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13.1
Originating Events
Earth, and life on it, originated billions of years ago. Scientists have pieced together a
scientific description of the initial conditions and events that may have resulted in the
origin of life. Much ongoing research focuses on testing hypotheses about the origins and
evolution of the earliest life forms.
Primordial Earth
Figure 1
Thermal mud pools, such as those
in Yellowstone National Park, are
suggestive of conditions on the
surface of primordial Earth.
Earth, when it formed some 4.6 billion years ago, was extremely hot.
Heat generated by asteroid impacts, internal compression, and
radioactivity melted most of the rocky material. Dense materials,
composed of such heavy elements as iron and nickel, formed Earth’s
inner core, while less dense materials formed a thick mantle. The
least dense rock, composed mostly of lighter elements, floated on
the surface and cooled to form a crust (Figure 1).
Hot gases formed Earth’s primitive atmosphere. When, after
some 500 million to 800 million years, the asteroid bombardment
slowed and surface temperatures cooled below 100°C, vast quantities of water vapour condensed. Hundreds of years of torrential
rains pooled in surface depressions to form ocean basins. The
atmosphere of primordial Earth would have contained large
amounts of nitrogen gas, carbon dioxide, carbon monoxide, and
water vapour. Other hydrogen compounds—such as hydrogen
sulfide, ammonia, and methane—would have been present. It is
probable, though not certain, that this early atmosphere also contained hydrogen gas.
Oxygen gas is highly reactive and, with the high temperatures present then, would have
combined with many other elements to form oxides; for this reason, the atmosphere
would have contained little, if any, free oxygen gas. The surface of Earth would have
been exposed to many intense sources of energy: radioactivity, intense ultraviolet light,
visible light, and cosmic radiation from a young Sun; heat from volcanic activity; and electrical energy from violent lightning storms.
Organic Molecules
primary abiogenesis theory that
the first living things on Earth arose
from nonliving material
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In the mid-1930s, the Russian biochemist Alexander Oparin and British biologist J.B.S.
Haldane independently proposed the theory of primary abiogenesis—that the first
living things on Earth arose from nonliving material. They reasoned that the first complex chemicals of life must have formed spontaneously on a primordial Earth and, at some
point, arranged themselves into cell-like structures with a membrane separating them
from the outside environment.
Although extremely harsh, the early conditions on Earth were ideal for triggering
chemical reactions and the formation of complex organic compounds. What molecules
might have formed from the reactions of gases in the primordial atmosphere? In 1953,
the Nobel Prize–winning astronomer Harold Urey and his student, Stanley Miller, investigated possible reactions. Their apparatus modelled the water cycle by using a condenser to produce precipitation and a heater to cause evaporation. Since Urey and Miller
suspected that the early atmosphere would have contained water vapour, ammonia, and
methane and hydrogen gases, they combined these gases and exposed them to electrical
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Section 13.1
sparks, thereby modelling early conditions on Earth (Figure 2). After one week, 15% of
the original carbon in the methane had been converted to a variety of compounds,
including aldehydes, carboxylic acids, urea, and—most interestingly—two amino acids:
glycine and alanine.
More recent evidence suggests that the specific combination of gases chosen by Urey
and Miller was not likely to have existed in the primordial atmosphere. In response,
many other scientists have continued this investigation with experiments that use the combination of gases now thought to have been present. These experiments have produced
an even greater variety of simple organic compounds, including essential sugars, all 20
amino acids, many vitamins, and all four nitrogenous bases found in RNA and DNA. The
most abundant nitrogenous base, adenine, was the easiest to produce under laboratory
conditions. These results suggest that many of the building blocks of life likely formed
spontaneously in Earth’s primordial environment.
electrodes
to vacuum
pump
DID YOU
KNOW
?
Building Blocks from Space
Evidence from outer space offers
new information regarding Earth’s
history. The Murchison meteorites
discovered in Australia are rich in
amino acids, and many asteroids
and comets contain significant
quantities of organic compounds.
These findings provide additional
evidence of the formation of
important organic compounds in
the solar system. Some scientists
hypothesize that heavy bombardment of the young Earth by such
bodies may have been the source
for much of the organic material
that would eventually form the
first living cells.
spark discharge
CH4
NH3
H2O
H2
gases
water out
condenser
water in
water droplets
boiling water
water containing organic
compounds
liquid water in trap
Figure 2
The Miller and Urey experimental apparatus
Chemical Evolution
For the first molecules to have produced living cells, they had to have been able to form
more complex chemical and physical arrangements. Polymerization of early monomers
may have occurred in numerous ways. Monomers may have become concentrated on
hot surfaces as water evaporated, and the increased concentrations and heat energy
may have triggered polymerization reactions. Under similar conditions, in 1977, Sidney
Fox at the University of Miami was able to trigger the spontaneous production of
thermal proteinoids consisting of chains of more than 200 amino acids. Other scientists have discovered that such materials as clay particles and iron pyrite form electrostatically charged surfaces that are also capable of binding monomers and catalyzing
polymerization reactions. These findings suggest mechanisms for the formation of the
first polymers. Could any polymers have then influenced their own formation?
