Chapter 14 Origin of Life
1. Define spontaneous generation, and list some of the observations that led people to think that
life could arise from nonliving things.
A. Spontaneous generation - Before the seventeenth century it was widely thought that
living things could also arise from nonliving things.
B. Observations
1) Redi’s Experiment - 1668 - hypothesis that meat kept away from adult flies would
remain free of maggots
a. The experimental group consisted of netting-covered jars that contained meat
b. The control group consisted of uncovered jars that also contained meat
c. The netting allowed air to enter and prevented flies from landing on the meat
d. After a few days, maggots swarmed over the meat in the open jars, while the
net-covered jars remained free of maggots
e. Redi’s experiment showed convincingly that flies come only from eggs laid by
other flies
2) Spallanzani’s Experiment - 1700s - designed an experiment to test the hypothesis
of spontaneous generation of microorganisms
a. Spallanzani hypothesized that microorganisms formed not from air but from
other microorganisms
1) Spallanzani reasoned that boiling broth in a flask would kill all the
microorganisms in the broth, on the inside of the glass, and in the air in the
flask
2) For his experimental group, Spallanzani boiled clear, fresh broth until the
flasks filled with steam
3) He then sealed the flasks by melting their glass necks closed while the broth
was hot
4) The control-group flasks of broth were left open
5) The broth in the sealed flasks remained clear and free of microorganisms,
while that in the open flasks became cloudy due to contamination with
microorganisms
6) Spallanzani concluded that the boiled broth became contaminated only when
microorganisms from the air entered the flask
7) Spallanzani’s opponents, however, objected to his method and disagreed
with his conclusions
a) They claimed that Spallanzani had heated the experimental flasks too
long, destroying the “vital force” in the air inside them
b) Air lacking this “vital force,” they claimed, could not generate life
c) Thus, those who believed in spontaneous generation of microorganisms
kept the idea alive for another century
3) Pasteur’s Experiment
a. made a curve-necked flask that allowed the air inside the flask to mix with air
outside the flask
b. The curve in the neck of the flask prevented solid particles, such as
microorganisms, from entering the body of the flask
c. Broth boiled inside the experimental curvenecked flasks remained clear for up
to a year
d. But when Pasteur broke off the curved necks, the broth became cloudy and
contaminated with microorganisms within a day
e. Pasteur reasoned that the contamination was due to microorganisms in the air
f. With Pasteur’s experiment, the principle of biogenesis became a cornerstone of
biology - The principle of biogenesis states that all living things come from
other living things – there is no such thing as spontaneous generation
2. Outline the modern scientific understanding of the formation of Earth.
A. The Formation of Earth
1) Evidence from computer models of the sun suggests that about 5 billion years ago,
our solar system was a swirling mass of gas and dust
2) Over time, most of this material collapsed inward, forming the sun
3) The remaining gas, dust, and debris circled the young sun
4) The planets are thought to have been formed from repeated violent collisions of
this space debris
5) During its 400-million-year-long period of formation, Earth grew increasingly
large as it was bombarded by debris
6) These collisions not only added to Earth’s mass but also released a great deal of
thermal energy
7) Each collision between the young and growing Earth and a large piece of space
debris, for example, would have released enough energy to melt the entire surface
of Earth
8) The estimated age of Earth, more than 4 billion years, is about 700,000 times as
long as the period of recorded history
9) Eventually, Earth’s surface cooled and formed a rocky crust and water vapor
formed the oceans where life is believed to have started
B. Calculating the Earth’s age
1) Relative dating
a. Tells us the sequence in which events occurred, not how long ago they
occurred
b. Looks at sequential layers of sedimentary rocks –law of superposition
states that in an un-deformed sequence of sedimentary rocks, each bed is
older than the one above it and younger than the one below it
2) Absolute dating
a. Use radiometric (radioactive) dating
b. Radioisotopes decay and give off radiation because they have unstable
nuclei (radioactive decay) - This results in isotopes that are smaller and
more stable
c. Scientists measures the proportion of radioactive isotopes (radioisotopes)
to their stable isotopes in the sample
d. Since the radioisotope has been decaying at a constant rate since it
