
• Raises key questions
- rate of evolutionary change
- nature of process
• Fossil record not precise enough for definitive
answers
The Burgess Shale
provides an
exceptional view of
life’s diversity at the
beginning of the
Paleozoic. Some
forms survive today,
others are very ancient
history.
If one could rewind
and re-play the tape of
life, would the
outcome be the same?
Causes of Extinctions
• Not just species, but families and phyla disappear
• Most taxa that ever lived are extinct
• Causes include:
- evolution into descendent form
- due to changes in physical environment
- due to appearance of biologically superior life
forms (predaton, competition)
• These are surface answers
Mass Extinction
• “a relatively brief period of time in which more
species go extinct than usual.”
• Five major ME mark end of:
Ordovician, Devonian, Permian, Triassic, Cretaceous
• K – T event is best known
- end of age of reptiles
- 63-66 mya
- asteroid evidence: iridium, crater
• Opportunity – adaptive radiation
This Iridium signal
led a Berkeley
physicist to propose
that the impact of a
huge meteor some
60-65 mya caused
the K-T extinction
event and the
extinctions of the
dinosaurs.
Clocks in Molecules
Evolution and Natural Selection
• The history of life involves
enormous change
• Over time, life has become
more diverse and more
complex
• Extinction is commonplace
• The Earth’s geological and
biological histories are
intertwined