24.1. Origin
of Life
A. Chemical evolution is the increase in complexity of chemicals
leading to the first cells.
1. While today life only comes from life, the first cells had to arise from an
increased complexity of chemicals.
2. The earth came into being about
4.6 billion years ago.
3. Heat from gravitation and
radioactivity formed the earth in several layers with iron and nickel in
a liquid
core, silicate minerals in a semi-liquid mantle, and upwellings of volcanic
lava formed a crust.
B. The Atmosphere Forms
1. The earth's size provides a gravitational field strong enough to hold an
atmosphere.
2. Earth's primitive
atmosphere differed from current atmosphere, consisting of:
a. water
vapor (H2O),
b. nitrogen
(N2),
c. carbon
dioxide (CO2),
d. small
amounts of hydrogen (H2), and
e. carbon
monoxide (CO).
3. Primitive atmosphere was formed
by volcanic out-gassing characteristic of the young earth.
4. The primitive atmosphere
contained little free oxygen (O2) and was a reducing atmosphere as opposed
to the
oxidizing atmosphere of today.
a. A
reducing atmosphere lacks free O2 and allows formation of complex organic
molecules.
b. An
oxidizing atmosphere contains free O2 and inhibits formation of complex organic
molecules.
5. The earth was so hot that H2O
only existed as a vapor in dense, thick clouds.
6. As the earth cooled, H2O vapor
condensed to form liquid H2O, and rain collected in ponds, etc.
7. The earth's distance from the sun
allows H2O to exist in all phases: solid, liquid, and gas.
8. NASA photos seem to confirm that
earth is bombarded by comets adding substantial water vapor.
C. Monomers Evolve
1. Larger comets and meteorites have pelted the earth perhaps carrying organic
chemicals.
2. A meteorite from Mars (ALH84001)
landed on earth 13,000 years ago; may have fossilized bacteria.
3. Aleksandr Oparin's 1938
Hypothesis
a. Suggested
organic molecules could be formed in the presence of outside energy sources:
1) methane (CH4)
2) ammonia (NH3)
3) hydrogen (H2)
4) water (H2O)
b.
Experiments performed by Stanley Miller and others show these gasses in the
primitive atmosphere
reacted with one another to produce small organic molecules.
4. Lack of oxidation and decay
allowed organic molecules to form a thick, warm organic soup.
5. Ammonia may have been scarce;
undersea thermal vents, however, produce much ammonia and additional
natural
reactions form peptides.
D. Polymers Evolve
1. Newly formed organic molecules polymerized to produce larger molecules and
macromolecules.
a. Gunther
Wachtershauser and Claudia Huber have secured peptides using iron-nickel
sulfides under
vent-like conditions.
b. Such
minerals have a charged surface that attracts amino acids and provides
electrons so they bond together.
2. Protein-first Hypothesis
a. Sidney
Fox demonstrated amino acids polymerize abiotically if exposed to dry heat.
b. Amino
acids collected in shallow puddles along the rocky shore; heat of the sun
caused them to form
proteinoids (i.e., small polypeptides that have some catalytic
properties).
c. When
proteinoids are returned to water, they form cell-like microspheres
composed of proteins.
d. This
assumes DNA genes came after protein enzymes; DNA replication needs protein
enzymes.
3. They Clay Hypothesis
a.
Cairns-Smith suggested that amino acids polymerize in clay, with radioactivity
providing energy.
b. Clay
attracts small organic molecules and contains iron and zinc atoms serving as
inorganic catalysts
for polypeptide formation.
c. Clay collects
energy from radioactive decay and discharges it if temperatures or humidity
changes.
d. If RNA
nucleotides and amino acids became associated so polypeptides were ordered by
and helped
synthesize RNA, then polypeptides and RNA arose at the same time.
