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Early Evolution of Life
Major events in early life include the evolution of prokaryotes, photosynthesis,
eukaryotes, multicellularity, and the colonization of land.
Alethopteris fossil.
Fossilized leaves of Alethopteris sp., and extinct plant that lived in the Carboniferous period.
Sinclair Stammers/Science Source.
Topics Covered in this Module
Early Life on Earth
Major Objectives of this Module
Give the date the first prokaryotes appear in the fossil record and how they were identified.
Describe the geologic and biologic effects of the evolution of photosynthesis.
Relate endosymbiont theory to the evolution of eukaryotes.
Explain how species evolved adaptations to life on land.
page 380 of 989
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Principles of Biology
74 Early Evolution of Life
When did life begin? What did the earliest life forms look like? When did
plants and animals appear on Earth? Evidence for early life on Earth comes
from geology and the fossil record.
Early Life on Earth
Scientists use radiometric dating to determine how old fossils are based on
how much the radioactive isotopes they contain have decayed. The history
of Earth is customarily divided into three eons: the Archaean, Proteozoic,
and Phanerozoic (Figure 1). The first single-celled organisms appeared in
the Archaean eon. The first eukaryotes and multicellular organisms
appeared in the Proterozoic eon. Animals appeared toward the end of this
eon, but most of their evolution occurred during the Phanerozoic eon, which
covers approximately the last half billion years and is further divided into the
Paleozoic, Mesozoic, and Cenozoic eras. Note how small a fraction of
Earth's history includes humans. If the history of Earth were an hour, humans
would have appeared in the last two-tenths of the last second.
contents
Figure 1: History of life on Earth.Fossils tell the history of life on Earth.
The changes scientists observe in the life forms preserved as fossils are
the basis of the divisions of the geologic time scale.
© 2014 Nature Education All rights reserved.
Figure Detail
The first cells to evolve were prokaryotes.
Scientists think that Earth formed with the rest of the solar system about 4.6
billion years ago. At first, the new planet would have been bombarded by
rocks and ice hurtling through space, and the repeated collisions would have
generated a large amount of heat. Sometime after this bombardment slowed
down, the planet cooled enough that the water vapor in its atmosphere could
condense, forming oceans. The atmosphere was probably made up primarily
of carbon dioxide and nitrogen, but volcanic eruptions might have contributed
methane, ammonia, and hydrogen sulfide. One hypothesis of the origin of life
is that the first living organisms were anaerobic archaebacteria that evolved
in the hydrothermal vents near submarine volcanoes, where anaerobic
bacteria still live.
The first evidence of life in the fossil record comes from prokaryotes: singlecelled microorganisms containing DNA but no nuclei or membrane-bound
organelles. Fossilized prokaryote communities bound very thin layers of
sediment together into rocks called stromatolites. The oldest known
stromatolites are about 3.5 billion years old. Because these fossils represent
large groups of prokaryotes, the first single prokaryotes might have evolved
much earlier. For about the next 1.5 billion years, they were the only life
forms on the planet.
Future perspectives.
Prokaryotes continue to interest scientists because, despite being small, they
are highly adaptable — as evidenced by the ongoing evolution of multi-drug
resistant bacteria. Recent research shows that the structure of prokaryotes is
more complicated than once thought. Although they do not have
membrane-bound organelles, prokaryotes do have distinct subunits that
perform individual functions. For instance, prokaryotes have organized
structures called micro-compartments. Micro-compartments produce energy
for cellular metabolism and carry out other important processes within the
cell. They are made out of proteins and structurally resemble viruses.
Micro-compartments might have evolved independently to look like viruses,
or they may have evolved from viruses that the prokaryotes took in and lived
with in symbiosis. These compartments are so small that it is difficult to
examine their structure even using electron microscopy, but genetic analyses
might help determine their origins. Viruses called bacteriophages are known
to "infect" bacteria, but some bacteria cannot survive without phages. Phage
genes encode many proteins that help bacteria defend themselves, for
instance, by producing toxins. Phages increase the toxicity or the survival of
several strains of bacteria, including some Escherichia coli, Streptococcus mitis, and
Salmonella enterica, suggesting the two microorganisms may commonly live in
mutually beneficial relationships.
