Evolution
Evolution is a process that affects living things. It is the alteration of life through time.
Evolution – the Janet and John version
Living things have a set of characteristics…
All living organisms possess a set of characteristics, for a human this means, two arms,
two legs, a flat face with nose in the middle, two forward directed eyes and a mouth etc.
There are thousands of these characteristics in an individual, not just external ones, but
others that are not so obvious (e.g. The use of iron in the blood, the production of
enzymes to digest food.)
…………organisms in the same species may differ; this is variation.
Even though all of the organisms belonging to the same species may possess the same set
of characteristics, there will be a range of values for each characteristic within the
species. Think about height, some people will be very tall and others will be very short,
but most people will have a height close to the average. Other characteristics do not have
a continuous spread of values, instead characteristics such as eye colour are discrete; they
can be one thing or another (brown, blue, green). This difference between the
characteristics in individuals is known as variation.
Characteristics are controlled by genes.
The characteristics possessed by an individual are controlled by genes. Genes are lengths
of a molecule called D.N.A., which are found on strands called chromosomes present in
the nuclei of cells.
Genes are inherited from the parents………
During reproduction, the genes from the parents are ‘copied’, ‘mixed’ and passed on to
the offspring, so the young inherit characteristics from the previous generation.
………but there may be changes, called mutations.
Occasionally, during the ‘copying’ process mistakes are made, this is called a genetic
mutation. This results in genes with a different set of instructions. Often this has no
effect on the characteristics but sometimes this is not the case. The genetic mutation may
result in a change in the characteristics.
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Characteristics may give organisms an advantage, this is called natural selection.
Some characteristics will give the individual an advantage in the struggle to survive. For
example a rabbit with long legs may be able to run a little faster than the other rabbits. It
is more likely to survive if the rabbits are chased by a fox, so we say it has been selected
for survival. This is an example of natural selection.
Advantageous characteristics may be inherited by the offspring.
An organism that has survived can pass its genes onto its offspring. There is a good
chance that the offspring will possess some of the advantageous characteristics of the
parent, so they too will have an advantage in the struggle to survive.
New species arise through this process of selection and inheritance.
Over a very long period of time, through generation after generation, many new
characteristics can become common in the population. Eventually, the individuals
become so different from the originals, that a new species has been formed. This is
called speciation.
In this way new species develop from older species and this is evolution.
The pattern of evolution
Darwin thought that evolution was a slow and gradual process of change. He envisaged
that the changes built up slowly in small steps, generation after generation; this
hypothesis is called gradualism. The alternative model is known as punctuated
equilibrium, in this hypothesis it is thought that the changes are sudden and dramatic.
M odels for evolution
Gradualism
Punctuated equilibrium
From our knowledge of genetics we know that both of these models are possible. The
only way to determine which is correct is to look at the evidence – the fossil record. One
particular group of trilobite fossils seems to show that gradualism is the mechanism.
Successive trilobite fossils show a gradual increase in the number of segments. But most
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of the fossil record does not show the same. We see particular organisms present in some
rocks, then younger rocks show startlingly different fossils. Is this punctuated
equilibrium or simply due to the many big gaps in the fossil record?
Major turning points in the history of life
1. Life in the Precambrian.
The Precambrian is the most mysterious time in the history of the Earth. It represents the
time from the formation of the planet 4.6 billion years ago to the start of the Cambrian
600 million years ago. Initially the Earth could not support life; its surface was too
violent and bombarded by harmful radiation from the sun. Eventually the oceans
condensed and provided an environment in which life could begin. The origin of life is a
mystery. There are two main schools of thought. The most popular theory is that life
began from the combination of amino acids found in the early seas. A scientist (Stanley
Miller) has shown that complex chemicals could form from simple building blocks under
the right conditions (electrical discharges, such as in a storm). An alternative theory is
called the Panspermia hypothesis. This suggests that the early Earth was “seeded” by
biological material from space, delivered in the dust from passing comets. For a long
time this idea was not accepted because it sounded rather like science fiction, but one of
the most influential believers in this hypothesis (a Professor of Astronomy in Cardiff)
recently found evidence of bacteria in the upper atmosphere which may have come from
space.
