Theory of
Evolution
THEORY OF
EVOLUTION
• The theory of evolution is scientists
explanation of how living things began.
• Also called organic evolution
• LIVING ORGANISMS HAVE CHANGED
GRADUALLY OVER TIME TO FORM NEW
ORGANISMS THAT ARE MORE COMPLEX.
Jean-Baptiste de Lamarck
• Jean-Baptiste de Lamarck (lived before
Pasteur) – French scientist credited with one of
the first theories of evolution
• Believed that organisms change over time from
simple to more complex by passing on
acquired traits to offspring.
• Believed that as environment changed,
organisms changed their behavior to survive.
As they used a body part for survival, it would
improve and be passed down. If they no
longer needed a body part or organ, it would
go away.
Giraffes
Example: Lamarck
believed that giraffes
got their long necks in
order to eat leaves
found in tall trees. They
continued to stretch
their necks and over
time the necks grew
and were passed down
to offspring.
In the Beginning – Some
Say
• The atmosphere of the early earth was very harsh. It
was extremely hot and wet and contained little free
oxygen. It was not suitable for life.
But where did the first
living thing come from?
• Organic evolution is when one living thing comes
from another. But where did the first living thing
come from?
• Alexander Operin and J. B. S. Haldane developed a
hypothesis (Operin-Haldane Hypothesis) that life
started from chemicals in the atmosphere reacting
to form organic compounds. This is chemical
evolution.
• They never tested the hypothesis.
Organic Molecules
• In1953, Stanley Miller and Harold Urey created an
experiment that showed atmospheric gases could
react with one another to form small organic
molecules.
• They placed ammonia, methane, and hydrogen in
a closed container. Then they heated it and
added electricity to simulate lightning on Earth.
They found amino acids and sugars after a few
days.
Next step
• The next step in evolution was for organic molecules
to develop lipid membranes around them.
• These were called protocells
First true cells
• The first cells were anaerobic (without oxygen),
heterotrophic (cannot make their own food)
prokaryotes (simple cell with no nucleus or
organelles) that used chemosynthesis to produce
energy.
• Chemosynthesis – “ate” organic compounds for
food.
• The first cells would have been classified as
bacteria.
Second Cells
• Eventually, these bacteria cells evolved to create
their own food.
• These are called autotrophs – some use
chemosynthesis and others use photosynthesis
(carbon dioxide plus water yields glucose).
o Because photosynthesis releases free oxygen, many
scientists believe that these photosynthetic bacteria
formed the atmosphere we know today.
Oxygen – Not so good
• Free oxygen is very destructive because it reacts
with everything.
• Prokaryotes would have had to develop a way to
protect themselves. Scientists theorize that they
evolved to use the oxygen to produce energy.
Now they would be called aerobic cells.
Prokaryotes to Eukaryotes
• How did cells continue to evolve to become
complex eukaryotes with organelles?
• Anaerobic cell engulfs aerobic cell (eats it)
• The aerobic bacteria that is eaten becomes what is
now a mitochondria (creates energy for the larger
cell) = aerobic eukaryotic cell
Endosymbiotic Theory
• This is a symbiotic relationship – both partners
benefit
• Then an aerobic eukaryote engulfed a
photosynthetic bacterium which turned into a
chloroplast (another symbiotic relationship).
• This formed an autotrophic eukaryote that gave rise
to plant cells.
Endosymbiotic Theory
summarized
• Anaerobic Cell eats Aerobic cell (bacteria)
• Aerobic cell becomes mitochondria and now we
have an aerobic eukaryotic cell
• Some become animal cells
• Others eat a photosynthetic bacteria
• Photosynthetic bacteria becomes a chloroplast.
• Now we have photosynthetic eukaryotic cells that
become plant cells.
Other Theories
• Panspermia – belief that life originated in outer
space
• Organic compounds were found in a meteorite
that landed in Australia in 2008
Fox
• Sidney Fox – Performed experiments that caused
amino acids to form spherical membranes that are
semipermeable – could have formed protocells.
Not Yet
• Scientists have been unable to explain how RNA or
DNA formed as of yet to confirm any type of
chemical evolution theory.
Evidence of Evolution
2
The theory of evolution offers an explanation for the existence of
all living organisms on the Earth today and in the past
It supposes that present day organisms have all been derived
from organisms that lived in the past
By a series of very small changes over millions of years new
species have developed from previous species*
Over a period of about 3000 million years, many new species have
been produced and many have become extinct.
We know a great deal about the organisms that lived millions
of years ago from studying their fossilised remains.
3
Fossil formation
There are many ways in which an organism can be fossilised
One of these ways is shown in the next sequence of slides
In principle, a fossil is formed when an organism dies, its body is
enclosed in mud, or sand. The soft parts decay but some of
the hard parts (skeleton, shells, seeds) are preserved
The mud or sand eventually becomes rock and the hard parts of the
organism are mineralised.