Photosynthesis changed the atmosphere.
Oxygen makes up 21% of the atmosphere now, but when life first emerged,
the atmosphere contained virtually no oxygen (Figure 2). Oxygen levels first
began to rise dramatically between 2.5 to 2 billion years ago. During this
time, photosynthetic prokaryotes emerged and diversified in the oceans, and
they released more and more oxygen as a byproduct of photosynthesis. At
first, the oxygen that these cyanobacteria produced remained dissolved in
ocean water. However, eventually, the water became saturated with oxygen,
so that additional oxygen produced by the photosynthesizing prokaryotes
was released into the atmosphere. Other abrupt shifts in the levels of oxygen
in the atmosphere occurred following the emergence of photosynthetic
eukaryotes. The development of photosynthetic eukaryotes led to algae,
which are the dominant photosynthesizers in the world's oceans, and
eventually to land plants.
Figure 2: Atmospheric oxygen levels on Earth over the last 4 billion
years.
This schematic represents our understanding of how oxygen in Earth's
atmosphere has changed throughout the history of life and notes when
organisms that photosynthesize evolved. (Modified from Xiong, J. &
Bauer, C. E.. Complex evolution of photosynthesis. Annual Review of Plant
Biology 53, 503-521 (2002).
© 2002 Annual Reviews Modified from Xiong, J. & Bauer, C. E..
Complex evolution of photosynthesis. Annual Review of Plant Biology 53,
503-521 (2002). doi: 10.1146/annurev.arplant.53.100301.135212. Used
with permission.
After the dramatic increase in atmospheric oxygen, the environment became
much less hospitable to the organisms that had evolved in the presence of
very low levels of atmospheric oxygen. Oxygen is reactive and can damage
cells and alter biochemical processes. As oxygen concentrations increased
in Earth's atmosphere, a very large proportion of the prokaryotic species that
thrived in anaerobic conditions went extinct, and species that were more
tolerant of the new oxygenated conditions thrived. Eventually, lineages of
these more tolerant organisms evolved the ability to utilize oxygen during
respiration as a means to release stored chemical energy. A majority of the
organisms that we are most familiar with (including ourselves) descended
from these oxygen-reliant lineages.
As photosynthetic prokaryotes, probably similar to cyanobacteria found on
Earth today, colonized the oceans, they released more and more oxygen.
Bacteria reproduce exponentially — the number of individuals doubles after
every reproductive cycle — and so with their increased numbers, the oxygen
on Earth rose very rapidly. At first, the oxygen that these cyanobacteria
produced dissolved in the oceans. Then it became concentrated enough to
react with iron, creating the red rocks we see today. Eventually, the oceans
became saturated with oxygen, so that additional oxygen "gassed out" and
built up in the air. Atmospheric oxygen levels rose — at first gradually, then
steeply. The fastest change was probably driven by photosynthesis in
eukaryotes as well as prokaryotes. Photosynthesis dramatically changed
Earth's atmosphere.
This dramatic "oxygen revolution" created an environment that was likely
inhospitable to most of the life forms that existed on Earth at that time. So,
how did the rise in atmospheric oxygen influence evolutionary history?
Oxygen is reactive. In certain forms, it damages cells and enzymes. As
oxygen concentrations increased in Earth's atmosphere, many prokaryotes
probably died. Any cells that could use oxygen productively were more likely
to survive and reproduce. This selective pressure ultimately led to the
evolution of oxygen-reliant eukaryotes.
Test Yourself
Carbon dioxide (CO2) levels in Earth's atmosphere have increased by about 40% since 1850.
On a geologic time scale, that's a dramatic change over a very short time. What were the
effects of the oxygen revolution about 2.5 billion years ago? How might those relate to today’s
problems with increasing CO2 in Earth's atmosphere?
