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Chapter 18
Origin and
History of Life
• By 4.6 BYA, our solar system was formed
• As Earth formed, heavier atoms like iron and nickel
became molten liquid core, dense silicate minerals
became the semiliquid mantle, and volcanic lava
formed crust
• Mass of Earth large enough to retain a gaseous
atmosphere
• Early atmosphere contained inorganic chemicals like
water vapor, carbon dioxide, hydrogen, ammonia,
hydrogen sulfide, and carbon monoxide
• Must have been a reducing atmosphere meaning
mostly free of oxygen
• As surface cooled, water vapor condensed to liquid
water and rain in time formed the oceans
• Closer to the sun and all water would have evaporated
• Farther from the sun and all water would have frozen
• Earth is struck by 5-30 ice comets the size of a house
every minute
• Hypotheses on evolution of monomers:
• Monomers came from outer space, monomers came
from reactions in atmosphere, or monomers came
from reactions at hydrothermal vents
• Organic molecules have been confirmed in some
meteorites
• In 1953, Stanley Miller using ideas of A.I. Oparin and
J.B.S. Haldane carried out a very famous experiment
• Oparin and Haldane suggested early atmosphere gases
in presence of strong energy sources could produce
monomers
• Energy sources include: heat from volcanoes and
meteorites; radioactivity from isotopes; electricity
from lightning, solar, and ultraviolet radiation
• Oparin’s idea is called abiotic synthesis-the formation
of sugars, amino acids, nucleotide bases from
inorganic molecules
• Miller placed a mixture resembling early atmospheremethane, ammonia, hydrogen, and water- in a closed
system
• He heated it and circulated it past an electric spark
• After a week, the mixture contained a variety of
amino acids and organic acids
Fig. 18.1
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
electrode
electric
spark
stopcock for
adding gases
CH4
NH3
H2
H2O
stopcock for
withdrawing liquid
condenser
boiler
heat
gases
hot water out
cool water in
liquid droplets
small organic molecules
• In cells, organic monomers join to form polymers in
the presence of enzymes, which are proteins
• Wachtershauser and Huber have shown that amino
acids will form peptides in the presence of iron-nickel
sulfides
• Sidney Fox’s experiment showed that amino acids
polymerize abiotically in dry heat
• If amino acids in ocean collected in shallow puddles
along the shore
• Heat of sun could cause formation of proteinoids
(small peptides with some catalytic properties)
• In lab, proteinoids returned to water form
microspheres
• Fox’s protein-first hypothesis assumes that DNA
genes came after protein enzymes arose
• Graham Cairns-Smith hypothesized that clay helped
cause polymerization of monomers to produce both
proteins and nucleic acids at the same time
• RNA-first hypothesis suggests that only RNA was
needed to progress toward formation of the first cell
• 1989 Nobel prize to Cech and Altman for discovery
that RNA can be both substrate and an enzyme
• Some viruses have RNA genes
• Protocell had preceded true cell and had to have a
membrane
• If lipids combine with microspheres, a lipid-protein
membrane is formed
• Under appropriate conditions, macromolecules give
rise to units called coacervate dropplets which absorb
substances
• Lipids placed in water naturally organize themselves
into double-layered bubbles about the size of a cell
• These are known as liposomes
• Heterotrophic hypothesis suggests that protocell feed
on simple organic molecules
• Today cell performs protein synthesis in order to
produced the enzymes that allow DNA to replicate
• Central dogma of genetics is DNA directs protein
synthesis and information flows from DNA to RNA to
protein
• RNA-first hypothesis says RNA evolved first and first
true cell had RNA genes
• Protein-first hypothesis says proteins were the first of
the three to evolve
• Cairns-Smith proposes that polypeptides and RNA
evolved simultaneously so first cell had RNA genes
that replicated with help of proteins
Fig. 18.4
Biological Evolution
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
cell
DNA
RNA
origin of
genetic code
protocell
plasma
membrane
polymers
Chemical Evolution
polymerization
small organic molecules
energy abiotic
capture synthesis
inorganic chemicals
outgassing
from
volcanoes
early Earth
• Fossils are remains and traces of past life
• Traces include trails, footprints, burrows, worm casts,
or preserved droppings
• Paleontology is the science of discovering and
studying the fossil record
• Sedimentation is a process of accumulating particles
that settle and under pressure form rock
• As particles settle, stratum (a recognizable layer) in
the stratigraphic sequences forms
• Any given stratum is older than the one above it and
younger than the one below it
• Relative dating of fossils:
• Each stratum of the same age contains certain index
fossils that serve to identify deposits of the same time
in different parts of the world
• These index fossils are used in relative dating
methods
Fig. 18.5
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
first appearance of Homo sapiens
(11:59:30)
Age of Dinosaurs
formation of Earth
land plants
oldest
multicellular
fossils
10
12
11 midnight 1
P.M. A.M.
oldest known rocks
2
9
3
8
4
7
5
6
6
7
5
a.
oldest
eukaryotic
fossils
4
9
1
P.M. A.M.
free oxygen
in atmosphere
b.