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thermal proteinoids a polypeptide
that forms spontaneously when
amino acids polymerize on hot surfaces; proteinoids are able to selfassemble into small cell-like
structures in cool water
The Evolutionary History of Life
587
ribozymes an RNA molecule able
to catalyze a chemical reaction
The most fundamental characteristic of living things is organized self-replication. To
self-replicate, molecules must demonstrate catalytic activity, that is, the ability to influence a chemical reaction. But can a molecule act as a catalyst for its own formation? In
the 1980s, Thomas Cech, working at the University of Colorado, discovered RNA molecules, called ribozymes, that act as catalysts in living cells. In other experiments, simple
systems of RNA molecules have been created that are able to replicate themselves. In
1991, while working at the Massachusetts Institute of Technology, chemist Julius Rebek,
Jr., created synthetic nucleotidelike molecules that could replicate themselves—and
make mistakes, which resulted in nonliving molecular systems that mutated and underwent a form of natural selection in a test tube. As demonstrated by such experiments,
Earth’s first self-replicating and evolving systems may have been RNA molecules. RNA
is also likely to have been the first hereditary molecule. Its catalytic activity and the roles
of tRNA, mRNA, and rRNA suggest that it is likely to have played a direct role in the
synthesis of proteins. Current scientific thinking about DNA is that it evolved later, perhaps by the reverse transcription of RNA.
Formation of Protocells
liposomes spherical arrangements
of lipid molecules that form spontaneously in water
ACTIVITY 13.1.1
Observing Liposome Formation
(p. 629)
How do nonliving collections of
phospholipids behave in water?
In this activity, you can model the
formation of cell-like structures as
the process may have occurred
on primordial Earth.
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The evolution of self-replicating molecular systems and cell-like structures is a vital area
of investigation among scientists who study the origin of life. All living things are composed of cells. For chemicals in cells to remain concentrated enough for metabolic
processes to occur, they must be separated from the surrounding dilute environment. How
might the first membranes have formed and arranged themselves into cell-like packages with an interior separated from the surrounding environment?
Lipid membranes can and do form spontaneously. Because of their hydrophobic tails,
fatty acids and phospholipids naturally arrange themselves into spherical doublelayered liposomes, or clusters. These can increase in size by the addition of more lipid
and, with gentle shaking, can form buds and divide. Their membranes also act as a semipermeable boundaries, so that any large molecules initially trapped within them, or produced by internal chemical activity, are unable to escape, thereby increasing in
concentration. Although they are not alive, they can respond to environmental changes
or reproduce in a controlled way, which means protocells do share many traits of living
cells. Additional experimental evidence has shown that semipermeable liquid-filled
spheres can also form from proteinlike chains. Researchers have discovered that if amino
acids are heated and placed in hot water, they form proteinoid spheres, which are capable
of picking up lipid molecules from their surroundings, as shown in Figure 3. These protocells are also able to store energy in the form of an electrical potential across their
membrane, a trait found in all living cells. Although these findings are the subject of
debate and many unanswered questions remain, there is evidence that chemical evolution could have given rise to molecular systems and cellular structures that are characteristic of life.
Figure 3
Liquid-filled spheres can
form spontaneously by
various protein mixtures
in water.
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Section 13.1
Prokayotic Organisms: The First True Cells
The oldest known fossils of cells on Earth—accurately dated to 3.465 billion years ago—
were found in western Australia in layered formations called stromatolites. These microscopic fossils resemble present-day anaerobic cyanobacteria (Figures 4 and 5). Even the
world’s oldest-known sedimentary rock formations located in Greenland—dating to
3.8 billion years ago—show chemical traces of microbial life and activity.
(a)
(b)
Figure 4
(a) Microfossils dating to 3.5 billion years ago were discovered by J. William Schopf of UCLA.
They closely resemble present-day cyanobacteria (b).