Submit
How did eukaryotes evolve?
The earliest identifiable fossil eukaryotes date from about 2.1 billion years
ago. Unlike prokaryotes, eukaryotes have a membrane-bound nucleus and
membrane-bound organelles. A wide variety of single-celled eukaryotes exist
today including algae, yeasts, and protists, such as amoebae. A eukaryote
has DNA enclosed in a nucleus, an endoplasmic reticulum that participates
in protein synthesis, a cytoskeleton that allows it to change shape to engulf
other cells and transport materials, and mitochondria that use oxygen and
glucose to produce energy-containing molecules for the cell. The process of
using oxygen and glucose to generate energy-packed molecules is called
aerobic respiration. This ability would have allowed eukaryotes to take
advantage of the sudden availability of oxygen after the oxygen revolution.
How did eukaryotes evolve from prokaryotes? The nuclear envelope and
endoplasmic reticulum resemble the cell membrane in composition, and they
probably evolved from the cell membrane folding in on itself. The
mitochondria indicate a more complicated story.
A controversial idea becomes well accepted.
It might be surprising to learn that every cell in the human body contains an
organelle derived from bacteria. This idea was certainly very controversial
when it was first proposed by Dr. Lynn Margulis. In 1967, Dr. Margulis wrote
a paper about the origin of eukaryotes that described the endosymbiont
theory of eukaryotic evolution — the theory that mitochondria (the
organelles that conduct aerobic respiration in eukaryotes) evolved from
free-living bacteria that had been engulfed by early cells. Scientists widely
disregarded her idea at the time, and Margulis's paper was rejected by 15
journals before the Journal of Theoretical Biology published it. Her ideas stood the
test of time, however, as the results of more and more experiments were
conducted that supported her now widely-accepted theory.
Margulis was not the first person to suggest the idea that eukaryotic
organelles might have arisen through a process of endosymbiosis, but she
had more evidence than earlier scientists. In the 19th century, the botanist
Andreas Schimper observed division of the chloroplasts of green plants (the
organelles that conduct photosynthesis). He noticed how they resembled
cyanobacteria, the primitive free-living microorganisms that photosynthesize.
In the early 1900s, the botanist Konstantin Mereschkowski proposed that
chloroplasts evolved from symbiotic cyanobacteria, bacteria living inside cells
to the benefit of both organisms. Around the same time, the scientist Ivan
Emanuel Wallin suggested that mitochondria evolved from bacteria. Neither
of these ideas received much attention at the time. But then in the 1960s,
scientists used new microscopic techniques to observe the interior of
mitochondria and realized that mitochondria have DNA arranged in circular
chromosomes. Bacterial DNA is arranged the same way.
Margulis noted that mitochondria self-replicate by fission, as bacteria do,
using their own DNA, which is arranged in a circular chromosome about the
size of a bacterial chromosome. What other evidence did Margulis gather?
Mitochondria are about the same size as bacteria. They have ribosomes,
enzymes, and transport systems much like those of bacteria, and they use
these to make their own proteins. The mitochondria have a double
membrane, and the innermost mitochondrial membrane has a composition
similar to a bacterial membrane.
BIOSKILL
Genomic Sequencing Informs Phylogenetic Analysis
The technique of DNA sequencing had not yet been developed when
Margulis published her landmark paper, but today sequencing is often used
to determine how organisms are related to each other. Scientists compare
the amino acid sequence of a protein in different species to look for the
number of differences between them. Figure 3 shows a portion of the
superoxide dismutase protein. This protein helps protect cells from the
damaging effects of oxygen.
Figure 3: Comparing proteins.
Mitochondria evolved from proteobacteria and chloroplasts evolved from
cyanobacteria. An ancestral cell lineage might have sequentially picked
up first one and then the other.
© 2014 Nature Education All rights reserved.
Transcript
Test Yourself
Explain how the data in Figure 3 support the endosymbiotic theory.
