Chapter 20 Outline and Terms
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20.1. Origin of Life (p. 325)
A. Chemical evolution is the increase in complexity of chemicals leading to the first cells (Fig. 20.1)
1. The earth came into being about 4.6 billion years ago.
2. Heat from gravitation and radioactivity formed the earth in several layers with iron and nickel in a liquid core, silicate minerals in a semiliquid 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 water vapor (H2O), nitrogen (N2), carbon dioxide (CO2), small amounts of hydrogen (H2) and carbon monoxide (CO).
3. Primitive atmosphere was formed by volcanic outgassing 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.
C. Small Organic Molecules Evolve
1. Aleksandr Oparin's 1938 Hypothesis
a. Suggested organic molecules could be formed in the presence of outside energy sources.
b. Experiments performed by Stanley Miller and others show how gases of the primitive atmosphere reacted with one another to produce small organic molecules. (Fig. 20.2) [transp. 117]
2. Lack of oxidation and decay allowed organic molecules to form a thick, warm organic soup.
D. Macromolecules Evolve and Interact
1. Newly formed organic molecules polymerized to produce larger molecules and macromolecules.
2. RNA-first hypothesis
a. Only the macromolecule RNA was needed at the beginning to form the first cell.
b. Thomas Cech and Sidney Altman discovered that RNA can be both a substrate and an enzyme.
c. Also, some viruses have RNA genes.
d. RNA would carry out processes of life associated with DNA (in genes) and protein enzymes.
e. Supporters of this hypothesis label this an RNA world some 4 billion years ago.
3. 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 protein.
d. This assumes DNA genes came after protein enzymes; DNA replication needs protein enzymes.
4. The 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 temperature 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.
E. A Protocell Evolves (Fig. 20.3)
1. Before the first cell arose, there would have been a protocell.
2. A protocell is a cell-like structure with a lipid-protein membrane and carries 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 semipermeable-type membrane.
d. A protocell could have contained only RNA to function as both genetic material and enzymes.
F. Protocells Were Heterotrophs
1. The protocell likely was a heterotrophic fermenter living on the organic molecules in the organic soup that was its environment; this suggests heterotrophs preceded 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.
2. First protocells may have used preformed ATP, but as supplies dwindled, natural selection favored cells that could extract energy from carbohydrates to transform ADP to ATP.
3. As there was no free O2, it is assumed that protocells carried on a form of fermentation.
4. First protocells had limited ability to break down organic molecules; it took millions of years for glycolysis to evolve.
5. Fox has shown that a microsphere has some catalytic ability and Oparin found that coacervates incorporate enzymes if they are available in the medium.
G. 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 a 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 a 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 protocell was capable of reproduction, it became a true cell and biological evolution began. (Fig. 20.4) [transp. 118]
20.2. History of Life (p. 328)
A. History of earth can be displayed as a 24-hour span starting at midnight. (Figure 20.5) [transp. 119]
1. Prokaryotes do not appear until 5 A.M.; eukaryotes are present at 4 P.M.; multicellular forms start at 8 P.M.; land is invaded at 10 P.M.; and humans appear 30 seconds before the end of the day.
2. This relative timetable corresponds to the fossil record spread over 4.5 billion years.
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 is a process that has been going on since the earth was formed and is an accumulation of particulate material forming a stratum or recognizable layer in a stratigraphic sequence.
5. Sequence indicates age of fossils; a stratum is older than the one above it, younger than one below it.
6. Paleontology is the study of fossils that results in knowledge about the history of life.
C. Dating Fossils Relatively
1. Geologists discovered from an early time that a stratum of the same age might contain the same fossil; therefore, fossils could be used for the purpose of relative dating of strata.
2. 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 fossil must be of the same age.
D. Dating Fossils Absolutely
1. Radioactive dating is 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 (14C) is a radioactive isotope contained within organic matter.
a. Half of the carbon-14 (14C) will change to nitrogen-14 (14N) every 5,730 years.
b. Comparing 14C radioactivity of a fossil to a modern organic matter calculates age of the fossil.
c. After 50,000 years, 14C radioactivity is so low it cannot be used to measure age accurately.
