Lines of Evidence
for Common
Ancestry
Evidence of
Common Ancestry
Fossils
Chronology
in Fossil
Record
Transitional
Forms
Homologies
Pace of
Evolution
Gradual
Anatomical
Punctuated
Biogeography
Molecular
Cellular
Developmental
Genetic
Endosymbiosis
Embryological
Vestigial
Structures
Distribution
of Living
Things
Plate
Tectonics
The Fossil Record
• The fossil record provides evidence about the
history of life on Earth
• It shows how different groups of organisms have
changed over time
The Fossil Record
• Fossils can provide evidence for evolution by dating
the rocks in which they were found
• The fossils from different time periods can be
compared and the changes in the organism’s form
noted
The Fossil Record
• What are some possible causes of
stasis (no change in fossils), gradual
change, sequential nature of the layer,
and sudden appearance and
disappearance of fossils
o Natural selection happens over many generations,
extinction, there are some layers missing from erosion, etc.
Relative Dating
• The age of the fossil is determined by comparing its
placement with that of fossils in other layers of rock
Numerical Dating
• Scientists calculate the age of a sample based on
the amount of remaining radioactive isotopes it
contains
Transitional Forms
• Fossils or organisms that show the intermediate
states between an ancestral form and that of its
descendants are referred to as transitional forms
Miller and Urey
• Stanley Miller and Harold Urey experiments suggest
how mixtures of the organic compounds necessary
for life could have arisen from simpler compounds
present on a primitive earth
• Miller and Urey used a mixture of nitrogen,
hydrogen, methane, and ammonia in their
experiments because this mixture of gases
resembles Earth’s early atmosphere
Miller and Urey
• Miller and Urey produced amino acids by passing
sparks through a mixture of hydrogen, methane,
ammonia, and water
• This suggested how simple compounds found on
the early earth could have combined to form
organic compounds needed for life
Gradualism
• Slow and steady nature of biological change
• In many cases, the fossil record confirms that
populations of organisms did change gradually
over time
Punctuated Equilibrium
• Pattern of long, stable periods interrupted by brief
periods of more rapid change
Homologies
• Organisms that have features homologous to the
features of other living organisms, as well as to
fossils, provide evidence that organisms living today
are linked to each other through common
ancestors
Homologies
• Similar characteristics due to relatedness are known
as homologies
• Homologous structures derived from a common
ancestral form
Homologies
• There are five types of homologies
that provide evidence of
common ancestry
o
o
o
o
o
Anatomical
Molecular (genetic)
Cellular
Developmental (embryological)
Vestigial structures
Homologies
• Hoatzin chicks have claws on their wings, as do
some chickens and ostriches
• This reflects the fact that bird ancestors had clawed
hands
Homologies
Anatomical Homologies
• Organisms that are closely related to one another
share many anatomical similarities
• There are four methods scientist
use to study homologies and/or
similarities in genetics
o
o
o
o
Compare protein similarities
DNA hybridization
Sequencing DNA
Comparing epigenetics of species: looking at
which genes are turned on and off in organisms
Genetic Similarities
• One way that researchers assess protein similarities is
by harnessing the immune system’s ability to
recognize foreign proteins
• For example, the immune system of a rabbit will
recognize a human protein as foreign and will
mount an attack against it by making antibodies
specific to that protein.
• The more similar the proteins from the two species
(human and chimpanzee) are, the stronger this
second attack will be
Genetic Similarities
• Research revealed the remarkable similarity
between the proteins of humans and those of other
great apes
• Fewer protein differences corresponded to shorter
times of separation
• Researchers estimated that humans, chimpanzees,
and gorillas shared a common ancestor only 5
million years ago—a much shorter length of time
than was commonly accepted at the time.
DNA Hybridization
• Each DNA molecule is made of two strands
of nucleotides
• If the strands are heated, they will separate—and as
they cool, the attraction of the nucleotides will
make them bond back together again
• To compare different species, scientists cut the DNA
of the species into small segments, separate the
strands, and mix the DNA together
DNA Hybridization
• When the two species’ DNA bonds together, the
match between the two strands will not be perfect
since there are genetic differences between the
species
• The more imperfect the match, the weaker the
bond between the two strands
• These weak bonds can be broken with just a little
heat, while closer matches require more heat to
separate the strands again.
DNA Hybridization
• DNA hybridization can measure how similar the DNA
of different species is—more similar DNA hybrids
“melt” at higher temperatures
• When this technique was applied to primate
relationships, it suggested that humans and
chimpanzees carried DNA more similar to one
another’s than to orangutans’ or gorillas’ DNA.
DNA Sequencing
• DNA sequencing provides evidence of evolutionary
relationships
• Scientists compare DNA sequences of different
species to determine similarities and difference in
their genomes. Similarities indicate common
ancestry
Homologies at the
Cellular Level
• All living things are fundamentally alike
• At the cellular and molecular level living things are
remarkably similar
• These fundamental similarities are most easily
explained by evolutionary theory: life shares a
common ancestor
• All organisms are made of cells, which consist of
membranes filled with water containing genetic
material, proteins, lipids, carbohydrates, salts, and
other substances
Endosymbiosis
• The eukaryotic cell possibly evolved from
prokaryotic organisms engulfing (through
endocytosis) other prokaryotes that had specialized
functions such as converting solar energy to sugar
(chloroplast) and converting sugars to forms of
energy (mitochondria) that could be used by the
cell.
Endosymbiosis
Developmental
Homologies
Developmental
Homologies
• Vertebrate embryos in early development are very
similar
• Early human embryos have structures like gills; also
have a tail
Developmental
Homologies
• Some species of living snakes have hind limb-buds
as early embryos but rapidly lose the buds and
develop into legless adults
• The study of developmental stages of snakes,
combined with fossil evidence of snakes with hind
limbs, supports the hypothesis that snakes evolved
from a limbed ancestor
Vestigial Organ
• Vestigial organs are structures that have apparently
lost most or all of their ancestral function in a given
species
• As the function of the trait is no longer beneficial for
survival, the likelihood that future offspring will inherit
the "normal" form of it decreases
Biogeography
• Biogeography is the study of how species are
scattered across the planet and how this
happened
• Only in the 1960s, as scientists carefully mapped the
ocean floor, were they able to demonstrate the
mechanism that made continental drift possible—
plate tectonics.
Biogeography
• Russel Wallace, a naturalist, began his travels in
1848 through the Amazon and Southeast Asia
• On his journeys, he sought to demonstrate that
evolution did indeed take place, by showing how
geography affected the ranges of species
Biogeography
• In 1915 the German geologist Alfred Wegener was
struck by the fact that identical fossil plants and
animals had been discovered on opposite sides of
the Atlantic
• Since the ocean was too far for them to have
traversed on their own, Wegener proposed that the
continents had once been connected
• Wegener found that the distributions of fossils of
several organisms supported his theory that the
continents were once joined together.