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EVIDENCE OF COMMON ANCESTRY
reflect
Are birds related to dinosaurs? Years ago, this question
spawned heated debates and controversy among many
scientists. Today, the scientific community generally accepts
the idea that birds and dinosaurs share a common ancestor.
What changed? Did scientists find evidence to support this
theory? If so, what sort of evidence did they find?
ancestor: an organism
from which other
organisms evolved
The Fossil Record
Many pieces of evidence support the theory that
birds and dinosaurs are related. One such piece of
evidence, the fossil record, is the accumulation of
all fossilized remains that scientists have collected
around the world. While the fossil record provides
evidence to support the theory of common ancestors
among different organisms, it is not a complete
record.
From fossils, scientists can identify the habitats and
ecosystems that extinct organisms once occupied.
They can also group similar extinct organisms
together and arrange them in the order in which they
lived, from oldest to youngest. The relative age of a
fossil can be determined by comparing its location
to other fossils within the same rock layers. Older
fossils appear in rock layers below younger fossils.
Scientists also use radioactive dating to determine
the age of fossils. Radioactive materials have a fixed
rate at which they decay. By measuring how much
radioactive material is left in a fossil, scientists know
how much has decayed and can then determine
how long it took to decay.
Fossils can give clues about how organisms changed over time. Scientists study the
similarities and differences in bones, shell shapes, or other features over time. For
example, scientists discovered melanosomes in an ancient bird feather. Melanosomes are
structures that contain melanin, which helps determine a creature’s coloring. Scientists also
found melanosomes in the feathery areas of a dinosaur fossil, which suggests an ancestral
link between birds and dinosaurs.
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EVIDENCE OF COMMON ANCESTRY
Sometimes the fossil record reveals shifts from one entire species to another. After
comparing the skeletons of birds to those of Coelurosaurian dinosaurs, paleontologists
concluded that many birds are likely direct descendants of the
group called coelurosaurs (especially the Velociraptors.) They
paleontologist: a
point to characteristics such as the S-shaped neck, hollow
scientist who studies
bones, large eye sockets, five or more vertebrae in the hip, and
prehistoric life
many other similarities.
what do you think?
Take a look at the photographs below. Do you think the elephant on the left is a direct
descendant of the woolly mammoth on the right? Explain your reasoning.
Studying Homologies of Different Organisms
A 2005 study of mitochondrial DNA confirmed that woolly mammoths and African and
Asian elephants share a common ancestor. Scientists compared the mitochondrial
DNA sequences of both animals and found strong similarities.
Mitochondrial DNA is particularly valuable to make these
common ancestor:
determinations because only the mother passes mitochondrial
a predecessor that a
DNA down to her offspring. Scientists looked at the DNA
creature shares with
sequences of each organism and compared their similarities
another creature.
or differences. Similarities in anatomical structure, gene
sequences, developmental stages, or any other criteria that
point to a common ancestor are called homologies.
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EVIDENCE OF COMMON ANCESTRY
Paleontologists have compared bone structures in organisms for years to determine
common ancestry among organisms. A likeness between the bone structure of one creature
and that of another may indicate a common ancestor. Mammals, birds, and reptiles all show
similar anatomical patterns.
Although the function of each is different, this diagram
shows similarities in the anatomical structure of forelimbs
in mammals (bat and human), birds (penguin), and
reptiles (alligator).
Species with similar origins also show similar embryonic development patterns.
Paleontologists can learn a lot about a species by studying the embryo and patterns of
development. For example, certain snake embryos have small buds that look like limbs.
The buds disappear in later developmental stages. This suggests that snakes evolved from
an ancestor that had limbs.
look out!
Although evidence may indicate certain
information, it may not be readily accepted. One
hypothesis that has received mixed reactions is
the idea that Tyrannosaurus rex is a predecessor
of the chicken. Paleontologists found protein
(collagen) in a 68 million-year-old T. rex bone.
In 2007, they reported that five of the seven
collagen fragments closely matched the collagen
of chickens.