© Henry W. Robinson/Visuals Unlimited
first
photosynthetic
organisms
8
3
2
free oxygen
in atmosphere
oldest fossils
(prokaryotes)
noon
12
10
11
1 second = 52,000 years
1 minute = 3,125,000 years
1 hour = 187,500,000 years
• Absolute dating methods relay on radioactive isotopes
to assign an actual date to a fossil
• All isotopes have a particular half-life, the length of
time for half of the radioactive isotope to change into
another stable element
• Carbon 14 will change to Nitrogen 14 in 5730 years
• Compare the amount of Carbon 14 in fossil to amount
of Carbon 14 in a modern sample
• Amount of Carbon 14 left can be converted to number
of half-lives since death of organism and thus the age
of the fossil
• After 50,000 years, C14 level in fossil is too low to
measure
• For older fossils, other isotopes with longer half-lives
can be used
• Geologic timescale is a result of the study of fossils in
strata
• Timescale divides history of Earth into eras, then
periods, and epochs
Fig. 18.7
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
thermophiles
Archaebacteria
halophiles
3
ARCHAEA
methanogens
7
Animals
5
1
Fungi
heterotrophic
protists
4
first cells
6
EUKARYA
Protists
photosynthetic
protists
mitochondria
8
Plants
chloroplasts
2
BACTERIA
aerobic bacteria
Bacteria
photosynthetic bacteria (produce oxygen)
other photosynthetic bacteria (do not produce oxygen)
3.5 BYA
2.2 BYA
1.4 BYA
543 MYA
•
•
•
•
•
•
•
•
Precambrium time:
About 87% of geologic timescale
Life arose and first cells came into existence
First identifiable chemical fingerprints of complex
prokaryotes from Greenland dated 3.8 BYA
Oldest prokaryotic fossils found in western Australia
dating 3.46 BYA resemble current cyanobacteria
Before continents, only volcanic rocks rose above
water
These stromatolites today have cyanobacteria in their
outer surface
Cyanobacteria in ancient stromatolites added oxygen
• By 2.0 BYA, oxygen in atmosphere made way for
aerobic bacteria as new metabolic pathways evolved
• Atmosphere changed from reducing to oxidizing one
• Oxygen formed ozone that filtered out ultraviolet
radiation allowing land-dwelling organisms
Fig. 18.8
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
20
10 mm
0
a. Stromatolites
b. Primaevifilum
a: Courtesy J. William Schopf; b: © Francois Gohier/Photo Researchers, Inc.
• Eukaryotic cell, originated around 2.1 BYA, is nearly
always aerobic and contains a nucleus and
membraneous organelles
• Evidence for endosymbiotic theory that says
nucleated cell engulfed prokaryotes that became
organelles
• Present mitochondria and chloroplasts are in size
range of bacteria
• Mitochondria and chloroplasts have their own DNA
and make some of their own protein
• Mitochondria and chloroplasts divide by binary
fission
• Outer membrane of mitochondria and chloroplasts
differ- outer membrane like eukaryotic cell and inner
like prokaryotic cell
• Fossils from Arctic Canada dating 1.4 BYA are
multicellular protists
• Some colonial protists form specialized gametes for
sexual reproduction
• Ediacaran invertebrate fossils, 630-545 MYA, are
thought to be soft-bodied invertebrates with many
different forms
• Not known if they gave rise to animals or simply
became extinct
• Paleozoic era lasts about 300 million years
• It includes three major mass extinctions
• Extinction is total disappearance of all members of a
species or higher taxonomic group
• Mass extinction is disappearance of large number of
species or higher taxonomic groups
• Cambrian period begins 542 MYA and was filled with
invertebrate animals
• All of today’s groups of animals trace history to
Cambrian
• Molecular clock is based on principle that mutations
in certain parts of genome occur at a fixed rate and are
• So number of DNA base differences tells how long
two species have been evolving separately
• Cambrian seafloors were dominated by now extinct
trilobites
• Trilobites were ancient arthropods with thick, joined
armor
• During Ordovician period, algae moved to fresh water
and then to damp land
• First land plants were nonvascular and remained short
like current day mosses
• Fossils of seedless vascular plants date to Silurian
period
• In Carboniferous period, club mosses, horsetails, and
seed ferns grew large and formed in warm swamps
• Current coal deposits are the remains of these plants
• Arthropods- spiders, centipedes, mites, and
millipedes- all preceded insects on land
• Insects enter fossil record in Carboniferous period
• Evolution of wings was an advantage allowing insects
to radiate into diverse habitats
• The jawless fishes began in the Ordovician period
• Jawed fishes appear in the Silurian period
• Fish are ectothermic, aquatic vertebrates with gills,
scales, and fins