4. RNA-first Hypothesis
a. Only the
macromolecule RNA was needed at the beginning to lead to the first cell.
b. Thomas
Chec and Sidney Altman discovered that RNA can be both a substance and an
enzyme.
c. RNA would
carry out processes of life associated with DNA (in genes) and protein enzymes.
d.
Supporters of this hypothesis label this an "RNA world" 4 billion
years ago.
E. A Protocell Evolves
1. Before the first cell arose, there would have been a protocell.
2. A protocell would
have with a lipid-protein membrane and carry on energy metabolism.
3. Fox showed that if lipids are
made available to microspheres, lipids become associated with
microspheres
producing a lipid-protein membrane.
4. Oparin demonstrated a protocell
could have developed from coacervate droplets.
a. Coacervate
droplets are complex spherical units that spontaneously form when
concentrated mixtures
of macromolecules are held in the right temperature, ionic composition and pH.
b.
Coacervate droplets absorb and incorporate various substances from the
surrounding solution.
c. In a
liquid environment, phospholipid molecules spontaneously form liposomes,
spheres surrounded by
a layer of phospholipids; this supports a semi-permeable-type membrane.
d. A
protocell could have contained only RNA to function as both genetic material
and enzymes.
5. If a protocell was a
heterotrophic fermenter living on the organic molecules in the organic soup
that
was its
environment; this suggests heterotrophs proceeded autotrophs.
a. A heterotroph
is an organism that cannot synthesize organic compounds from inorganic
substances and
therefore must take in preformed organic compounds.
b. An autotroph
is an organism that makes organic molecules from inorganic nutrients.
6. If the protocell evolved at
hydrothermal vents, it would be chemosynthetic and autotrophs would have
preceded
heterotrophs.
7. First proto-cells may have used
preformed ATP, but as supplies dwindled, natural selection favored cells
that could
extract energy from carbohydrates to transform ADP to ATP.
8. As there was no free O2, it is
assumed that protocells carried on a form of fermentation.
9. First proto-cells had limited
ability to break down organic molecules; it took millions of years for
glycolysis
to evolve.
10. Fox has shown that a microsphere
has some catalytic ability and Oparin found that coacervates incorporate
enzymes if
they are available in the medium.
F. A Self-Replication System Evolves
1. In living systems, information flows from DNA 
RNA protein;
this sequence developed in stages.
2. The RNA-first hypothesis suggests
that the first genes and enzymes were RNA molecules.
a. These
genes would have directed and carried out protein synthesis.
b. Ribozymes
are RNA that acts as enzymes.
c. Some
viruses contain RNA genes with protein enzyme called reverse transcriptase that
uses RNA as
a template to form DNA; this could have given rise to the first DNA.
3. The protein-first hypothesis
contends that proteins or at least polypeptides were the first to arise.
a. Only
after protocell develops complex enzymes could it form nucleic acids from small
molecules.
b. Because a
nucleic acid is complicated, the chance that it arose on its own is minimal.
c.
Therefore, enzymes are needed to guide synthesis of nucleotides and then
nucleic acids.
4. Cairns-Smith suggests that
polypeptides and RNA evolved simultaneously.
a. The first
true cell would contain RNA genes that replicated because of the presence of
proteins; they
become associated in clay in such a way that the polypeptides catalyzed RNA
formation.
b. This
eliminates the chicken-and-egg paradox; both events happen at the same time.
5. Once the proto-cell was capable
of reproduction, it became a true cell and biological evolution began.
a. After DNA
formed, the genetic code still had to evolve to store information.
b. Because
the current code is subject to less errors than other possible codes, and
because it minimizes
mutations, it likely underwent a natural selection process.
6. Most biologists suspect life
evolved in basic steps.
a. Abiotic
synthesis of organic molecules such as amino acids occurred in the atmosphere
or at hydrothermal vents.
b. Monomeres
joined together to form polymers at seaside rocks or clay, or at vents; the
polymers could
have been proteins or RNA or both.
c. Polymers
aggregated inside a plasma membrane to make a protocell that had limited
ability to grow;
if it developed in the ocean it was a heterotroph, if at a hydrothermal vent, a
chemoautotroph.
d. Once the
protocell contained DNA genes, or RNA molecules, it was a true cell.