There are a number of problems with finding fossils from the Precambrian. The great age
of the rocks means that there is a very high probability that they will have been destroyed
at some point in the past. The other problem is that the organisms living at this time were
small and soft bodied. The oldest fossils found are around 3.6 – 3.8 million years old,
they belong to the super-kingdom called the Prokaryotes. These are single –celled
organisms with no nucleus including bacteria and blue-green algae. The earliest
prokaryotes would have simply consumed the amino acids in the water around them, later
the blue-green algae evolved, which were capable of photosynthesis.
Fossils from the Precambrian include stromatolites. These are sedimentary rocks that
seem to contain narrow bands, building up to form mounds. We can see modern
stromatolites forming in a place called Shark Bay in Australia. They are formed by a mat
of photosynthetic bacteria, sticking the sediment together with some form of mucus. If
the bacteria become covered by more sediment (such as when the tide deposits sand), the
bacteria work their way up to the surface so they are exposed to the sunlight. A new
layer of stuck sediment forms and the process is repeated many times.
The lack of oxygen in the early atmosphere meant that the bacteria were anaerobic,
gaining energy from chemical reactions that did not require the use of oxygen (e.g.
sulphur reduction). Some of these anaerobic bacteria still live in anoxic environments
(such as deep sediments) today. As time went on the simple prokaryotes evolved into the
more complex eukaryotes. These are organisms that have distinct organelles such as
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nucleus, mitochondria, chloroplasts etc. This evolutionary step may have happened when
one type of prokaryotes incorporated another type into its body. Instead of providing
food, they may have worked together, forming a more complex organism.
The simplest animals are the protozoans. These are single-celled creatures that do not
photosynthesise. Fossils from Zambia suggest that these gave rise to the multicellular
animals such as sponges sponges about 1000 million years ago and by about 700 million
years ago many invertebrates including creatures like jelly fish lived in the sea. A famous
group of fossils from Precambrian rocks are called the Ediacaran fauna. These include a
group called medusoids, rather like jellyfish. One set of these fossils, which are about
700 million years ago, is found in Charnwood Forest in England.
2. Life in the Cambrian – advanced invertebrates, development of hard parts.
The Cambrian period is a very important time in the history of life. The fossil record
shows that between 600 and 500 million years ago, there was a sudden and rapid increase
in the number of species. This is called the Cambrian explosion. One of the most
important developments was the evolution of organisms with the ability to extract
dissolved chemicals from the surrounding seawater and to use them to form hard outer
coverings called exoskeletons. Probably the best examples are the trilobites, a large
group of arthropods with a jointed exoskeleton. They were particularly abundant during
the Cambrian.
One very important group of fossils come from a quarry in Canada containing a rock
called the Burgess Shale. This is an example of
exceptional preservation because the fine-grained
sediment was able to preserve the organisms living
on the seabed in incredible detail. Even soft tissue
was preserved. Many of the organisms present in
the Burgess Shale were unfamiliar to scientists, and
they were so strange that it
is quite difficult to interpret
how they lived. One
particular fossil called
Hallucigenia was actually originally interpreted upside down;
scientist thought that spines on its back were used for walking
and the tentacles waved around in the water. The diagram
shows the correct (?) interpretation for the animal.
3. The evolution of the vertebrates.
One of the animals found as a fossil in Cambrian rocks possesses a flexible cord running
along its back. This animal is the most primitive chordate – the group containing the
vertebrates. Vertebrates are animals with a brain in a skull and a series of bones forming
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the backbone. The backbone provides protection for a spinal cord, which connects the
brain to the other parts of the body.
The first of the five groups of vertebrates to evolve were the fishes. Fish are coldblooded animals that live in water extracting oxygen from the water using gills. The
most primitive fish had no jaws, just a permanently open mouth, some of these fish still
survive today, such as the lamprey, which attaches itself to larger fish by means of the
mouth. Around 400 million years ago, fish with jaws evolved and during the Devonian
there were many species of fish filling the world’s oceans and lakes; one genus, called
Dunkleosteus grew to 9 metres in length and by examining the massive armoured jaws
we can interpret its mode of life as a formidable predator. One group of fish did not
evolve to use a skeleton of bone, instead their skeletons are composed of the softer,
lighter material called cartilage. These are the sharks and rays, a group that has
successfully populated the seas since the Devonian. Most modern fish belong to the
group known as the bony fishes whose fins are supported by bones but another group of
bony fishes had fins that were mainly flesh. They evolved about 390 million years ago
and they are though to be the ancestors of the land vertebrates.