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EVIDENCE OF COMMON ANCESTRY
Other researchers questioned this conclusion and the research process. No other scientist
had ever found protein that had survived even one million years, let alone 68 million. Could
the proteins have come from contamination? A computerized process determined the
match. Some scientists wondered if different results might have occurred if the instructions
were written differently. Acceptance of this hypothesis will take more research and
evidence.
Biogeography
Biogeography is another type of evidence that supports the
theory of evolution by natural selection. Biogeography is
the study of past and present geographical distribution of
species.
Organisms have characteristics that allow them to survive
in their environment. If the environment changes, some
species may evolve over time to cope with the differences.
Sometimes part of a population is geographically separated
from the main group. The isolated population is forced to
adapt to different conditions and evolve separately. The
lemur is an example. Some scientists believe lemurs crossed
from Africa to the island of Madagascar millions of years
ago by floating on vegetation. By 20 million years ago, the
continents had drifted so far apart that the lemurs became
permanently isolated on Madagascar.
The lemur is an
example of an animal
whose ancestors
were geographically
separated from their
native population.
Continental drift, the breaking apart and movement of continents from one massive
landform named “Pangaea,” explains why many species evolved into new species. As the
land masses of Pangaea drifted apart, newly isolated
species developed characteristics that set them apart
from their common ancestor.
Crocodiles have not changed
from their original forms in
200 million years. They are an
example of stasis.
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EVIDENCE OF COMMON ANCESTRY
Rates and Patterns of Evolution
Charles Darwin proposed the theory of evolution by natural selection in 1859, which
suggested that all creatures descended from common ancestors. His theory contended
that organisms evolve through natural selection, a gradual process by which subsequent
generations of organisms develop characteristics that help them survive in their
surrounding conditions through mutations. Mutations that
mutation: an
benefit an organism are passed on to subsequent generations.
abrupt change in
However, it is important to note that natural selection is not a
the genotype of an
deliberate process. Organisms do not choose which traits to
organism
pass on to their offspring—it is a natural process that favors
certain traits in a particular environment. Organisms that do
not have these favorable characteristics will gradually decrease in number, and the gene
pool of the population changes. As Darwin’s theory was accepted, so was the theory of
gradualism. This is the idea that evolution occurs gradually. Changes or adaptations appear
slowly and sequentially over time.
In 1972, Niles Eldredge and Stephen Jay Gould contended that the fossil record shows
very few gradual changes and intermediate forms of evolutionary change. They proposed a
new theory of evolutionary change consisting of two general ideas: stasis and punctuated
equilibrium. Eldredge and Gould stated that some species tend to stay in a state of stasis,
or a period during which no evolutionary changes occur for a long time. Stasis can last
for millions of years. When evolutionary change does occur, it happens quickly in bursts,
called punctuated equilibrium. Punctuated equilibrium results in new species appearing
suddenly in the fossil record and leaves little fossil evidence behind of the transition from
the ancestral species to the new form.
The Origin of Life and the Complexity of Cells
Much of this lesson has been devoted to understanding how Earth’s organisms have
changed over time. But how did Earth’s first, single-celled organisms arise? Which scientific
theories explain the origin of life on Earth?
In the 1920s, two scientists working in different countries proposed the same hypothesis on
the origin of life. Russian biochemist Alexander Oparin and British geneticist J.B.S. Haldane
proposed that life began spontaneously from combinations of nonliving matter exposed to
energy from lightning, volcano eruptions, or the Sun.
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EVIDENCE OF COMMON ANCESTRY
Oparin and Haldane proposed that life began in the oceans where organic molecules were
everywhere, thus resulting in a “primordial soup.” The two scientists suggested that Earth’s
early atmosphere lacked oxygen. Instead, it was composed of nitrogen, hydrogen, and
other compounds, including water vapor, ammonia, and methane. Life could more easily
arise in this environment because electrons from energy sources such as lightning could
react with molecules such as nitrogen to form organic compounds. An oxidized atmosphere
would have simply absorbed these electrons. Their ideas became known as the OparinHaldane Hypothesis.