formed according to its half-life, scientists can determine how old the
sample is
e. Depending on the isotope, half-lives vary from a fraction of a second to
billions of years
1. Half-life – the amount of time it takes for ½ of the radioisotope
to decay to its stable daughter isotope form
3. Describe the formation of the first organic compounds
A. The first organic compounds
1) All of the elements found in organic molecules today are thought to have existed on
Earth and in the rest of our solar system when Earth formed, but how did they come
together to form something necessary to sustain life? - “Primordial Soup”
2) Russian scientist Oparin and British scientist Haldane suggested the earth’s ocean
contained large amounts of organic molecules, which came from the dissolving of
materials that were in our atmosphere
a. Oparin and American scientist Harold Urey also suggested that the earth’s
atmosphere didn’t contain oxygen, but was rich in nitrogen (N2), H2O, ammonia
(NH3), and methane (CH4) and because there was no oxygen to accept electrons
from these molecules, they could react with other hydrogen-rich molecules
b. This hypothesis was tested by Stanley Miller and Harold Urey in 1953
1. He put all of these gases into a device and simulated lightning using sparks and
within a few days, he found amino acids, fatty acids, and other hydrocarbons
(things essential to life)
3) Big problems with this experiment
a. We’ve found fossils that are older than we thought that life had begun
b. There was no ozone layer on earth, so any ammonia and methane used in the making
of organic molecules would have been burned up by UV radiation
B. New Model: The bubble model
1) Louis Lerman, a geophysicist, said that the key processes happened in
bubbles on the ocean’s surface
2) Steps
a. Gases were trapped in underwater bubbles (the gases came from undersea
volcanoes)
b. Gases underwent chemical reactions
c. Gases were ejected into the atmosphere
d. Gases underwent further reactions, using UV radiation and
lightning
e. Simple and complex compounds fell into the oceans
C. Organic chemicals became complex
1) How did amino acids link to form proteins? How did nucleotides form the
long chains of DNA that store instructions for making proteins?
2) Short chains of RNA have self assembled within water
a. It is thought that perhaps once RNA was made, it acted like an enzyme, and
catalyzed the assembly of the first proteins
3) Microspheres or Coacervates may have led to cells
a. Scientists believe that microspheres, which are spherical, short chains of amino
acids (proteins) gathered together in tiny vesicles (membranes), may have been
the first step toward cellular organization
b. Coacervates – collections of droplets that are composed of molecules of different
types, such as amino acids and sugars
c. Coacervates can grow and microspheres can bud, suggesting replications, but
where are the genes?
4) Origin of heredity – The first life forms
a. Most agree that double stranded DNA evolved after RNA
b. RNA molecules have varying shapes; DNA doesn’t
c. The special folding of RNA suggests that RNA may behave like proteins and
catalyze reactions
d. Ribozyme – an RNA molecule that can act as an enzyme to promote a specific
chemical reaction
5) Question: when did the first organism arise? How?
a. Some microspheres that contained RNA developed a means of transferring
information to offspring (DNA)
b. These may have been the first prokaryotes
c. Prokaryotes are the oldest group of organisms – probably
anaerobic heterotrophic at first due to the lack of oxygen in the atmosphere
4. Explain how photosynthesis and aerobic respiration are thought to be related.
A. Some forms of life had become photosynthetic by 3 billion years ago
1) The 3.5-billion-year-old microfossils found in Australia are probably of
photosynthetic unicellular organisms that are related to modern cyanobacteria, a
group of photosynthetic unicellular prokaryotes
B. Oxygen, a byproduct of photosynthesis, was damaging to many early
unicellular organisms
1) About 2.5 billion years ago, photosynthesis by cyanobacteria began adding
oxygen to Earth’s atmosphere and the sun caused the O2 to bond and form O3
(ozone) which blocks the UV light from the sun enabling organisms to move
from the ocean to the land
2) Oxygen, however, could destroy some coenzymes essential to cell function
3) Within some organisms, however, oxygen bonded to other compounds, thereby
preventing the oxygen from doing damage
4) This bonding was one of the first steps in aerobic respiration
2) Thus, an early function of aerobic respiration may have been to prevent the
destruction of essential organic compounds by oxygen
5. Define endosymbiosis, and explain why it is important in the history of eukaryotes.
A. How did such a complex type of cell evolve from the simple
prokaryotes?