Submit
BIOSKILL
Why would early prokaryotes have engulfed other prokaryotes and continued
to live symbiotically with them? These organelles produce energy-containing
molecules that help the host cells function. Chloroplasts or, more generally,
plastids convert energy from sunlight to usable forms. Mitochondria convert
energy from glucose and oxygen to usable forms. Either might have
harnessed a previously unavailable energy source for their host cells, such
as methane, which would have helped the cells function anaerobically.
Engulfing a prokaryote that could make use of rising oxygen levels would
have conferred a selection advantage because the host cell could survive
the oxygen revolution. The theory of serial endosymbiosis proposes that
multiple endosymbiotic events occurred in the same cell lineage (Figure 4).
Figure 4: Serial endosymbiosis.
Serial endosymbiosis. Mitochondria evolved from proteobacteria and
chloroplasts evolved from cyanobacteria.
© 2014 Nature Education All rights reserved.
Transcript
Test Yourself
What kind of evidence probably indicates that photosynthetic eukaryotes evolved after other
(heterotrophic) eukaryotes?
Submit
Future perspectives and open questions.
Endosymbiosis is not something that only happened once, by chance, early
in evolutionary history. Eukaryotes are known to have engulfed other
eukaryotes. For instance, different types of algae have evolved through this
process of secondary endosymbiosis. Sequencing genomes of species that
have evolved through secondary endosymbiosis could help scientists
understand how the process works. In 1981, Norman Weeden proposed that,
because the plastid (e.g., chloroplast) genome is not big enough to encode
all the proteins the plastid uses, the plastid must have transferred many of its
genes to the host cell's genome during endosymbiosis. According to one
current estimate, 90% of the proteins a plastid uses are made by the host
genome and imported from the cytoplasm. So the DNA an organelle carries
into a host cell does not necessarily stay in one place but can infiltrate the
nucleus.
A 2002 study by William Martin and colleagues compared 24,990 proteins
produced by the flowering plant Arabidopsis, a model organism for scientific
research, with the sequenced genomes of cyanobacteria and other
microorganisms. The researchers found that genes derived from
cyanobacteria make up a surprisingly large fraction of the eukaryotic genome
— about 18%. The proteins these genes produce are active in cell growth,
transcription, division, transport systems, and cellular metabolism.
Cyanobacteria apparently contributed much more to plant genomes than
only chloroplasts.
If chloroplasts have inserted their DNA into host genomes, what about
mitochondria? Mitochondrial DNA has been identified within the nuclear DNA
of grasshoppers, shrimp, rats, cats, monkeys, and humans. How does DNA
from organelles move into the host genome, and why do some genes stay in
the organelle genome? How did bacterial genes shape our evolution? These
questions drive areas of active research.
How did multicellularity evolve?
Single-celled eukaryotes evolved into multicellular eukaryotes. The earliest
known examples are multicellular algae, which appear in the fossil record
about 1.2 billion years ago. Another estimate comes from DNA sequencing.
Scientists estimate that, based on the amount of change among their DNA
sequences, multicellular eukaryotes probably diverged from a common
ancestor about 1.5 billion years ago. After the first few algae, a long gap
appears in the fossil record. More complex organisms do not show up for
another 500 million years. Between 575 and 535 million years ago, fossils of
soft-bodied multicellular creatures emerge. They are called the Ediacaran
biota because the fossils were found in Edicara Hill, Australia. Some had
simple circular forms and others had segments, like worms. The largest were
over 3 feet (1 m) long. They probably fed on algae, strained plankton, or
scavenged dead organisms. Other fossils found in the Doushantuo formation
in China may represent embryos or adults from extinct animal groups. Given
the gap, though, why did it take so long for multicellular organisms to
appear?
Geologic evidence indicates that between 750 and 580 million years ago,
Earth went through a series of ice ages. At times, the oceans and land were
covered by sheets of ice. Most organisms would have been able to live only
near deep-sea hydrothermal vents or perhaps in areas near the equator that
were not covered in ice. The first multicellular organisms seem to have
evolved around the time the "snowball Earth" melted (Figure 5).
