Transcript complex life.2004

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
The Beginning
• Emergence of life from non-life
- (Oparin and Miller experiments)
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Respiration – energy from organic matter
Self-replication
First cell (3.5 bya)
Accumulation of oxygen in atmosphere and
aerobic respiration – 1 bya
• Aerobic respiration and ozone layer
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.
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 chemosythesis 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
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.
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
New Arrivals, Diversification, and Decline
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 (pred, comp)
• 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.
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
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