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Chapter Introduction
The Origin of Earth
17.1 The Big Bang
17.2 Early Earth
Evolution of Life on Earth
17.3 The Beginnings of Life
17.4 Chemical Evolution
17.5 Biological Evolution
The Record of the Rocks
17.6 Microfossils and Prokaryotes
17.7 Eukaryotes
Chapter Highlights
Chapter Animations
Learning Outcomes
By the end of this chapter you will be able to:
A Describe the origin of the universe and probable
conditions on early Earth.
B Evaluate hypotheses about the origin of life
and identify the probable characteristics of early
life-forms.
C Distinguish between chemical and biological
evolution.
D Describe the fossil record for prokaryotes and
for eukaryotes.
The Origin of Life
 What is required for
life to begin?
 How did life arise on Earth?
This photo shows stromatolite structures formed
by cyanobacteria, in Shark Bay, Australia.
The Origin of Life
• Of all the questions scientists
investigate, the origin of life
on Earth probably evokes
the most public curiosity
and controversy.
• Where did life come from?
• When did it begin?
• How did the first living
things look?
This photo shows stromatolite structures formed
by cyanobacteria, in Shark Bay, Australia.
The Origin of Earth
17.1 The Big Bang
• Measurements of light coming from deep space
indicate that the stars and galaxies are rapidly
moving away from each other.
• In the 1920s, Edwin Hubble studied light from
distant galaxies to test the idea that the size of the
universe is stable.
The Origin of Earth
17.1 The Big Bang (cont.)
• The length of any wave (including light) changes if
the object that is producing it moves toward or away
from the object receiving the wave.
– When a source of light moves away from the
observer, the wavelength of the light increases
and the visible light is a bit redder—a redshift.
– When the source of light moves toward the
observer, the wavelength of the wave decreases,
and the visible light is a bit bluer—a blueshift.
The Origin of Earth
17.1 The Big Bang (cont.)
• Hubble developed his theory of the expanding
universe after measuring red shifts in the light from
distant galaxies.
Photographed by the Hubble
Telescope, the spiral galaxy
NGC 4414 is 60 million lightyears away. Measurements of
light from such distant galaxies
provide evidence that the
universe is expanding.
The Origin of Earth
17.1 The Big Bang (cont.)
• Evidence indicates that, about 15 billion years ago,
the whole universe was concentrated in one
superdense mass that exploded.
• That explosion, called the big bang, hurled matter
and energy into space.
• Gravity pulled some of the matter together to form
galaxies, stars, and planets.
• Clumps of matter around our own star, the Sun,
became planets.
The Origin of Earth
17.1 The Big Bang (cont.)
• The best current estimate, based on the analysis of
rocks and meteorites, is that Earth formed about
4.6 billion years ago.
• The Moon may have formed when a meteor
collided with Earth, sending a large chunk of
Earth into space.
• Meteors are thought to be bits of material left over
from the formation of our solar system.
Geologists have worked
out a history of Earth from
evidence in its rocks. This
timeline relates several
major biological events to
the history of Earth.
The Origin of Earth
17.1 The Big Bang (cont.)
The history of Earth—and life—
compressed into the distance
between the tip of a person’s
nose and the tip of the fingernail
on the index finger. The history
of our species, Homo sapiens,
would disappear with one pass
of a nail file.
The Origin of Earth
17.2 Early Earth
• The decay of radioactive elements such as
uranium (U), thorium (Th), and some isotopes of
potassium-40 is the primary source of heat energy
within Earth.
• The entire planet was probably hot when it first
formed, and as the outer crust cooled, gases
escaping from the planet’s hot interior formed a
primitive atmosphere.
The Origin of Earth
17.2 Early Earth (cont.)
• The early atmosphere likely originated from volcanic
gases: mostly nitrogen (N2), carbon dioxide, water
vapor, and hydrogen (H2), with small amounts of
carbon monoxide (CO).
• All geologic evidence indicates that oxygen gas (O2)
probably was not present in the early atmosphere.
The Origin of Earth
17.2 Early Earth (cont.)
• Oxygen began to accumulate in the atmosphere
after the first photosynthetic organisms started to
produce it about 2.1–2.4 billion years ago.
• Modern levels of oxygen in the atmosphere were
probably reached about 360 million years ago as
plants became abundant on land.
Evolution of Life on Earth
17.3 The Beginnings of Life
• The surface of Earth 4.6 billion years ago would
have been hostile to modern life.
– Organic compounds do not form easily in
an atmosphere rich in nitrogen and carbon
dioxide.
– Ultraviolet radiation from the Sun bathed the
surface of Earth.
– There were extreme temperature variations.
– The was a scarce supply of oxygen gas.