Figure 5: Multicellular fossil.
This Proterozoic fossil is of one of the earliest animals. Philip Donoghue
and colleagues used X-ray microscopy to show how early multicellular
embryos develop.
© 2006 Nature Publishing Group Donoghue, P. C. J., et al. Synchrotron
X-ray tomographic microscopy of fossil embryos. Nature 442, 680-683
(2006) doi:10.1038/nature04890. Used with permission.
Between 535 and 525 million years ago, during the Cambrian period of the
Paleozoic era, an amazing diversity of life forms appears in the fossil record.
Animals with hard body parts, bones, and shells show up for the first time.
Many of these fossils were found in the Burgess Shale in British Columbia,
Canada. About half of the current animal phyla arrive in the record during
this time, including:
Arthropods: invertebrates with exoskeletons, including insects and
spiders
Echinoderms, such as sea stars and sand dollars
Chordates, which include all vertebrates, such as fish, snakes, birds,
and mammals.
This period of sudden diversification is called the Cambrian explosion (Figure
6). Although most animals from this time look nothing like any animal on
Earth today, they were the ancestors of most present-day animals. Early
predators appearing in the Cambrian were longer than 3 ft (1 m) with claws
to capture prey. The prey displayed defenses such as body armor and sharp
spines. How could natural selection have resulted in tougher skin or sharper
teeth?
Figure 6: The Cambrian explosion.
The Cambrian period, approximately 555 to 500 million years ago, was
marked by a huge diversification of animal types. The ancestors of almost
all of current animal phyla first appeared during the Cambrian, including
the chordates, echinoderms, mollusks, arthropods, annelids, and
brachiopods.
© 2012 Nature Education All rights reserved.
Figure Detail
Life eventually adapted to land.
Fossils show that photosynthetic prokaryotes lived on damp land over a
billion years ago, but fungi, plants, and animals did not evolve until much
later — about 500 million years ago. How did organisms evolve to live
successfully on land? One major feature that emerged among successful
terrestrial photosynthetic organisms is that they evolved structures to avoid
drying out. Many current plants have a waxy coating to limit evaporation of
water, as well as a vascular system to take in water from soil. By 420 million
years ago, land plants about 4 inches (10 cm) tall had evolved with a
vascular system that transported water and nutrients up from the ground. By
370 million years ago, more complex plants with true roots and leaves
appeared. Roots are specialized to anchor the plant and transport water and
nutrients, and leaves are specialized for photosynthesis. How could natural
selection have resulted in this specialization of cells?
Fungi evolved along with plants in symbiotic relationships, like chloroplasts
with host cells. Fossils show that fungi lived on the roots of plants almost as
soon as plants colonized land. Most plants today harbor fungi in their root
systems. The fungi help the plants absorb water and minerals, and the plants
provide nutrients to the fungi.
Arthropods, the group of animals that includes insects, were the first animals
to colonize land. They appear in the fossil record about 420 million years
ago. Next were tetrapods, about 365 million years ago. Tetrapods are
four-limbed animals that probably evolved from a particular group of fish. All
mammals descended from tetrapods, including humans. Upright posture
evolved uniquely and separately for biped tetrapods, such as hominids and
birds. Arthropods and animal descended from tetrapods are now the most
widespread and diverse groups of animals, including creatures as different
as (among arthropods) butterflies, mosquitoes, spiders, and crabs and
(among tetrapods) alligators, birds, moles, elephants, bats, kangaroos, dogs,
and humans.
The phenomenon of having a drink "go down the wrong way" (i.e., choking)
appears to be a problem with human anatomy. It also represents an outcome
of evolutionary history and a link to the developmental journey from embryo
to fully developed human being. What is the origin of this less-than-ideal
anatomy? The oral cavity leads to both the esophagus, the tube down which
food travels to reach the stomach, and to the trachea, the tube down which
air travels to reach the lungs. The reason these are connected is because
they develop from similar tissues. The development of these tissues is linked
to how these structures evolved in the first place. Understanding the
evolutionary relationships between organisms, and the process through
which complex structures evolved, provides insight into the anatomy,
physiology, and behaviors that exist today.