24.2.
History of Life
A. Macroevolution is the large scale pattern of change taking place over
very long time spans.
B. Fossils Tell a Story
1. A fossil is the remains or traces of an organism preserved in
sediment or rock.
2. The vast majority of dead
organisms are consumed by scavengers or decompose.
3. The great majority of fossils are
found embedded in or recently eroded from sedimentary rock.
4. Sedimentation has
been going on since the earth was formed; it is an accumulation of particles
forming
a stratum, a
recognizable layer in a stratigraphic sequence.
5. Sequence indicates age of
fossils; a stratum is older that the one above it, younger than the one below
it.
6. Paleontology is the
study of fossils that results in knowledge about the history of life.
C. Relative Dating of Fossils
1. Strata of the same age in England and Russia may have different sediments.
2. However, geologists discovered
that strata of the same age might contain the same fossils.
3. Therefore, fossils could be used
for the purpose of relative dating of strata.
4. A particular species of fossil
ammonite is found over a wide range and for a limited time period;
therefore,
all strata in the world that contain this are of the same age.
D. Absolute Dating of Fossils
1. Radioactive dating is one method used to determine the
absolute age of fossils in years.
2. Radioactive isotopes have a
half-life, the time it takes for half of a radioactive isotope to change
into a
stable element.
3. Carbon 14 is a
radioactive isotope contained within organic matter.
a. Half of
the carbon 14 will change to nitrogen 14 every 5,730 years.
b. Comparing
Carbon 14 radioactivity of a fossil to a modern organic matter calculates age
of the fossil.
c. After
50,000 years, Carbon 14 radioactivity is so low it cannot be used to measure
age accurately.
4. It is possible to use potassium
40 and uranium 238 to date rocks and infer the age of a fossil.
a. Half of
the potassium 40 will change to argon 40 every 1.3 billion years.
b. The ratio
of uranium 238 to lead 207 can be used to date rocks older than 100 million
years.
E. The Precambrian
1. Life begins in the Precambrian from 570 million years ago to
4.6 billion years ago.
a. The
Precambrian encompasses 87% of the geologic time scale.
b. Early
bacteria probably resembled Archaea that live in hot springs today.
c. 3.8
billion years ago the first chemical fingerprints of complex cells occur; at
3.5 BYA, prokaryotic
cells appear in stromatolites.
d. By 2
billion years ago, oxygen levels were high enough that anaerobic prokaryotes
were declining.
e.
Oxygen-releasing photosynthesis caused the atmosphere to become oxidizing
rather than reducing.
f.
Accumulation of O2 caused extinction of anaerobic organisms and rise of aerobic
organisms.
g. O2 forms
ozone or O3 in the upper atmosphere, contributing to the ozone shield
and blocking
ultraviolet radiation from reaching the earth's surface; this allowed organisms
to live on land.
2. Eukaryotic Cells Arise
a. The
eukaryotic cell of 1.5 billion years ago is aerobic and contains a nucleus and
organelles.
b. Theory
of endosymbiosis: nucleated cells engulfed prokaryotes that became
various organelles.
3. Multicellularity Arises
a. It is not
known exactly when multicellular organisms appeared.
b.
Separating germ cells from somatic cells may have contributed to diversity of
organisms.
c. Fossils
of Ediacara Hills of Australia from about 600 MYA were soft-bodied primitive
invertebrates.
F. The Paleozoic Era
1. The Paleozoic era lasted over 300 million years, a very active
period with 3 major mass extinctions.
a. An extinction
is the total disappearance of a species or higher taxonomic group.
b. Mass
extinction is disappearance of large numbers of species or higher
groups in a short geological time.