1) about 2.0 and 1.5 billion years ago, a type of small aerobic eubacteria (a
prokaryote) entered and began to live and reproduce inside larger, anaerobic
prokaryotes
2) In 1966 researcher Lynn Margulis has proposed that what may have started as
an invasion became a successful, mutually beneficial relationship—called
endosymbiosis
3) It is thought that the aerobic prokaryote eventually gave rise to modern
mitochondria, which are the site of aerobic respiration in eukaryotic cells
B. Sometime later, there was a second successful invasion of preeukaryotic cells
1) This time, the invader was a relative of modern photosynthetic cyanobacteria
2) These invaders eventually gave rise to chloroplasts, the sites of photosynthesis
C. There is compelling evidence to support this hypothesis of eukaryotic
evolution
1) Both chloroplasts and mitochondria replicate independently from the replication
cycle of the cell that contains them
2) Chloroplasts and mitochondria contain some of their own genes, which are
different from those of the rest of the cell
a. These genes are found in the organelles themselves, in a circular piece of
DNA, an arrangement characteristic of prokaryotic, though not eukaryotic,
DNA
3) Both of these organelles are surrounded by a double membrane,
unlike most other organelles
a. Some scientists now think that the nucleus originated as a DNA virus that
also took up residence within a prokaryotic cell because it also has a double
membrane
Chapter 14 Origin of Life
 Before the 1600s, it was generally thought that organisms could arise from nonliving
material by spontaneous generation.
 Redi showed in 1668 that rotting meat kept away from flies would not produce new flies.
Maggots appeared only on meat that had been exposed to flies.
 Spallanzani showed in the 1700s that microorganisms would not grow in broth when its
container was heated and then sealed. This seemed to indicate that microorganisms that
cause food spoilage do not arise from spontaneous generation but, rather, are carried in the
air.
 Pasteur used a variation of Spallanzani’s design to prove that microorganisms are carried in
the air and do not arise by spontaneous generation.
 Earth is thought to have been formed by repeated collisions of debris moving through space.
 The mass number of an element is the total number of protons and neutrons in the nucleus.
The number of neutrons in atoms of an element can vary. Atoms with varying numbers of
neutrons are called isotopes.
 An isotope’s half-life is the time it takes for one-half of a sample of the isotope to decay.
 The ages of objects like fossils and rocks can be determined by measuring the amount of
radioactive decay that has occurred in radioactive isotopes found in the sample.
 The first simple organic compounds may have been formed at high temperatures on early
Earth in an atmosphere of ammonia, hydrogen gas, and water vapor.
 Macromolecules important to life may have been assembled from simple organic
compounds. Lightning may have supplied the energy for these chemical reactions.
 Some organic compounds may have been deposited on Earth by meteorites.
 Cell-like structures, including microspheres and coacervates, form spontaneously in certain
kinds of solutions. These structures do not show heredity.
 In addition to its role as a template for protein assembly, some RNA molecules can act as
enzymes.
 The first molecule that held hereditary information may have been RNA rather than DNA.
 RNA can assume different shapes, much as proteins do. These shapes depend on areas of
attraction between the RNA nucleotides.
 The first cells that formed on Earth were probably heterotrophic prokaryotes.
 The first autotrophic cells probably used chemosynthesis to make food.
 An important initial function of aerobic respiration may have been to bind oxygen and
prevent it from doing damage to early organisms.
 Eukaryotic cells may have evolved from large prokaryotic cells that were invaded by smaller
prokaryotic cells. These small prokaryotic invaders may have been the ancestors of
organelles, including mitochondria and chloroplasts.
Vocabulary List
Archaebacteria
Biogenesis
Carbon-14/carbon-12
Carbon-14’s half-life
Chemosynthesis
Coacervates
Cycanobacteria
Early atmosphere
Endosymbiosis
Evidence of endosymbiosis
First cells – anaerobic heterotrophic prokaryotes
First photosynthetic cells by 3 billion years ago
Half-life
How can we determine what happened so long ago?
Isotopes
Lynn Margulis
Microspheres
Oparin’s hypotheses
Ozone – required for life on land
Pasteur’s Experiment
Radioactive dating
Ribozyme
Second cells - chemosynthetic autotrophs such as archaebacteria
Sidney Fox
Spontaneous generation
Stanley L. Miller and Harold C. Urey
The atomic number
The estimated age of Earth
Third cells – aerobic eukaryotes
Thomas Cech