In 1953, Stanley Miller and Harold Urey
tested this hypothesis with the Miller/Urey
experiment. They built an apparatus to
simulate Earth’s early atmosphere, shown
in the diagram at right. They applied a
continuous electric current to mimic the
lightning storms on Earth. Within a week,
small amounts of carbon had formed into
organic molecules, and some had even
formed into small amino acid chains.
Amino acids are the building blocks of
proteins.
Many scientists are skeptical of this
theory, partly because lightning storms
probably did not occur often enough
to generate the quantities of organic
molecules necessary for the origin of life.
So, while amino acids and other organic
compounds may have been formed, they
would not have been formed in the amounts produced by the Miller/Urey experiment.
Iron-Sulfur World Theory
In 1985, German chemist Günter Wächtershäuser proposed the Iron-Sulfur World Theory.
He claimed that life began on the surfaces of mineral deposits near volcanic vents on the
ocean floor. There, carbon reacted with metals such as iron and nickel, creating organic
compounds (carbon monoxide, carbon dioxide, hydrogen cyanide, and hydrogen sulfide)
from gases spewed by the underwater volcanoes. This process took place under high
pressure and high temperatures. According to Wächtershäuser’s theory, these were the
perfect conditions for organic synthesis. An evolving cycle of chemical reactions eventually
formed acetic acid and pyruvic acid, two key elements in the citric acid cycle that organisms
use to make energy. This theory accounts for metabolism, or how chemical energy is
transferred within a cell, but it does not describe how the organism could duplicate itself.
For this, the RNA World hypothesis offers a possible explanation.
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EVIDENCE OF COMMON ANCESTRY
RNA World Hypothesis
The first two hypotheses explain how inorganic molecules
become organic monomers, such as amino acids, sugars,
phosphates, and bases. The more difficult problem was
describing how these simple chemical systems became complex
enough to form organisms—how did monomers become
polymers, such as proteins, carbohydrates, and lipids.
In the 1980s, American scientist Thomas Cech and his
colleagues discovered that RNA (ribonucleic acid) can act as
a catalyst, helping to drive the chemical reactions necessary
for many processes in an organism. Ribosomal RNA and
transfer RNAs are the primary molecules responsible for protein
synthesis. RNA is also able to store and pass on information,
allowing molecules to replicate, which is another essential
aspect of living organisms. Thus, the RNA World Hypothesis was
introduced, proposing that RNA existed before DNA. This theory
explains how simple molecules could become complex selfreplicating systems. However, scientists do not understand how
RNA was originally formed. Debate and research still continue
on many aspects of this theory.
The First Cells
Once the basic chemicals came together under the
right circumstances, how did they form into cells? Over
millions of years, primitive organic systems associated
with lipids that formed membranes that held them together
and separated them from the external environment.
The information-containing molecules (e.g. RNA) inside
the primitive cells enabled the cells to replicate. Many
scientists now believe the earliest forms of life were
ancient bacteria and archaea that could live in extreme
temperatures and thrive in an environment without oxygen.
catalyst: a
substance that helps
start or increase the
rate of a reaction
Did life begin in extreme
environments like this
thermal pool?
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EVIDENCE OF COMMON ANCESTRY
Scientists in the Spotlight: Lynn Margulis
In the 1960s, biologist Lynn Margulis proposed the endosymbiotic
theory. She suggested that mitochondria, which convert food
into energy in cells, and chloroplasts, which convert sunlight into
energy in plant cells, have more in common with prokaryotes
than eukaryotes. In fact, she suggests that mitochondria
and chloroplasts were once free-living bacteria that evolved a
mutualistic relationship with early eukaryotic cells. Over time, the
reproductive cycle of the endosymbiont became completely tied
to that of the host, and the endosymbionts lost the ability to live
outside of the host cell. Support for the endosymbiotic theory is
found in the DNA and ribosomes of mitochondria – both of which
are similar to that found in Rickettsia, a parasitic endosymbiont.