2. The Cambrian period
saw marine algae flourish; an increase in diversity of marine invertebrate
fossils
occurred,
probably as a result of the presence of exoskeletons.
a. Today's
invertebrates all trace their ancestry to the Cambrian period, and possibly
earlier.
b. A molecular
clock, based on a fixed rate of changes in base pair sequences, allows
us to trace backward
how long current species have evolved separately.
c. Why
fossils are easy to find in the Cambrian but not before is complex question;
most likely the animals
evolved earlier but without outer skeletons.
d. Perhaps
the evolution of exoskeletons was due to the presence of plentiful O2 in the
atmosphere.
e.
Exoskeletons may have been due to the increase pressures of predation.
f. Cambrian
sea floors were dominated by trilobites, now extinct.
3. Invasion of Land
a. In the Ordovician
period, marine algae expanded to freshwater.
b. Fungi
associated with plant roots to form mycorrhizae, allowing plants to live on
bare rocks.
c. In the Silurian
period, vascular plants invaded land and later flourished in warm
swamps in the
Carboniferous period.
d. Spiders,
centipedes, mites, and millipedes preceded the appearance of insects on land.
e. The Devonian
period (360-408 million years ago) is called the Age of Fishes and saw
jawless and then
jawed fishes, including both cartilaginous and ray-finned fishes.
f. The Carboniferous
period (286-360 million years ago)
1) This was an age of coal-forming forests with an abundance of club mosses,
horsetails, and ferns.
2) "Age of the Amphibians" because amphibians diversified.
3) The evolution of wings on insects in the Carboniferous allowed insects to
radiate into a diverse group.
4) Primitive vascular plants and amphibians were larger and more abundant
during the Carboniferous
period; climate change to colder and drier began the process that produced coal.
G. The Mesozoic Era
1. Although there was a mass extinction at the end of the Paleozoic, evolution
of some plants and animals
continued
into the Mesozoic era (66.4 - 245 million years ago).
2. The Triassic period
(208-245 million years ago) was the first of three periods of Mesozoic.
a.
Gymnosperms became dominant.
b. One group
of reptiles, theraspids, had fist mammal features.
c. Reptiles,
including dinosaurs expanded.
d. corals
and mollusks dominated seas.
3. The Jurassic period
(144-208 million years ago) was the second of three periods of Mesozoic.
a. Cycads
and ginkgoes flourished; called the "Age of Cycads."
b. Many
dinosaurs flourish in sea, on land and in air.
c. Recent
finds of Caudipteryx confirms birds descended from dinosaurs.
4. The Cretaceous period
(66.4 - 144 million years ago) was last period of Mesozoic.
a. Another
new Chinese fossil, Jeholodens, reveals early mammal with long
snout but sprawling hind limbs.
b. This
ended in a mass extinction in which dinosaurs, most reptiles, and many marine
organisms perished.
H. The Cenozoic Era
1. The Cenozoic (66.4 million years ago to present) is divided into the
Paleogene and the Neogene periods.
2. During the Cenozoic era,
mammals with hair and mammary glands diversified and human evolution began.
3. In the Paleogene period (24-66
million years ago) mammals diversified tremendously from origins in
the
Mesozoic; flowering plants formed vast tropical forests.
a. During
the Paleocene epoch (58-66 million years ago), angiosperms
diversified; birds diversified
greatly; mammals diversified and primitive primates, herbivores, carnivores,
and insectivores appeared.
b. In the Eocene
epoch (37-58 million years ago), subtropical forests with heavy
rainfall thrived; all
modern orders of mammals represented.
c. The Oligocene
epoch (24-37million years ago) began with significant mammalian
extinction; many
modern families of flowering plants evolved; browsing mammals and monkey-like
primates appeared.