4. It is possible to use potassium-40 (40K) and uranium-238 to date rocks and infer the age of a fossil.
a. Half of the potassium-40 (40K) 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. How the Story Unfolds (Table 20.1)
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. 3.5 billion years ago the first prokaryotic cells appear in stromatolites. (Fig. 20.9)
d. By 2 billion years ago, the first oxygen-releasing cells appeared.
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 also contributed the ozone shield, blocking ultraviolet radiation from reaching the earth's surface to allow organisms to live on land.
2. Eukaryotic Cells Arise
a. The eukaryotic cell of 2.1 billion years ago is aerobic and contains a nucleus and organelles.
b. Theory of endosymbiosis: nucleated cells engulfed prokaryotes that became various organelles.
c. By 600 million years ago, multicellular organisms appeared.
d. Separating germ cells from somatic cells may have contributed to diversity of organisms. (Fig. 20.8)
e. Fossils of Ediacara Hills of Australia may be primitive jellyfish, sea pens, and segmented worms.
3. Complexity Increases in Paleozoic
a. The Paleozoic era lasted 300 million years, a very active period with 3 major mass extinctions.
b. 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. (Fig. 20.9)
1) Perhaps the evolution of exoskeletons was due to the presence of plentiful O2 in the atmosphere.
2) Exoskeletons may have been due to the increase pressures of predation.
c. In the Ordovician period, marine algae continued to flourish;
invertebrates spread and diversified; jawless fishes as first
vertebrates appeared; ended with first mass extinction recorded
in the fossil record.
(Table 20.1)
d. In the Silurian period plants invaded land.
1) During the Silurian period, low-lying primitive vascular plants appeared on land.
2) Also, the first jawed fished appeared during this period.
e. The Devonian period (360-408 million years ago) saw the
first seed ferns; jawed fishes diversified and began to dominate
the seas; the first insects and amphibians appeared; ended in
a mass extinction.
(Table 20.1)
f. The Carboniferous period (286-360 million years ago) was the age of great coal-forming forests with an abundance of club mosses, horsetails, and ferns; "Age of the Amphibians"---amphibians diversified; first reptiles appeared; the first great radiation of insects occurred during this period. (Fig. 20.10)
1) Primitive vascular plants and amphibians were larger and more abundant during the Carboniferous period; climate change began the process that produced coal.
2) Insects appeared and flourished to become the largest animal group today.
g. In the Permian period (245-286 million years ago) conifers appeared; reptiles diversified; amphibians declined; ended with a mass extinction in which most species in seas and on land perish. (Table 20.1)
4. Dinosaurs Rule in the Mesozoic
a. 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).
b. The Triassic period (208-245 million years ago) saw cycads and ginkgoes appear; the forests of gymnosperms and ferns dominated; first mammals appeared; first dinosaurs appeared; corals and mollusks dominated seas; ended in mass extinction of many organisms in the seas and some on land; dinosaurs and mammals survived.
c. The Jurassic period (144-208 million years ago) saw cycads and ginkgoes flourish and is called the "Age of Cycads"; dinosaurs flourish (Fig. 20.11a); birds appear.
d. Cretaceous period (66.4-144 million years ago) flowering
plants spread and increased in dominance; coniferous trees declined;
placental animals appeared; modern insect groups appeared; ended
in a mass extinction in which all (or nearly all) dinosaurs, most
reptiles, and many marine organisms perished.
(Fig. 20.16)
1) During the early Cretaceous period, dinosaurs were still diversifying.
2) A mass extinction occurred at the end of the Cretaceous that eliminated dinosaurs.
5. Mammals Take Over in the Cenozoic
a. The Cenozoic (66.4 million years ago to present) is divided into the Paleogene and Neogene periods.
b. Only during Cenozoic era did mammals, with hair and mammary glands, diversify and human evolution begin.
c. In the Paleogene period (24-66 million years ago) mammals diversified tremendously, beginning at the size of a mouse; flowering plants formed vast tropical forests.
d. During the Paleocene epoch (58-66 million years ago, angiosperms diversified; birds diversified greatly; mammals diversified---primitive primates, herbivores, carnivores, and insectivores appeared.
e. In the Eocene epoch (37-58 million years ago), subtropical forests with heavy rainfall thrived; all modern orders of mammals represented; ended with significant mammalian extinction, which continued into Oligocene epoch.
f. The Oligocene epoch (24-37 million years ago) began with
significant mammalian extinction; many modern families of flowering
plants evolved; browsing mammals and monkeylike primates appeared.