Like mitochondrial DNA, chloroplast DNA is also similar to
prokaryotic photosynthetic cyanobacteria.
prokaryote: singlecelled organism
without a nucleus or
organelles
eukaryote:
organism (usually
multicellular) with a
defined nucleus and
membrane-bound
organelles
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EVIDENCE OF COMMON ANCESTRY
What Do You Know?
Read each example in the left column of the chart. Then choose a term from the list below
the chart that BEST matches each example. Use each term only once.
Example
Humans and chimpanzees share some DNA and protein
sequences.
Whales, bats, and birds have the same bone that connects
their limbs to the middle of their bodies.
Fossils of a shelled organism show slow changes in form
over time within the species.
A combination of lightning and an atmosphere composed of
nitrogen, hydrogen, and other compounds, including water
vapor, ammonia, and methane results in organic compounds.
A scientist is studying the distribution of extinct and modern
ferns in North America.
Chicken, pig, and fish embryos all have pharyngeal folds.
A series of reactions in underwater volcanoes results in the
formation of pyruvic acid and acetic acid.
Fossils of a bird species show no changes in form over
millions of years, and then a new, similar bird species
appears in the fossil record.
RNA acts as a catalyst, helping long molecules to form and
cells to replicate.
Matching Term
List of Terms:
• Biogeography
• Developmental Homology
• Structural Homology
• Gradualism
• RNA World Hypothesis
• Iron-Sulfur World Theory
• Molecular Homology
• Punctuated Equilibrium
• Oparin-Haldane Hypothesis
what do you think?
Analyze the endosymbiotic theory and propose a depiction of the early earth if early
eukaryotes had not developed an endosymbiotic relationship with chloroplast and
mitochondria.
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EVIDENCE OF COMMON ANCESTRY
connecting with your child
Evolutionary Trees
To help your child learn more about common
ancestry, work together to create an
evolutionary tree. Also called a phylogenetic
tree or a tree of life, this diagram shows
how species may be related to each other
through physical characteristics, genetics,
or character traits. According to Darwin’s
theory of evolution by natural selection, all
species share a common ancestor.
First have your child choose 12 organisms
to include in his or her evolutionary tree,
including three aquatic animals, three land
mammals, three insects, and three reptiles.
Search for photographs of these creatures
on the Internet or in old magazines. Get
a piece of large poster board, a ruler,
glue, and a pencil. This evolutionary tree
will focus on the physical characteristics
of organisms. Brainstorm with your child
different characteristics to use on the tree.
Some ideas include:
• presence or absence of a backbone
• presence of hair, fur, or scales
• warm or cold bloodedness
• lungs or gills
• external structures such as fins, claws,
or hooves
• food preference (omnivore, carnivore, or
herbivore)
Turn the poster board to a landscape
format. Write “Common Ancestor” at the
bottom of the poster board and draw a large
“Y” above the term. Leave plenty of room
above the “Y” to draw your tree “branches.”
Now choose a characteristic for each side
of the “Y” (e.g., presence or absence of a
backbone). Separate the 12 organisms into
two piles according to this characteristic.
Now look at the organisms in each group
and choose a characteristic to separate
each of the two groups into smaller groups.
You might find that one group, such as the
organisms without backbones, cannot be
further separated. If this is the case, glue
these organisms onto the correct side of
the “Y” at the top. For any group(s) that
can be further separated, draw a “V” on the
correct side(s) of the “Y” at the top. Place
the organisms on either side of the “V.” For
characteristics such as hair, fur, or scales,
you will need to draw an extra line or two
on the “V” – one line for each form of the
characteristic.
Continue with this process until only one
organism is left and glue the picture of the
organism at the end of the branch. For ideas
about how evolutionary trees are drawn and
organized, you may want to do an Internet
search prior to this activity using the term,
“evolutionary tree.”
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EVIDENCE OF COMMON ANCESTRY
Here are some questions to discuss with your child:
• What surprised you the most about the relationships of the organisms in the
evolutionary tree?
• What surprised you the least?
• Do you think an evolutionary tree based on genetic information might look different than
the tree based on physical characteristics? Explain your reasoning.
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