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Evolution: The Evidence
Lines of evidence: The science of
• At the heart of
evolutionary theory is
the basic idea that life
has existed for
billions of years and
has changed over
• Overwhelming
evidence supports
this fact.
Lines of evidence: The science
of evolution
• The history of living things is documented
through multiple lines of evidence that
converge to tell the story of life through
Lines of evidence: The science
of evolution
Fossil Evidence
Chemical Similarities
Distribution in time
and space
Fossil Evidence
• The fossil record
provides snapshots of
the past that, when
assembled, illustrate a
panorama of
evolutionary change
over the past four
billion years.
• The picture may be
smudged in places and
may have bits missing,
but fossil evidence
clearly shows that life is
old and has changed
over time.
Fossil Evidence
• Fossils or organisms that
show the intermediate states
between an ancestral form
and that of its descendants
are referred to as transitional
• There are numerous
examples of transitional
forms in the fossil record,
providing an abundance of
evidence for change over
Transitional Forms
• Pakicetus, is described as an
early ancestor to modern
• Although pakicetids were
land mammals, it is clear that
they are related to whales
and dolphins based on a
number of specializations of
the ear, relating to hearing.
• The skull shown here
displays nostrils at the front
of the skull.
Transitional Forms
• A skull of the beluga whale
that roams the seas today has
its nostrils placed at the top of
its skull.
• It would appear from these
two specimens that the
position of the nostril has
changed over time and thus
we would expect to see
intermediate forms
(transitional form).
Transitional Forms
• Note that the nostril
placement in Aetiocetus
is intermediate between
the ancestral form
Pakicetus and the
modern beluga — an
excellent example of a
transitional form in the
fossil record!
Chemical Similarities
• Living things on earth are fundamentally
similar in their basic chemical composition.
Chemical Similarities
• All living things on earth share the ability to
create complex molecules out of carbon
and a few other elements.
• In fact, 99% of the proteins,
carbohydrates, fats, and other molecules
of living things are made from only 6 of the
92 most common elements.
• This is not a mere coincidence.
Chemical Similarities
• All plants and animals receive their
specific characteristics from their parents
by inheriting particular combinations of
• Molecular biologists have discovered that
genes are, in fact, segments of DNA
molecules in our cells.
Chemical Similarities
• These segments of DNA contain
chemically coded recipes for creating
proteins by linking together particular
amino acids in specific sequences.
Chemical Similarities
• All of the tens of thousands of types of
proteins in living things are made of only
20 kinds of amino acids.
• Despite the great diversity of life on our
planet, the simple language of the DNA
code is the same for all living things.
• This is evidence of the fundamental
molecular unity of life.
• Evolutionary theory predicts
that related organisms will
share similarities that are
derived from common
• Similar characteristics due to
relatedness are known as
homologies or homologous
anatomic structures.
• Homologies can be revealed
by comparing the anatomies
of different living things,
looking at cellular similarities
and differences, studying
embryological development,
and studying vestigial
structures within individual
• In the following photos of plants, the leaves
are quite different from the "normal" leaves
we envision.
• Each leaf has a very different
shape and function, yet all are
homologous structures, derived
from a common ancestral form.
• The pitcher plant and Venus'
flytrap use leaves to trap and
digest insects.
• The bright red leaves of the
poinsettia look like flower
• The cactus leaves are modified
into small spines which reduce
water loss and can protect the
cactus from herbivory.
• Another example of
homology is the
forelimb of
(vertebrates with
• Frogs, birds, rabbits and
lizards all have different
forelimbs, reflecting their
different lifestyles.
• But those different forelimbs
all share the same set of
bones - the humerus, the
radius, and the ulna.
• These are the same bones
seen in fossils of the extinct
transitional animal,
Eusthenopteron, which
demonstrates their common
Homologies: Embryonic
• The embryonic
development of all
vertebrates shows
similarities as you
can see from these
Gill Slits
Distribution in time and space
• Understanding the history of life on
Earth requires a grasp of the depth
of time and breadth of space.
• We must keep in mind that the time
involved is vast compared to a
human lifetime and the space
necessary for this to occur includes
all the water and land surfaces of the
• Establishing chronologies, both
relative and absolute, and
geographic change over time are
essential for viewing the motion
picture that is the history of life on
Distribution in time and space
• The age of the Earth and its
inhabitants has been
determined through two
complementary lines of
• relative dating
• numerical (or radiometric)
Relative Dating
• Relative dating places fossils in a
temporal sequence by noting their
positions in layers of rocks, known
as strata.
• As shown in the diagram, fossils
found in lower strata were typically
deposited first and are deemed to be
older (this principle is known as
• Sometimes this method doesn't
work, either because the layers
weren't deposited horizontally to
begin with, or because they have
been overturned.
Relative Dating
• By studying and
comparing strata
from all over the
world we can learn
which came first
and which came
next, but we need
further evidence to
ascertain the
specific, or
numerical, ages of
Numerical Dating
• Numerical dating relies on
the decay of radioactive
elements, such as
uranium, potassium,
rubidium and carbon.
• Very old rocks must be
dated using volcanic
• By dating volcanic ash
layers both above and
below a fossil-bearing
layer, as shown in the
diagram, you can
determine "older than X,
but younger than Y" dates
for the fossils.
Numerical Dating
• Sedimentary rocks less
than 50,000 years old can
be dated as well, using
their radioactive carbon
• Geologists have
assembled a geological
time scale on the basis of
numerical dating of rocks
from around the world.
• The distribution of
living things on the
globe provides
information about the
past histories of both
living things and the
surface of the Earth.
• This evidence is
consistent not just with
the evolution of life,
but also with the
movement of
continental plates
around the worldotherwise known as
plate tectonics.
• Marsupial mammals are
found in the Americas as well
as Australia and New Guinea,
shown in brown on the map
at right.
• They are not found swimming
across the Pacific Ocean, nor
have they been discovered
wandering the Asian
• There appear to be no routes
of migration between the two
• How could marsupials have
gotten from their place of
origin to locations half a world
• Fossils of marsupials have
been found in the Antarctic
as well as in South America
and Australia.
• During the past few
decades scientists have
demonstrated that what is
now called South America
was part of a large land
mass called Gondwana,
which included Australia
and Antarctica.
• Gondwana split apart 160 to
90 million years ago.
• Marsupials didn't need a
migration route from one
part of the world to another;
they rode the continents to
their present positions.