Earth’s atmosphere
Evolution of Life on Earth
17.3 The Beginnings of Life (cont.)
• Popular scientific explanations for the beginning of
life include the following:
1. Life originated on some planet of another star
and traveled to Earth through space.
2. Life originated by unknown means on Earth.
3. Life evolved from nonliving substances through
interaction with the environment.
Evolution of Life on Earth
17.3 The Beginnings of Life (cont.)
• Many people believe that a supernatural force or
deity created life.
• That explanation is not within the scope of science,
therefore, such beliefs are not part of scientific
debates about the origin of life.
Evolution of Life on Earth
17.4 Chemical Evolution
• Most research on the origin of life focuses on the
idea that life evolved from nonliving substances by
interacting with the natural environment.
• In the 1920s, the Soviet scientist Alexander Oparin
and the British scientist J. B. S. Haldane separately
described this hypothesis in detail.
Evolution of Life on Earth
17.4 Chemical Evolution (cont.)
• Oparin and Haldane proposed:
– The early atmosphere consisted of
methane, ammonia, hydrogen, and
water vapor.
– Energy sources such as
radioactivity, lightning, cosmic
radiation from space, and heat
energy from volcanoes caused
gases in the atmosphere to
react, forming organic
compounds.
– Those compounds then
accumulated in the soup-like
hot oceans.
Evolution of Life on Earth
17.4 Chemical Evolution (cont.)
– They hypothesized that life
evolved by further chemical
reactions and transformations in
the complex organic soup.
– Both thought the first life-forms
were heterotrophs that fed
on the organic compounds in
the soup.
– These ideas are the basis of the
heterotroph hypothesis for the
origin of life.
Evolution of Life on Earth
17.4 Chemical Evolution (cont.)
• The Oparin-Haldane version of the heterotroph
hypothesis requires three major steps for the
origin of life:
1. There had to be a supply of organic molecules,
produced by nonbiological processes.
2. Some processes had to assemble those small
molecules into polymers such as nucleic acids
and proteins.
3. Other processes had to organize the polymers
into a system that could replicate itself, using the
organic molecules produced in step 1.
Evolution of Life on Earth
17.4 Chemical Evolution (cont.)
• In the 1950s, Harold Urey and Stanley Miller worked
on the problem of how organic compounds could
form in an inorganic environment.
• They were able to create organic compounds and
amino acids in an airtight apparatus under conditions
that might have existed on Earth 4.6 billion years ago.
A drawing of Miller’s apparatus
Evolution of Life on Earth
17.4 Chemical Evolution (cont.)
• More recent experiments have produced at least
13 of the 20 common amino acids found in proteins.
• Those experiments also have produced all the bases
found in DNA and RNA and even small amounts of
ribose, the sugar in RNA.
Evolution of Life on Earth
17.4 Chemical Evolution (cont.)
• Evidence from meteorites indicates that organic
compounds also form in space.
(a), Cyril Ponnamperuma and his associates
were the first to detect amino acids in a
meteorite when they analyzed a sample of
the Murchison meteorite (b).
Instruments flown through the
comet’s tail of Comet Halley
detected organic compounds.
Evolution of Life on Earth
17.4 Chemical Evolution (cont.)
• Some organic molecules also
may have formed at volcanic
vents deep in the ocean.
• Regardless of origin, there
is a plausible explanation for
step 1 of the Oparin-Haldane
hypothesis.
Evolution of Life on Earth
17.4 Chemical Evolution (cont.)
• For the formation of complex molecules to occur,
smaller molecules must been concentrated enough
to make them collide and react with each other.
• In 1985, A. G. Cairns-Smith suggested that clay
particles might have helped to form the first organic
polymers by catalyzing the bonding together of these
small molecules to form proteins and other polymers.
Repeating crystal structures in clay particles
(scanning electron micrograph) may have
served as a blueprint for assembling the first
biochemical polymers.
Evolution of Life on Earth
17.4 Chemical Evolution (cont.)
• The work by Cairns-Smith and others since allows
us to infer that polymers could have formed from
simple organic compounds in the prebiological soup.
• Self-replication, a central feature of living systems,
is the third and final step for the origin of life.
• Research into self-replication has led to the
hypothesis that life began in an “RNA world.”
Evolution of Life on Earth
17.4 Chemical Evolution (cont.)
• According to the RNA-world hypothesis:
– RNA served as both information molecule and
catalyst at first.
– DNA later became the main information molecule
and protein enzymes the primary biological
catalysts.
• So far, scientists have found no RNA that can
replicate itself completely.
Evolution of Life on Earth
17.4 Chemical Evolution (cont.)
• RNA molecules can
undergo simulated
Darwinian evolution in
the laboratory.
Beginning with a mixture of RNA molecules
with various sequences, directed evolution
allows a scientist to develop a ribozyme
(RNA enzyme) that can catalyze its own
replication or another desired reaction.
