Transcript BigBang - Warren`s Science Page

Chapter 20
Life’s Origin and Early Evolution
AP Biology
Spring 2011
Chapter 20.1
In the Beginning
Big Bang
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Big Bang: describes the instantaneous distribution of all
mater and energy throughout the universe
Big Bang
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Gaseous particles condensed into stars, where nuclear
reactions began giving off light
Dying stars left behind clouds of dust and gas, which
began to cool and form our solar system
As particles began clustering together and gaining mass,
pre-planetary objects started to form increasing
gravitational pull
About 4.6 billion years ago Earth and other
planets had formed
Conditions of Early Earth
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Volcanic eruptions
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Led to early atmosphere of water vapor, carbon dioxide,
nitrogen, and hydrogen
Free oxygen levels were low until about 2.2 billion years
ago
When the earth’s crust cooled, water condensed, rain
began and carried minerals from rocks in runoff
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Runoff carried nutrients into the seas where life began
Origins of the Building Blocks of Life
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Early experimentation demonstrated non-biological
mechanisms for producing organic molecules
Stanly Miller: used lab apparatus to demonstrate
synthesis of amino acids from hydrogen, methane,
ammonia, and water under abiotic conditions
Other possibilities: arrival of organic compounds from
outer space and synthesis of biological molecules near
hydrothermal vents in deep seas
Assembly of amino acids has been detected in carbon rich
meteorites
Origins of the Building Blocks of Life
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Still many questions about early earth conditions!
Chapter 20.2
How Did Cells Emerge
Origins of Proteins and Metabolism
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Metabolism: group of interacting molecules that copy
themselves repeatedly and use different molecules to
supply the reaction
Origins of Proteins and Metabolism
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Hypothesis #1: Proteins formed on clay tidal flats
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Clay would have attracted amino acids with an electrical
charge
Sunlight could have provided energy to form bonds
Origins of Proteins and Metabolism
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Hypothesis #2: Metabolic
pathways evolved at
hydrothermal vents
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Heat, minerals, and pressure
combined at the vents to create
conditions sufficient to induce
organic compound synthesis
Experimental evidence supports
part of each claim
Origin of the First Plasma Membranes
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Proto-cells were simple membrane bound sacs containing
nucleic acids that served as templates for proteins
Membrane bound sacs can form spontaneously,
incorporating proteins and fatty acids in conditions similar
to the clay tidal flats
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Why do we need plasma membranes?
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Origin of Genetic Material
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From accumulated organic compounds emerged
replicating systems consisting of DNA, RNA, and proteins
An “RNA world” may have preceded DNA’s dominance
as the main informational molecule
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Evidence supports this
Origin of Organic Material
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RNA still serves as enzymes in cells
rRNA catalyzes protein formation
DNA sequencing of rRNA shows very small variation
between prokaryotes and eukaryotes
Self-replicating ribosomes have been synthesized in the
lab
Origin of Organic Material
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How DNA entered the picture is not yet clear
DNA may have provided several advantages over RNA
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DNA is more stable and less susceptible to damage
DNA enabled storage of more information
DNA prevented viral attacks on RNA (until viruses adapted
new enzymes that worked on DNA)
Chapter 20.3
The First Cells
The Golden Age of Prokaryotes
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Common ancestor about 4.3 billion years ago
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Primitive fossils date back 3.2-3.5 billion years ago
Genomic analysis of living prokaryotes indicate an
important divergence 3.46 billion years ago that may have
produced a branch leading to bacteria and another
lineage leading to archea and eukaryotes
The Golden Age of Prokaryotes
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Evolution of the cyclic pathway of photosynthesis in
bacteria tapped renewable source of energy
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Sunlight
Large accumulations of these cells are seen today as fossils
(stromatolites)
The Golden Age of Prokaryotes
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Proterozoic eon (2.5 billion year ago)
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Noncyclic pathway had evolved among cyanobacteria that
were producing oxygen
Led to an oxygen rich environment with three important
consequences:
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An oxygen rich atmosphere stopped the nonliving synthesis of organic
matter
Free oxygen permitted aerobic respiration, which now became the
dominant energy-releasing pathway
Oxygen enriched atmosphere and helped
to screen UV rays allowing life to move onto
land
The Rise of Eukarotes
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The oldest complete eukaryotic fossils are about 2.1
billion years old and had organelles
Red Algae: earliest known organism
750 million years ago algal diversity increased
570 million years ago: earliest animal fossils
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Animals diversity increased during an adaptive radiation 543
million years ago
Led to all major animal lineages
Chapter 20.4
Where Did Organelles Come From?
Origin of the Nucleus, ER, and Golgi
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Is possible that the infoldings of the plasma membrane
seen in prokaryotic cells could have led to the narrow
channels that permit the separation of tasks and materials
seen in the ER of eukaryotic cells
Origin of the Nucleus, ER, and Golgi
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Importance of infoldings:
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Increases surface area for membrane bound
enzymatic/metabolic activity
Could have surrounded DNA to become nuclear envelope
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Protects cells from foreign DNA
Bacteria take up DNA from their surroundings and
viruses
Origin of Mitochondria and Chloroplasts
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According to theory of endosymbiosis:
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One species becomes a resident inside another cell to benefit
both
Origin of Mitochondria and Chloroplasts
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Aerobic bacteria could have become the mitochondria
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Mitochondria are similar in size to bacteria, have their own
DNA, divide independent of the cell, and the mitochondrial
inner membrane resembles a bacterial cell’s plasma membrane
Origin of Mitochondria and Chloroplasts
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Cyanobacteria could have become chloroplasts
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Chloroplasts resemble cyanobacteria in metabolism and have
DNA that is self-replicating
Glaucophytes are protists with unique photosynthetic
organelles resembling cyanobacteria with separate cell
walls
Evidence of Endosymbiosis
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Lab culture of Amoeba became infected with a bacterium
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Some cells died but others thrived and became dependent on
the invaders to make an essential enzyme for them
Chapter 20.5
Time Line for Life’s Origin and Evolution
Time Line
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