• Cartilaginous and ray-finned fishes appear in the
Devonian period called the age of fishes
• Some fish have lobed-fins
• Data suggests that lobe-finned fishes were ancestral to
amphibians and modern-day lobe-fin fishes
• Amphibians are thin-skined vertebrates that return to
water to reproduce
• Warm swamps of the Carboniferous period allowed
the radiation of amphibians with some as large as 6
meters
• Carboniferous is called the age of amphibians
• Permian period saw the weather turn cold and dry
bringing about a major mass extinction and ending the
Paleozoic era
• In the Mesozoic era, nonflowering seed plants, the
gymnosperms became dominant
• Cycads and related plants were so common in Triassic
and Jurassic periods that it is called age of cycads
• Reptiles with a scaly skin and shelled, land eggs,
underwent adaptive radiation
• In Jurassic, large flying reptiles called pterosaurs
dominated the air
• Great marine reptiles were common in the sea
• Dinosaurs controlled the land and prevented mammals
from radiating into most habitats
• Average dinosaur was about crow size
• Many giants evolved like Apatosaurus that was 4.5
meters at the hip and 27 meters long, weighing 40
tons
• May have benefited from favorable surface-area-tovolume ratio for retaining heat
• Some dinosaurs may have been endothermic
• At end of Cretaceous period, the dinosaurs became
victims of a mass extinction
C
• A group of dinosaurs, the theropods were bipedal with
an elongated mobile neck
• They most likely gave rise to the birds
• Archaeopteryx fossils show characteristics tie
dinosaur ancestry to the birds
• Cenozoic era usually divided into teritary period and
Quaternary period
• After Mesozoic era, mammals began adaptive
radiation into habitats left vaccant by dinosaurs
• Mammals are endotherms with hair which helps keep
body heat from escaping and mammary glands that
produce milk to feed their young
• Flowering plants called angiosperms become the
dominant plants
• Continental drift was confirmed in the 1960s
• Continents are not fixed so their positions in the
oceans changes over time
• During Paleozoic era, continents joined to form one
supercontinent called Pangaea
• Pangaea divided into two large subcontinents
Gondwana and Laurasia
• These then split into the continents of today
• Explains why coastlines of several continents are
mirror images of each other
• Also helps explain the unique distribution patterns of
several fossils
• Plate tectonics says the Earth’s crust is fragmented
into plates that float on a hot lower mantle layer
• Continents and ocean basins are part of these rigid
plates which move like conveyor belts
• At ocean ridges, seafloor spreading occurs as molten
mantle rock rises and material is added to the ocean
floor
• Seafloor spreading causes the continents to move a
few centimeters a year on the average
• At subduction zones forward edge of moving plate
sinks into mantle and is destroyed
• Forming deep ocean trenches bordered by volcanos or
volcanic island chains
• When two continents collide, result is a mountain
range like Himalayas
• Where two plates meet and scrape past one another a
transform boundary occurs
• San Andress fault in southern California is a
transform boundary
Fig. 18.15
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
North
America
Laurasia
Eurasia
North
America
Eurasia
Africa
India
Africa
India
South
America
South
America
Australia
Australia
Antarctica
(251 million years ago)
PALEOZOIC
(135 million years ago)
MESOZOIC
(65 million years ago)
Antarctica
Present day
CENOZOIC
• At least five mass extinctions have occurred- at ends
of Ordovician, Devonian, Permian, Trassic, and
Cretaceous periods
• Walter and Luis Alvarez proposed in 1977 that the
Cretaceous extinction when dinosaurs died out was
due to a bolide
• A bolide is an asteroid that explodes, producing
meteorites that fall to Earth
• Alvarez found that Cretaceous clay contains
abnormally high level of iridium, an element that is
rare on crust, but more common in asteroids and
meteorites
• Impact of a large meteorite could cause cloud of dust
that could block out sun and cause plants to freeze and
die
• A huge crater that might have been caused by the
meteorite was found in Caribbean Gulf of Mexico
region near Yucatan peninsula
Fig. 18.16
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
STATUS TODAY
mammals
mammal
s
birds
bird
s
dinosaurs
dinosaurs extinct
insects
insects
ammonoids
brachiopods
poriferans
ammonoids extinct
brachiopod
s
poriferans
CAMBRIAN
Major
Extinctions
% Species
Extinct
ORDOVICIAN
SILURIAN
DEVONIAN
CARBONIFEROUS
443.7 MYA
359.2 MYA
75%
70%
PERMIAN
TRIASSIC
JURASSIC
251 MYA 199.6 MYA
90%
60%
CRETACEOUS
TERTIARY QUARTERNARY
65.5 MYA
75%
PRESENT