Evolution of Life on Earth
17.5 Biological Evolution
• The boundary between chemical evolution and
biological evolution may be the formation of
self-replicating polymers.
• Life could be described as “a self-sustained
chemical system that is capable of undergoing
Darwinian, or biological, evolution.”
Evolution of Life on Earth
17.5 Biological Evolution (cont.)
• Biological evolution consists of three processes:
1. self-reproduction
2. mutation that can be inherited
3. natural selection
Evolution of Life on Earth
17.5 Biological Evolution (cont.)
• Cell theory holds that all life is made of cells and that
all cells come from preexisting cells.
• The origin of cells and membranes is still not clearly
understood.
• In the early 1980s, Carl Woese suggested that life
began on Earth before the planet was fully formed
as water droplets that functioned as primitive cells.
Evolution of Life on Earth
17.5 Biological Evolution (cont.)
• Other possibilities for the first life-forms include
proteins, DNA, and RNA.
• Amino acids in the oceans may have joined
spontaneously to form proteins.
• Many proteins are enzymes that catalyze the
synthesis of other substances, but there is no
known way for proteins to replicate themselves
without the help of RNA.
Evolution of Life on Earth
17.5 Biological Evolution (cont.)
• Short strands of RNA can self-replicate.
• RNA also stores information, directs the synthesis
of proteins, and has limited ability as a catalyst.
• In certain viruses, such as the virus that causes
AIDS, RNA also directs the synthesis of DNA.
Evolution of Life on Earth
17.5 Biological Evolution (cont.)
• A few scientists maintain that the first life-forms could
have functioned only if they contained both proteins
and nucleic acids.
• Viruses consist of DNA or RNA
surrounded by a protein coat.
• Viruses are so dependent on
their host cells that they probably
originated after their hosts and
their role in evolution is unclear.
Evolution of Life on Earth
17.5 Biological Evolution (cont.)
• All the ideas about the origin of life and the
nature of the first life-forms have supporters
and opponents.
The Record of the Rocks
17.6 Microfossils and Prokaryotes
• Some scientists have tried to investigate early lifeforms by searching for fossils.
• What may be the oldest known microfossils, or
fossil microorganisms, were discovered in 1993 in
northwestern Australia.
The Record of the Rocks
17.6 Microfossils and Prokaryotes (cont.)
• The fossils are of tiny, single-celled filaments that
resemble certain modern bacteria capable of
photosynthesis.
A 3.5-billion-year-old prokaryote
fossil in a stromatolite from western
Australia. This fossil contains the
oldest known cells.
The Record of the Rocks
17.6 Microfossils and Prokaryotes (cont.)
• The microscopic fossils were embedded in mineral
grains encased in a type of rock that formed almost
3.5 billion years ago.
• This discovery showed that life appeared on Earth
much earlier than previously thought.
The Record of the Rocks
17.6 Microfossils and Prokaryotes (cont.)
• The Australian fossils were found with domelike
structures called stromatolites that are composed
of many wafer-thin layers of rock.
• Today, limestone-secreting bacteria form similar
structures at Shark Bay, Australia.
The Record of the Rocks
17.6 Microfossils and Prokaryotes (cont.)
(a), Approximately 530 million-year-old fossil stromatolites from Wyoming. Note
the characteristic layered structure. The image is approximately 5 cm across.
(b), Modern living stromatolites in Shark Bay, Australia, built by cyanobacteria.
Each dome is about 30–100 cm in diameter.
The Record of the Rocks
17.6 Microfossils and Prokaryotes (cont.)
• Older microfossils will be very difficult to find as they
have melted, eroded, or otherwise changed until
they are no longer recognizable.
• These fossils are thought to be prokaryotes, but
their structures were probably less organized than
modern prokaryotes.
The Record of the Rocks
17.6 Microfossils and Prokaryotes (cont.)
• Carl Woese suggests that the first organisms may
have been methanogens—anaerobic bacteria that
obtain energy by using carbon dioxide to oxidize
hydrogen.
• Methanogens and related bacteria live today in
conditions thought to be like those of early Earth
such as near hydrothermal volcanic vents on
the seafloor.
The Record of the Rocks
17.7 Eukaryotes
• The first microfossils that may have been eukaryotes
are about 2.1 billion years old.
• Fossil evidence indicates that eukaryotic cells
became common by 750 million years ago.
• Eukaryotic cells contain membrane-enclosed
organelles such as a nucleus, mitochondria,
and chloroplasts.
The Record of the Rocks
17.7 Eukaryotes (cont.)
• Margulis proposes that eukaryotes originated from a
symbiosis between large anaerobic prokaryotes
and smaller aerobic or photosynthetic prokaryotes.