Although the oldest fossil bacteria resemble photosynthetic cyanobacteria, which use
oxygen, the very first prokaryotic cells would certainly have been anaerobic, as the atmosphere would then have contained little or no free oxygen. These first prokaryotic organisms would likely have relied on abiotic sources of organic compounds. They would
have been chemoautotrophic, obtaining their energy and raw materials from the metabolism of such chemicals in their environment as hydrogen sulfide, released at high temperatures and in large quantities from ocean-floor vents. These organisms would have
adapted to living under harsh conditions of extreme heat and pressure and may have
resembled present-day thermophilic archaebacteria. As the first cells reproduced and
became abundant, these chemicals would have gradually become depleted. Any cell that
was able to use simple inorganic molecules and an alternative energy source would have
had an advantage. Fossil evidence suggests that, by 3 billion years ago, photosynthetic
autotrophs were doing just that.
Although the first photosynthetic organisms may have also used hydrogen sulfide as a
source of hydrogen, those that used water would have had a virtually unlimited supply.
As they removed hydrogen from water, they would have released free oxygen gas into the
atmosphere—a process that would have had a dramatic effect. The accumulation of
oxygen gas, which is very reactive, would have been toxic to many of the anaerobic organisms on Earth. While these photosynthetic cells prospered, others would have had to
adapt to the steadily increasing levels of atmospheric oxygen or perish. Some of the oxygen
gas reaching the upper atmosphere would have reacted to form a layer of ozone gas,
having the potential to dramatically reduce the amount of damaging ultraviolet radiation
reaching Earth. At the same time, the very success of the photosynthetic cells would have
favoured the evolution of many heterotrophic organisms.
These early life forms and evolutionary stages produced the necessary conditions to
support the dramatic success of life on Earth powered and supplied by energy from the
sun and the chemical products of photosynthesis.
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stromatolites shaped rock formations that result from the fossilization of mats of ancient prokaryotic
cells and sediment
Figure 5
The actions of cyanobacteria began
forming these stromatolites in
western Australia 2000 years ago.
They are almost identical to those
containing the fossils discovered by
Schopf (Figure 4).
chemoautotrophic describes an
organism capable of synthesizing its
own organic molecules with carbon
dioxide as a carbon source and oxidizing an inorganic substance as an
energy source
heterotrophic describes an
organism unable to make its own
primary supply of organic compounds (i.e., feeds on autotrophs,
other heterotrophs, or organic
material)
The Evolutionary History of Life
589
DID YOU
KNOW
?
The Panspermia Theory
Proponents of the panspermia
theory suggest that life may have
arisen elsewhere in this solar system
and travelled to Earth on a meteorite
or comet. How much evidence is
there to support this theory? A
meteor from Mars made headlines
in 1996 when an examination with
a scanning electron microscope of
samples revealed objects resembling
bacteria fossils (Figure 6). Many
scientists, however, suspect these
structures are inorganic in origin.
What conditions on Mars might have
permitted or fostered abiogenesis?
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Figure 6
SUMMARY
Earliest Evolutionary Processes
•
Earth formed about 4.6 billion years ago. By about 4 billion years ago, less dense
compounds had cooled to form a solid crust, water vapour had condensed, and
ocean basins had filled.
•
Early anaerobic conditions on Earth likely resulted in the formation and polymerization of many small organic molecules.
•
Some RNA molecules act as catalysts for various reactions, including their own
replication. As such, they are likely candidates for the first hereditary molecular
systems.
•
Both lipid and protein compounds likely formed liquid-filled semipermeable
vessels spontaneously. These vessels have some of the same properties as cells.
•
The first cells, which evolved at least 3.8 billion years ago, resembled modern
prokaryotic cells. After photosynthetic prokaryotic cells evolved, at least 3 billion
years ago, oxygen gas began to accumulate in Earth’s atmosphere.
Section 13.1 Questions
Understanding Concepts
1. Review the theory of natural selection as described by
Darwin (Chapter 11, section 11.6). Given that chemical
evolution occurred before life existed on primordial Earth,
explain when you consider the process of natural selection to have begun.
2. Many scientists study chemical evolution. Suggest ways in
which selective forces might have acted on chemicals
before living cells existed on Earth.
3. Compare and contrast thermal proteinoids and liposomes.
4. In what way might the lack of oxygen in the early atmos-
phere have influenced the formation of both complex
organic molecules and the first living cells?
Applying Inquiry Skills
attempted to model chemical reactions in the atmosphere of
ancient Earth. Even though other scientists have cast doubts
on this particular combination of gases, why do we consider
the findings of the Urey–Miller experiment to be relevant?
6. (a) In Activity 13.1.1, why was lecithin used to model the
formation of protocells?
(b) Did your observations permit you to determine whether
the vesicles have double-layered membranes?
Making Connections
7. Some scientists have different perspectives on the earliest
evolutionary history on Earth. In print and electronic sources,
find out more about their research, evidence, reasoning, and
differing interpretations of experimental results.
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5. Suggest reasons that Urey and Miller selected the combina-
tion of gases they did for their experiment in which they
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