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20
LIFE'S ORIGIN AND EARLY EVOLUTION
Impacts, Issues: Looking for Life in All the Odd Places
A. Some organisms can live in extreme environments.
1. Thermus aquaticus can live in hot water at 176F.
a. A heat-resistant enzyme taken from this prokaryote is used in
PCR.
b. This has resulted in a new line of inquiry—bioprospecting.
2. Some organisms can survive near hydrothermal vents on the seafloor
(230F), others in acidic springs (pH near 0), and still others cling to
life on mountain glaciers.
B. Nanobes, one-tenth size of bacteria, are growing 3 miles
below Earth in 170 degree Celsius rocks.
20.1 In the Beginning
A. The big bang describes the instantaneous
distribution of all matter and energy throughout
the universe.
1. Gaseous particles condensed into stars, where nuclear reactions began
giving off light.
2. Dying stars left behind clouds of dust and gas, which began to cool
and form our solar system.
B. Conditions on the Early Earth
1. About 4.6 billion years ago remnants of exploding stars began to
condense into planets around the sun.
a. The earth was initially very hot, but cooled to form an outer
mantle and partially-molten core.
b. First atmosphere probably consisted of gaseous hydrogen,
nitrogen, carbon monoxide and carbon dioxide.
****Oparin/Haldane’s Hypothesis
c. Free oxygen levels were low until about 2.2 billion years ago.
2. When the earth’s crust cooled, water condensed, rains began, and
pools of chemicals began to form.
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C. Abiotic Synthesis of Organic Compounds
1. Within 200 million years life had originated on its surface, but
how?
a. Evidence from neighboring bodies in our solar system indicates
that precursors for building biological molecules must have
been present on the primitive Earth.
b. Energy in the form of sunlight, lightning, and heat from the
Earth's crust was also present.
2. Could large organic molecules have formed spontaneously and then
evolved into the molecular systems of life?
a. Stanley Miller used a lab apparatus to demonstrate synthesis of
amino acids from a mixture of hydrogen, methane, ammonia, and
water under abiotic conditions.
b. Other possibilities include the arrival of organic compounds from
outer space and synthesis of biological molecules near
hydrothermal vents in deep seas.
3. The assembly of proteins, DNA, and other complex organic
compounds could have been facilitated by clay templates that brought
them together in the same place and time.
a. Even if molecules were formed spontaneously, they would have
quickly hydrolyzed unless clay templates served to hold the
molecules together for condensation reactions.
b. Experiments by Sidney Fox showed that when amino acids are
placed in water and heated, they spontaneously order
themselves into small protein-like molecules which Fox called
"proteinoids."
20.2 How Did Cells Emerge?
A. Origin of Agents of Metabolism
1. The structure of the first proteins dictated their behavior, including
some which acted as enzymes, leading to metabolic pathways.
2. Possibly the porphyrin ring (a component of both chlorophyll and
cytochromes) was the electron transporter of the first metabolic
pathways.
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B. Origin of the First Plasma Membranes
1. The metabolism in living cells cannot occur without a barrier
against the chemical actions on the outside.
2. Proto-cells were probably membrane-bound sacs containing
nucleic acids that served as templates for proteins.
3. Membrane-bound sacs can form spontaneously, incorporating
proteins and fatty acids.
C. Origin of Self-Replicating Systems
1. From accumulated organic compounds emerged replicating systems
consisting of DNA, RNA, and proteins.
2. An RNA world may have preceded DNA's dominance as the main
informational molecule.
3. How DNA entered the picture is not yet clear, but we do know that
some reactions were more probable than others—not random.
4. Sidney Fox heated amino acids to form protein chains, which when
allowed to cool self-assembled into proteinoid spheres that were
selectively permeable.
5. David Deamer combined fatty acids and glycerol to form long-tail
lipid molecules that self-assembled into small, water-filled sacs
resembling cell membranes.
20.3 The First Cells
A. The Golden Age of Prokaryotes
1. The original prokaryote line split into archaebacteria, eubacteria,
and a line leading to eukaryotes.
2. Evolution of the cyclic pathway of photosynthesis in eubacteria
tapped a renewable source of energy—sunlight; large
accumulations of these cells are seen today as fossils known as
stromatolites.
