© Alan Richardson
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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.
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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
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5
living fish
sediment from river
fish skeleton partly buried by sediment
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more recent sediment collects
older sediment
becomes rock
fish skeleton fossilised
land raised above
water level
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recent rock
older rock
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fossilised skeleton exposed
earth movements
fracture rock
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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
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This is a fossil of a fish which lived 40 million years ago
© Alan Richardson
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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
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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
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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)
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
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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
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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
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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
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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
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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
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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
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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
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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
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Dave. ‘A bloke on the telly just said that before we
were humans we were monkeys’.
Mick. ‘What were we before that?’
Dave. ‘ I don’t know. I can’t even remember being a
monkey’
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Dave is wrong
Dave’s view of evolution
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Dave seems to think that evolution can take place in a
person’s lifetime. This is pretty bizarre, even for Dave.
He has also confused monkeys with apes. Our nearest
relatives are apes (chimpanzees and gorillas), not monkeys
He makes the common error that, e.g. fish, as we know
them today gave rise to amphibia and, by the same
reasoning, apes, as we know them, gave rise to humans
The correct interpretation of the evidence is that apes and
humans had a common ancestor which was neither ape nor
human
In the course of evolution, this common ancestor produced
two lines of descendants, culminating in modern day humans
and modern day apes
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The accepted view of evolution
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Question 1
Evolution has taken place over
(a) hundreds
(b) thousands
(c) millions
(d) billions
of years
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Question 2
Fossils can be formed only if total decay
(a) is prevented
(b) is accelerated
(c) is slowed up
(d) is immediate
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Question 3
Older fossils are found
(a) in the deepest rock layers
(b) in the shallowest rock layers
(c) only in rocks over 1 million years old
(d) evenly dispersed in all rock layers
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Question 4
Which of these statements is correct ?
Fossilised organisms
(a) are all totally different from present-day species
(b) are much the same as present-day species
(c) often show strong resemblances to present-day species
(d) are never related to other fossilised organisms
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Question 5
The order of appearance of vertebrates in the fossil
record is
(a) Birds and mammals; reptiles; amphibians; fish
(b) Fish; amphibians; reptiles; birds and mammals
(c) Fish; reptiles; amphibians; birds and mammals
(d) Amphibians; fish; reptiles; birds and mammals
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Question 6
Reptiles were most abundant
(a) 400 million years ago
(b) 300 million years ago
(c) 200 million years ago
(d) 100 million years ago
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Question 7
Archaeopteryx is a fossil which shows features
intermediate between
(a) reptiles and mammals
(b) reptiles and birds
(c) birds and mammals
(d) reptiles and amphibians
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Question 8
Which statement is correct ?
(a) Amphibia are derived from fish
(b) Amphibia evolved into reptiles
(c) Amphibia and fish share a common ancestor
(d) Reptiles evolved into mammals
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Answer
Correct
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Answer
Incorrect