(Fig. 20.12)
g. 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.
h. In the Miocene epoch (6-24 million years ago), grasslands spread as forests contracted; apelike mammals and grazing mammals flourished.
i. During the Pliocene epoch (2-6 million years ago, herbaceous angiosperms flourished; first hominids appeared.
j. 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. (Fig. 20.13)
k. The Holocene (0.01 million years ago to present) saw destruction of forests by humans that accelerates mass extinctions; age of human civilization.
20.3. Factors That Influence Evolution
A. The Earth's Crust
1. Dynamic, not immobile as was once thought.
2. In 1920, German meteorologist Alfred Wegener presents data from across disciplines supporting continental drift.
3. Continental drift is the movement of continents with respect to one another over the earth's surfaces.
4. 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. (Fig. 20.14)
5. The same geological structures (e.g., mountain ranges) are found in many of the areas where continents once touched.
6. Continental drift explains unique distribution patterns of several fossils (e.g., species of the seed fern Glossopteris).
7. Continental drift explains why same fossils (e.g., reptiles Cynognathus and Lystrosaurus) are found on different continents.
8. Continental drift helps to explain why Australia, South America, and Africa all have their own distinctive mammals; that is, the current mammalian biological diversity is the result of isolated evolution on separate continents.
9. 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. (Fig. 20.15a)
10. 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 seafloor spreading.
11. Seafloor spreading is lateral movement of oceanic crust away from ocean ridges due to material added to ocean floor.
12. Subduction zones are regions where oceanic crust collides with continental crust, causing the oceanic crust to descend into the mantle where it is melted. (Figs. 20.15a, c)
13. Where ocean floor is at the leading edge of a plate, a deep trench forms that is bordered by volcanoes or volcanic island chains.
14. Two continents colliding can form a mountain range (e.g., Himalayas are result of collision of India and Eurasia).
15. Transform faults are regions where two crustal plates meet and scrape past one another resulting in relatively frequent earthquakes. (Fig. 20.15b)
G. Exploring Mass Extinctions
1. An extinction is the total disappearance of the members of a species or some higher taxonomic group.
2. A mass extinction is disappearance of a large number of species or higher groups within a few million years.
3. After a mass extinction, remaining groups may undergo adaptive radiation as they spread out into empty habitats.
4. Mass extinctions have been attributed to tectonic, oceanic, and climatic changes. (Fig. 20.16) [transp. 120]
5. Five major mass extinctions occurred at ends of Ordovician, Devonian, Permian, Triassic, and Cretaceous periods.
6. 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 layer of iridium soot has been identified in the correct strata and a huge crater near the Yucatan is a possible impact site.
7. 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.
8. Continental drift contributed to Ordovician extinction; Devonian and Triassic extinctions may have been bolide events.
H. Microevolution Versus Macroevolution
1. When fossil evidence permits, paleontologists determine lineages of taxonomic groups.
2. A lineage is a line of evolutionary descent from a common ancestor.
3. Fossils have been found tracing the lineage of the horse from Hyracotherium to Equus.
4. Transitional links have been found linking some major groups but not others.
5. Paleontologists note slim chance of organisms becoming fossils; it is an incomplete fossil record.
6. One possible explanation is that many organisms may change little for long periods. (Fig. 20.17)
7. Stasis is a time of limited evolutionary change in a lineage.
I. Phyletic Gradualism Versus Punctuated Equilibrium [transp. 121]
1. Phyletic gradualism proposes evolutionary change with new species occurring gradually in an unbranched lineage. (Fig. 20.18a)
a. Slow, gradual change leads to speciation that may not necessarily be detected in fossil record.
b. However, some more transitional links would eventually be found in the fossil record.
2. Punctuated equilibrium proposes periods of rapid change with speciation followed by long periods of stasis. (Fig. 20.18b)
a. Supported by evidence that a long period of evolutionary stasis is interrupted by a brief period of speciation.
b. In this case, more transitional links would not be found in the fossil record.