Download Evidence of Common Ancestry

Evidence of Common Ancestry
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.
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 point to characteristics such as the S-shaped neck, hollow bones, large eye sockets, five or
more vertebrae in the hip, and many other similarities.
paleontologist: a scientist who
studies prehistoric life
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Evidence of Common Ancestry
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 determinations because only the
common ancestor: a
mother passes mitochondrial DNA down to her offspring. Scientists
predecessor that a
looked at the DNA sequences of each organism and compared their
creature shares with
similarities or differences. Similarities in anatomical structure, gene
another creature.
sequences, developmental stages, or any other criteria that point to a
common ancestor are called homologies.
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).
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Evidence of Common Ancestry
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.
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.
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 benefit an organism are passed on to subsequent
generations. However, it is important to note that natural selection is not mutation: an abrupt
a deliberate process. Organisms do not choose which traits to pass on
change in the genotype
of an organism
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 Oparin- Haldane 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 self- replicating systems.
However, scientists do not understand how RNA was originally
formed. Debate and research still continue on many aspects of this
theory.
catalyst: a substance
that helps start or
increase the rate of a
reaction
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
Did life begin in extreme
thrive in an environment without oxygen.
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 freeliving 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
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
•  Structural Homology
•  RNA World Hypothesis
•  Molecular Homology
•  Oparin-Haldane Hypothesis
•  Developmental Homology
•  Gradualism
•  Iron-Sulfur World Theory
•  Punctuated Equilibriumv
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
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.”
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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