When the rock is exposed as a result of earth movements or erosion,
the fossil remains can be dug out and studied.
The sediment eventually becomes rock
Fish B becomes a fossil much later than fish A
The deeper the rock layer, the older the fossil
Living fish B
Dies
Enclosed in sediment
Hard parts fossilised
Living fish A
Dies
Enclosed in sediment
Hard parts fossilised
4
5
living fish
sediment from river
fish skeleton partly buried by sediment
6
more recent sediment collects
older sediment
becomes rock
fish skeleton fossilised
land raised above
water level
7
recent rock
older rock
8
fossilised skeleton exposed
earth movements
fracture rock
9
When rock strata become exposed, it can be assumed that,
in most cases, the lowest layers are the oldest*
This means that the fossils of organisms preserved in the lowest
layers represent animals and plants that lived many millions
of years ago
rock strata of
increasing age
© Alan
Richardson
10
This is a fossil of a fish which lived 40 million years ago
© Alan Richardson
11
This is a reconstruction, from fossil remains, of an ‘armour-plated’
fish which lived 350 million years ago
dorsal fin
The fish which gave rise to fossils such as this, were very
different from today’s fish
12
By studying the type of fossils and the abundance of each species
in the different strata, it is possible to build up a picture of how the
populations of organisms changed over millions of years
The next slide shows how this picture appears for vertebrate animals
The width of each band represents the relative abundance of
the organisms as judged by their fossil remains.
For example, 100 million years ago there were many more species
of reptiles than there are today
Mammals do not appear at all in rocks older than about
3 million years
13
Relative number of species
mammals
birds
reptiles
100 million
years ago
amphibia
200 million
years ago
fish
300 million
years ago
o
o
o
o
o
o
o
400 million
years ago
o
o
o
o
o
o
o
o
o
o
o
o
o
o
o
o
o
o
o
o
o
o
o
o
o
o
o
o
o
o
o
o
Possible lines of evolution
Interpretation of the fossil record (1)
14
In interpreting the fossil record, it must be borne in mind that:
• fossils will be formed only in places and at times when conditions
are suitable for fossilization. So they may not be representative of
the organisms living at that time 1
• only the skeletons, shells and other parts resistant to decay will
be preserved 2
• fossils are found only if they are exposed by Earth movements or
erosion 3
• very few fossils have been found which show intermediate
stages between different groups of organism 4
Interpretation
15
Interpreting the fossil record (2)
Despite its gaps, the fossil record does show that, over millions
of years:
• organisms became more numerous and more diverse,
i.e. many more species came into existence 1
• organisms became more complex 2
• although very few organisms found as fossils are alive today,
there are many basic similarities in the body structure of the
fossils and similar organisms living today 3
• although they are few, there are fossils which show characteristics
intermediate between major groups4
16
claws
wing-like
forelimbs
teeth
thin
ribs
long tail
feathers
Replica of Archaeopteryx fossil;
half bird half reptile
© Alan Richardson
Reptile-like features
Bird-like features
18
Evidence from comparative anatomy
• Fish, amphibia, reptiles, birds and mammals all have
vertebral columns, skulls, brains, circulatory systems and
many other features in common
• This suggests that they all arose from a common ancestral
form rather than arising independently.
• The same reasoning can be applied to other groups of
organisms such as insects which have most of their anatomical
features in common
• Although these features are superficially different there is
a basic pattern underlying them all
17
Vertebrate limbs
The basic pattern of the vertebrate limb is represented by the
human arm skeleton shown below
ball and
socket joint
single bone
hinge joint
two
bones
group of
small
bones
five jointed
digits
Vertebrate forelimb
19
The forelimbs of the following vertebrates show the basic
pattern of limb bones with modifications which are adapted
to their methods of locomotion.
The basic pattern suggests they have evolved from a
common ancestor
Lizard
Dolphin
Bird
Bat
20
DNA evidence
Another important line of evidence for evolution comes
from DNA analysis
Any permanent change in form or function of an organism
must be preceded by a change in its DNA
Organisms which have much of their DNA in common must be
closely related, i.e. they have split from a common ancestor
comparatively recently (in geological terms)
For example, humans and chimpanzees have 99% of their DNA
in common which suggests a close relationship and relatively
‘recent’ divergence from a common ancestor
21
Evolutionary pathways
The next slide shows the widely accepted pathway of
evolution for plants and animals
Only a few of the main types of organisms are shown
22
Monocots
Herbs Shrubs Trees
Birds
Mammals
Reptiles
Dicots
Amphibia
Fish
Flowering plants
Conifers
Mosses
Insects
Ferns
Crustacea
Annelids
Liverworts
Mollusc
Flatworms
Coelenterates
Algae
Fungi
Multicellular
plants
Multicellular
animals
Single celled
organisms
26
The accepted view of evolution