Lungfishes belong to the fleshy finned fishes. They are able to move onto the land and to
gulp air if hot dry weather dries up pools and lakes. About 350 million years ago these
fishes evolved into the first amphibians. The fleshy fins developed into legs and the
gulping breathing technique of their ancestors developed into proper lungs. Some of the
amphibians grew to very large sizes (up to 9 metres) and until the evolution of the
reptiles; they were the dominant land animals.
Ichtyostega – an early amphibian
350 million years ago the amphibians gave rise to the reptiles, the first group of
vertebrates able to live permanently on land. Reptiles evolved to occupy many ecological
niches; their dominance of the Earth reached a peak during the Mesozoic era. The
terrestrial reptiles belong to the group known as the dinosaurs but other very large reptiles
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dominated the oceans (the plesiosaurs and ichthyosaurs) and even conquered the air (the
pterosaurs). Other reptile groups that survive today include the crocodiles, the tortoises
and turtles, the snakes and lizards. The four main groups of reptiles evolved around 270
million years ago.
The Reptiles' Family Tree
Dinosaurs,
crocodiles,
& pterosaurs.
Snakes
& lizards
Mammal-like
reptiles.
Tortoises
& turtles
Sea reptiles.
Ancient reptiles
Amphibians
Many scientists now believe that the dinosaurs were actually warm blooded and a recent
discovery in China suggests that some possessed feathers, probably as a form of
insulation. This is the point when birds are thought to have
evolved from the reptiles. The earliest fossil of a true bird is
Archaeopteryx from the late Jurassic. It is a remarkable fossil
because it shows many of the characteristics of both groups; the
feathered wings of a bird and the bony tail, teeth and clawed
fingers of a reptile. This fossil is another example of
exceptional preservation, found from a quarry of very finegrained limestone in Germany.
The mammal-like reptiles evolved around the start of the
Permian period and some of them developed into massive
carnivores like Dimetrodon. This animal had a huge “sail” of
skin supported by vertical bones, running down its back. This may have been an
adaptation for controlling its body temperature. The first mammals evolved from the
mammal-like reptiles about 190 million years ago. During the Mesozoic, the mammals
were small rodent-like animals but once the dinosaurs became extinct, they evolved to fill
many of the niches previously occupied by the dinosaurs. They are now the dominant
group of vertebrates.
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4. The evolution of land plants.
Plants initially evolved in the Precambrian as tiny unicellular algae, capable of
photosynthesis. They produced oxygen as a waste product and this gas built up in the
atmosphere. Eventually the oxygen built up to become a significant proportion of the
atmosphere. Oxygen molecules (O2) reacted together to form molecules of another gas
called ozone (O3). Ozone high in the atmosphere has the ability to prevent harmful ultraviolet radiation from reaching the Earth’s surface. The presence of an ozone layer was
crucial to the colonisation of the land.
True plants (those organisms with a cell surrounded by a cellulose cell wall) include the
simplest group; the algae. They evolved from blue-green algae about 1000 million years
ago. The first land plants to evolve were mosses, which appeared about 400 million
years ago. Algae are adapted to live in water, obtaining all their needs directly from the
water, mosses live in cushion-like clumps close to the ground so they remain wet, but to
colonise the land, plants must be able to transport materials around the body. Vascular
plants possess a transport system, several groups of vascular plants evolved during the
Palaeozoic including the ferns, horsetails and gymnosperms (seed ferns and conifers).
All of these groups reproduced using spores.
Until 120 million years ago there were no flowers. Flowering plants (the angiosperms)
evolved during the Mesozoic. The flowering plants are the most important land plants
today; they exist in many different environments reproducing by means of pollination.
This involves transferring pollen from one flower to another, this can occur by means of
the wind, or commonly using insects. The evolution of insect pollinated plants and the
evolution of insects is an interesting example of how two entirely different groups evolve
to benefit each other. The plants evolved flowers that were more attractive to insects
(e.g. colours, nectar, scent) and the insects evolved to become more efficient at obtaining
nectar (e.g. the coiled proboscis of a butterfly). Pollen is so small and produced in such
quantities that it is very widely spread by currents in the atmosphere. Pollen is composed
of a very tough material, resistant to destruction, and is one of the most common
microfossils found in sedimentary rocks. They are excellent as environmental and
climatic indicators. During the Tertiary period, the grasses evolved.
ESTA GEOTREX The Geology Teachers Resource Exchange Contributor: Owain Thomas
School Date: 22:04:05
Establishment: Amman Valley
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