Mammals breathe through lungs. Air passes through the nose and throat and
into the lungs, which are made up of tiny, delicate sacs called alveoli (see
Figure 7). Inhaled oxygen diffuses across the alveoli into tiny surrounding
blood vessels, and carbon dioxide (CO2), a waste product of aerobic
respiration, diffuses from the blood into the alveoli. When the CO2 is exhaled,
muscles compress the lungs to squeeze the air out of the alveoli.
Figure 7: Bronchi and lungs.
Bronchi branch off the trachea and further branch multiple times into
smaller and smaller air way vessels, the bronchioles (black and
gray-translucent staining). These branches are like a network of roots
penetrating deep into the lung that facilitate gas exchange inside lung
tissue. In this image, some blood vessels (red), branches of the
pulmonary artery, are also visible.
© 2004 Nature Publishing Group Monforte-Muñoz, H., Walls, R. L.
Intrapulmonary airways visualized by staining and clearing of
whole-lung sections: the transparent human lung. Modern Pathology 17,
22–27 (2004) doi:10.1038/modpathol.3800003. Used with permission.
The first respiratory organs were not lungs, however, but probably gills, the
tissues fish use to breathe. Gills are thin tissues folded like a fan. A fish
swallows water, pushing it out over the gills. As the water passes over the
gills, it transfers oxygen to the blood, just as oxygen passes to the blood
from the lungs. In the open air, gills collapse and dry out. In the lungs, the
surfaces also must remain moist.
A group of fish, called lungfish, has lungs as well as gills. Some of these fish
live in shallow ponds, and during the dry season they burrow into the mud
and breathe air. Most fish have organs called swim bladders that allow them
to swallow air to make their bodies lighter and better able to float. A fossil
fish called Tiktaalik, estimated to be 375 million years old, had gills, lungs, and
other characteristics that were more mammalian than fish-like: a flat skull
with eyes on top of its skull, a neck, shoulder bones, and ribs. It is
considered a possible "transition" fossil between lineages. Human embryos
in an early stage of development have structures that resemble gills. The
lungs first appear at day 26 of gestation as buds off the digestive tract. The
alveoli form in the last 2 weeks before birth. By age 8, a child has about 300
million alveoli.
Taken together, what does this evidence suggest? The lungs seem to have
evolved from gills, the system fish use to take in air from swallowed water. In
human embryos, the lungs form as offshoots of the digestive tract. Charles
Darwin thought swim bladders in fish had evolved into lungs, but now
scientists believe swim bladders evolved from lungs. Ancient vertebrates
better able to breathe air and live on land must have moved out of the water
onto land. Eventually the vertebrates that lived entirely on land lost the gills
they no longer needed. Some fish that live partly in mud retained both lungs
and gills, and other fish that live entirely in water evolved swim bladders in
addition to their gills. Mammals, birds, and reptiles all evolved different types
of lungs. For instance, the lungs of birds conduct air to sacs in their bones
and abdomen, and reptiles cannot breathe when food is in their mouths.
Test Yourself
Provide two pieces of evidence that suggest humans and fish have a common ancestor.
Submit
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Early Life on Earth
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Principles of Biology
74 Early Evolution of Life
Summary
Give the date the first prokaryotes appear in the fossil record
and how they were identified.
Single-celled prokaryotes, the first sign of life on Earth, appear in the fossil
record as stromatolites about 3.5 billion years ago. The best evidence is the
stromatolites, fossilized layers of these organisms trapped in sediment.
OBJECTIVE
Describe the geologic and biologic effects of the evolution of
photosynthesis.
Photosynthetic organisms dramatically increased atmospheric oxygen levels
between 2 and 2.5 billion years ago. This change created selective pressure
favoring organisms that used oxygen to produce energy-containing
molecules.