3. By the dawn of the Proterozoic eon (2.7 billion years ago), the
noncyclic pathway evolved among cyanobacteria that were
producing oxygen.
a. An oxygen-rich atmosphere stopped the further chemical origin
of living cells.
b. Free oxygen permitted aerobic respiration, which now became
the dominant energy-releasing pathway.
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B.The Rise of Eukaryotes
1. The oldest complete eukaryotic fossils are about 2.1 billion years
old and had organelles.
2. The earliest known organism is filamentous red algae.
20.4 Where Did Organelles Come From?
A. Origin of the Nucleus and ER
1. Prokaryotic cells have infoldings of the plasma membrane that may
have been the forerunners of the endoplasmic reticulum and nuclear
envelope.
2. The appearance of these organelles would have provided a
separation of the DNA and enzymes from the surrounding
cytoplasm.
B. Origin of the Mitochondria and Chloroplast
1. According to the theory of endosymbiosis, one species becomes a
resident inside another cell to the benefit of both.
2. Aerobic bacteria could have become the mitochondria;
cyanobacteria could have become chloroplasts.
C. Evidence of Endosymbiosis
1. A laboratory culture of Amoeba became infected with a bacterium;
some cells died but others thrived and became dependent on the
invaders to make an essential enzyme for them.
2. 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.
3. Chloroplasts are believed to have originated by endosymbiosis too;
they resemble cyanobacteria in metabolism and have DNA that is
self-replicating.
The rest of the notes are for information purposes only - not for quizzing!
20.5 Life in the Paleozoic Era (550 to 240 MYA)
A. During the Cambrian period, nearly all of the major phyla evolved;
most organisms lived on or near the sea floor (trilobites were a
dominant group).
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B. In the Ordovician period, the Gondwana continent drifted southward,
shallow marine environments were formed, reef organisms flourished,
and glaciers formed to trigger the first mass global extinction.
C. In the Silurian and Devonian periods, Gondwana drifted northward, reef
organisms recovered, predatory fishes flourished, and amphibians and
stalked plants were moving onto land.
D. In the Carboniferous period, major radiations of plants and animals
occurred as land masses were alternately flooded and drained; coal
deposits formed.
E. In the Permian period, insects, amphibians and reptiles flourished; the
formation of a supercontinent called Pangea caused the greatest of all
mass extinctions.
20.6 Life In the Mesozoic Era (240 to 65 million years ago)
A. Speciation On a Grand Scale
1. The Mesozoic is divided into the Triassic, Jurassic, and Cretaceous
periods.
2. Early in the Cretaceous, the supercontinent Pangea began breaking
up, favoring divergences and speciation on a grand scale, especially
flowering plants, insects, and reptiles.
B. Rise of the Ruling Reptiles
1. Early in the Triassic, rather small dinosaurs evolved from reptilian
lineage.
2. In time adaptive zones opened up, allowing proliferation of the huge
dinosaurs.
3. At the close of the Jurassic, many dinosaurs perished in a mass
extinction caused perhaps by plumes of molten material from
ruptures in the Earth's crust or impacts of asteroids.
4. Later in the era, superplumes caused global temperatures to
increase, which led to a proliferation of photosynthetic organisms.
5. At the close of the era, the last dinosaurs vanished in a mass
extinction that may have been due to the consequences of an asteroid
impact in Mexico.
20.7 Focus on Science: Horrendous End to Dominance
20.8 Life In the Cenozoic Era
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A. The breakup of Pangea resulted in major changes in land mass
configurations, climates, and adaptive zones.
B. During the Paleocene epoch, climates were wetter and warmer
allowing forests to extend farther north and south than they do today.
C. During the Eocene epoch, continued warming saw the emergence of
assorted mammals in new habitats.
D. From the Oligocene through the Pliocene, an abundance of grazing
animals thrived in the woodlands and grasslands.
E. Today the distribution of land masses favors species diversity, yet the
activities of human civilization that began about 50,000 years ago
have accelerated the pace of extinction.
2. Some types of coenzymes (enzyme helpers) have a structure
identical to that of RNA nucleotides; furthermore, nucleotide
precursors will self-assemble under conditions similar to those on
the early Earth.
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