• Eventually, the partners, or endosymbionts, lost the
ability to live independently.
• A large body of evidence supports Margulis’s
endosymbiont hypothesis that mitochondria and
plastids were once free-living prokaryotes.
The endosymbiont hypothesis
The Record of the Rocks
17.7 Eukaryotes (cont.)
• Some single-celled organisms that do not have all
the organelles usually found in eukaryotes may be
survivors of a transitional phase to eukaryotic life.
(a) Giardia lamblia, x16,000, has two haploid nuclei and no mitochondria.
(b) Paramecium, x160, has one large nucleus (stained brown here) and several
small nuclei (stained green here). The first eukaryotes may have resembled
these single-celled organisms.
Summary
• It is not possible to repeat historical events, such as the origin
of life, but inference helps scientists study such events and
draw plausible conclusions about how they occurred.
• The big bang probably occurred about 15 billion years ago.
• Planets, including Earth, formed around the Sun about 4.6
billion years ago. Gases escaping from within Earth formed a
primitive atmosphere.
• Scientists hypothesize that chemical evolution, driven by a
variety of energy sources, led to the origin of life on Earth.
• Catalytic RNA may have served as both information molecule
(before DNA) and functional molecule (before proteins).
• According to one definition, living systems must be able to
reproduce, must be subject to mutations that can be passed
on to offspring, and must be subject to natural selection.
Summary (cont.)
• The first living thing may have been a “naked” information
molecule such as RNA, DNA, or protein. It may also have been
a cell-like structure.
• The first life-forms were probably heterotrophs that lived on
organic compounds in Earth’s oceans.
• The oldest fossils appear similar to modern bacteria.
• Photosynthesis probably evolved very early, but significant
levels of oxygen did not accumulate in the atmosphere until
about 1 billion years ago.
• The oldest known fossils of eukaryotes are about 2.1 billion
years old and they became abundant by 750 million years ago.
• Mitochondria and plastids may have arisen as free-living
prokaryotes that occupied host cells and increased their
energy yield.
• There are more questions than data relating to the origin of
life on Earth.
Reviewing Key Terms
Match the term on the left with the correct description.
___
stromatolites
a
___
methanogens
d
___
symbiosis
b
___
endosymbiont
e
___
hydrothermal
c
a. fossils containing some of the
earliest known prokaryotes
b. an ecological relationship
between organisms of two
species that live together in
direct contact
c. referring to hot water vents on the
ocean floor
d. anaerobic bacteria that produce
methane as a by-product of their
metabolic processes
e. an organism having a mutually
beneficial relationship with a
host organism while living in the
host’s body
Reviewing Ideas
1. What are the three popular scientific
explanations of how life started on Earth?
The three popular scientific explanations of the
origin of life are: (1) Life originated on some
planet of another star and traveled to Earth
through space. (2) Life originated by unknown
means on Earth. (3) Life evolved from nonliving
substances through interaction with the
environment.
Reviewing Ideas
2. What is the endosymbiont hypothesis?
Lynn Margulis’ endosymbiont hypothesis states that
mitochondria and plastids (such as chloroplasts)
originated as free-living prokaryotes. Eukaryotes
originated from a symbiosis between large
anaerobic prokaryotes and smaller aerobic or
photosynthetic prokaryotes. The large cells
absorbed the smaller ones (or alternatively, small
parasitic cells bored into larger ones). Eventually,
the internal partners, or endosymbionts, lost the
ability to live independently.
Using Concepts
3. What is the significance of determining the rate
of expansion of the universe?
By determining the rate of the universe’s
expansion, scientists can work backward to
determine the time at which the universe was much
smaller. Evidence indicates that, about 15 billion
years ago, the whole universe was concentrated in
one superdense mass that exploded in an event
called the big bang.
Using Concepts
4. When did photosynthesis began on Earth?
How do we know this?
Oxygen began to accumulate in the atmosphere only
after the first photosynthetic organisms started to
produce it, about 2.1–2.4 billion years ago. At first,
the oxygen released by photosynthesis would have
combined with other elements, such as iron, instead
of remaining free in the atmosphere. By dating the
oxygen-containing compounds such as iron oxides
that are present in the oldest rocks, we can estimate
the approximate beginning of photosynthesis.
Synthesize
5. In our search for the origin of life, what
evidence may support the theory that life
is not unique to Earth?
Amino acids have been found in meteorites and in
the tail of Comet Halley. This proves that these
compounds are not unique to Earth. If organic
compounds originally were the basis of life on
Earth, they could very well have performed the
same role elsewhere in the universe.
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Chapter Animations
Earth’s atmosphere
A drawing of Miller’s apparatus
The endosymbiont hypothesis
Earth’s atmosphere
A drawing of Miller’s apparatus
The endosymbiont hypothesis
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