OBJECTIVE
Relate endosymbiont theory to the evolution of eukaryotes.
Mitochondria and chloroplasts are widely believed to have evolved from
bacteria that entered eukaryotic cells through endosymbiosis. These
organelles not only have their own circular DNA, like bacteria, but contribute
DNA to the nuclear genome, have double membranes and make their own
proteins. Researchers are investigating this process and its effects.
OBJECTIVE
Explain how species evolved adaptations to life on land.
About 500 million years ago, some organisms moved from the oceans to
land. Based on fossil, developmental, and anatomical evidence, scientists
think humans and other mammals evolved from fish and lungs evolved from
gills.
OBJECTIVE
Key Terms
endosymbiotic theory
Widely accepted theory that states mitochondria and chloroplasts were once
free-living prokaryotes prior to incorporation inside cells.
serial endosymbiosis
Theory that multiple endosymbiotic events occur in the same cell.
References
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revealed. Heredity 90, 2–3 (2003). doi:10.1038/sj.hdy.6800204 (article)
Effros, R. M. Anatomy, development, and physiology of the lungs. GI Motility
online (2006). doi:10.1038/gimo73 (article)
Hunter, P. Not so simple after all: A renaissance of research into prokaryotic
evolution and cell structure. EMBO reports 9, 224–226 (2008).
doi:10.1038/embor.2008.24 (article)
Kump, L. R. The rise of atmospheric oxygen. Nature 451, 277–278 (2008).
doi:10.1038/nature06587 (link)
Maxmen, A. Friendly bacteria fight the flu. Nature (2011).
doi:10.1038/news.2011.159 (link)
Steinman, M. & Hill, R. Sequence Homologies Among Bacterial and
Mitochondrial Superoxide Dismutases. PNAS 70, 3725–3729 (1973). (link)
Timmis, J. N. et al. Endosymbiotic gene transfer: Organelle genomes forge
eukaryotic chromosomes. Nature Reviews Genetics 5, 123–135 (2004).
contents
doi:10.1038/nrg1271 (article)
Vargas-Parada, L. Mitochondria and the immune response. Nature Education 3,
15 (2010). (link)
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Principles of Biology
74 Early Evolution of Life
Test Your Knowledge
1. When do the first cells appear in the fossil record?
1.2 billion years ago
2.3 billion years ago
535 million years ago
4.6 billion years ago
3.5 billion years ago
2. How did photosynthetic prokaryotes dramatically change Earth's atmosphere?
They increased carbon dioxide levels by burning fossil fuels.
They increased carbon dioxide levels through respiration.
They increased oxygen levels through photosynthesis.
They reduced nitrogen levels by fixing nitrogen.
They reduced methane levels through anaerobic respiration.
3. Which of the following is NOT a way in which mitochondria and bacteria are
similar?
Mitochondrial genes resemble proteobacterial genes.
Their ribosomal structures and RNA are similar.
They both produce all the proteins they need.
They are similar in size.
They both have DNA arranged in circular chromosomes.
4. After Lynn Margulis's endosymbiont theory met opposition, what new evidence
emerged in support of the theory?
lipid composition analysis
light microscopy
electron microscopy
DNA sequencing
transitional fossils
5. What is the main distinction between prokaryotes and eukaryotes?
Eukaryotes have chloroplasts, and mitochondria and prokaryotes have neither.
Only eukaryotes have ribosomes.
Eukaryotes have a nuclear membrane.
Eukaryotes are multicellular; prokaryotes are unicellular.
Eukaryotes have more DNA and are therefore more complex than prokaryotes.
6. Which of the following would NOT likely have occurred without the evolution of
photosynthetic organisms?
Life would be much less diverse.
Most organisms would have gone extinct due to the increase in oxygen.
Mitochondria-containing cells would not have evolved.
There would be no oxygen on Earth.
All of the other choices are potential consequences of a photosynthesis-free world.
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IN THIS MODULE
Early Life on Earth
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Test Your Knowledge
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