4. During the Neogene period
(24 million years ago to present), primates evolved into monkeys, apes, and
then humans;
major climatic shifts occurred; grasslands were replaced by forests, which put
pressure on
primates who
were adapted to living in trees, causing some primates to evolve to a
nonarboreal existence.
a. In the Miocene
epoch (6-24 million years ago), grasslands spread as forests
contracted; apelike mammals
and grazing mammals flourished.
b. During
the Pliocene epoch (2-6 million years ago), herbaceous
angiosperms flourished; first hominids appeared.
c. Pleistocene
epoch (0.01-2 million years ago) was beginning Ice Age and contributed
to significant
mammalian extinction; herbaceous plants spread; modern humans arise and may
have contributed to extinction.
d. The Holocene
(0.01 million years ago to present) saw destruction of forests by humans that
accelerates
mass extinctions; age of human civilization.
24.3.
Factors That Influence Evolution
A. Continental Drift
1. Earth's crust is dynamic, not immobile as was once thought.
2. In 1920, German meteorologist
Alfred Wegener presented data from across disciplines supporting continental
drift.
3. Continental drift was confirmed
in 1960's; the continents move with respect to one another.
4. At 225 MYA, continents were
joined to form one supercontinent called Pangaea which later divided
into
Gondwanaland and Laurasia and then split to form today's configuration.
5. Continental drift explains
why the coastlines of several continents (e.g., the outline of the west coast
of
Africa and
that of the east coast of South America) are mirror images of each other.
6. The same geological structures
(e.g., mountain ranges) are found in many areas where continents once touched.
7. Continental drift explains unique
distribution patterns of several fossils (e.g., species of the
seed fern Glossopteris).
8. Continental drift explains why
same fossils (e.g., reptiles Cynognathus and Lystrosaurus)
are found
on different
continents.
9. Continental drift explains why
Australia, South America, and Africa have distinctive mammals; current
mammalian
biological diversity is the result of isolated evolution on separate
continents.
10. Plate tectonics is
the study of the behavior of the earth's crust in terms of moving plates that
are formed
at ocean
ridges and destroyed at subduction zones.
11. Ocean ridges are
ridges on ocean floors where oceanic crust forms; regions in oceanic crust
where
molten rock
rises and material is added to the ocean floor results in sea floor spreading.
12. Sea floor spreading is
lateral movement of oceanic crust away from ocean ridges due to material added
to ocean
floor.
13. Subduction zones
are regions where oceanic crust collides with continental crust, causing the
oceanic
crust to
descend into the mantle where it is melted.
14. Where ocean floor is at leading
edge of a plate, a deep trench forms bordered by volcanoes or volcanic
island
chains.
15. Two continents colliding form a
mountain range (e.g., Himalayas are result of collision of India and Eurasia).
16. Transform boundaries
are regions where two crustal plates meet and scrape past one another resulting
in
relatively frequent earthquakes.
B. Mass Extinctions
1. Five mass extinctions occurred at ends of Ordovician, Devonian,
Permian, Triassic, and Cretaceous periods.
2. Mass extinctions have been
attributed to tectonic, oceanic, and climatic changes.
3. Walter and Louis Alvarez, in
1977, proposed that Cretaceous extinction was due to the
aftereffects of a
bolide (an
asteroid that explodes producing meteorites) striking earth.
a. A layer
of iridium soot has been identified in the correct strata.
b. A huge
crater near the Yucatan is the impact site.
4. David Raup and John Sepkoski
proposed, in 1984, that marine fossils show mass extinctions every 26
million years,
in periodicity with astronomical movement through the galaxy.
5. Continental drift contributed to Ordovician
extinction; Gondwanaland arrived at the south pole and glaciers
chilled
oceans and land until Gondwanaland drifted away from pole.
6. Devonian extinction
may have been bolide event; this saw end to 70% of marine invertebrates.
7. Permian extinction
was very severe; 90% of ocean species and 70% of land species disappeared
perhaps
due to
excess of carbon dioxide due to change in ocean circulation.
8. Triassic extinction
has been attributed to meteorite collision with earth; a crater in Central
Quebec may
have been
impact site.