© Alan Richardson

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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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 theorganism are mineralised.

When the rock is exposed as a result of earth movements or erosion,the fossil remains can be dug out and studied.

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Fossil formation

Living fish A

Dies

Enclosed in sediment

Hard parts fossilised

Living fish B

Dies

Enclosed in sediment

Hard parts fossilised

Fish B becomes a fossil much later than fish A

The sediment eventually becomes rock

The deeper the rock layer, the older the fossil

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Process of fossilisation

living fish sediment from river

fish skeleton partly buried by sediment

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fish skeleton fossilisedolder sedimentbecomes rock

more recent sediment collects6

Layers of sediment

land raised abovewater level

recent rock

older rock

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earth movements fracture rock

fossilised skeleton exposed

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Fractured rock

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 lowestlayers represent animals and plants that lived many millionsof years ago

rock strata of increasing age

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Rock strata

© Alan Richardson

© Alan Richardson

This is a fossil of a fish which lived 40 million years ago10

Fossil fish

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 speciesin the different strata, it is possible to build up a picture of how thepopulations 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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Vertebrate sequence

Relative number of species

400 millionyears ago

fish

amphibi

a

reptiles

birds

mam

mal

s

100 millionyears ago

200 millionyears ago

300 millionyears ago

o

o

o

o

o

o

o

oo

o

o

o

o

o

oo

o

o

o

o

oo

o

oo

oo

o

oo

o

o

o

o

o

o

o

o

o

Possible lines of evolution

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Vertebrate fossil record

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 conditionsare suitable for fossilization. So they may not be representative ofthe 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 orerosion 3

very few fossils have been found which show intermediate stages between different groups of organism 4

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Interpretation (1)

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 thefossils and similar organisms living today 3

although they are few, there are fossils which show characteristics intermediate between major groups4

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feathers

claws

thinribs

teeth

long tail

wing-likeforelimbs

Reptile-like features

Bird-like features

Replica of Archaeopteryx fossil; half bird half reptile © Alan Richardson

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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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Comparative anatomy

Vertebrate limbs

The basic pattern of the vertebrate limb is represented by the human arm skeleton shown below

ball and socket joint

hinge joint

single bone two bones

group of small bones

five jointed digits

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Vertebrate forelimb

The forelimbs of the following vertebrates show the basic pattern of limb bones with modifications which are adapted to their methods of locomotion.

Lizard Bird

Dolphin Bat

The basic pattern suggests they have evolved from a common ancestor

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Modified limbs

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

DNA evidence20

DNA evidence

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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Evolutionary pathways

Mammals

Flatworms

Annelids

Insects

Coelenterates

Fish

Birds

Reptiles

Crustacea

Amphibia

Single celled organisms

Monocots Herbs Shrubs Trees

Dicots

Flowering plants Conifers

Mosses

Liverworts

Algae

Ferns

Fungi

Multicellular plants

Multicellularanimals

Mollusc

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Evolutionary pathways 2

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 and Mick

Dave’s view of evolution

Dave is wrong

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Incorrect evolutionary sequence

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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Discussion

The accepted view of evolution

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‘Correct’ evolutionary sequence

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

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