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Environments Through Time
1. Evidence from early earth indicates the first life forms survived changing habitats during the
Archaean and Proterozoic eons.
Identify that geological time is divided into eons on the basis of fossil evidence of different life forms
 A relative timescale assembled from sedimentary sequences was divided according to major changes
in the types of rocks present and the appearance/disappearance of fossils from layer to layer.
 Some boundaries in the timetable correspond to the apparently sudden extinction of a significant
amount of the earths species (Cretaceous-Tertiary = dinosaurs; Permian-Triassic = mass extinction)
 The term Hadean (not universal) refers to that period of time for which there is no rock record (therefore
no fossil evidence) which began with the origin of the earth 4.6 billion years ago
 The Archaean eon has the oldest known rocks and signifies the start of cyanobacteria
 The beginning of the Proterozoic was originally the oldest discernable fossil stromatolites although
earlier life has now been found
 Proterozoic/Phanerozoic boundary roughly marks rapid diversification and complexification in the
types of life forms (i.e. hard exo/skeletons appear = more fossils as soft bodied organisms do not fossilise easily )
Define cyanobacteria as simple photosynthetic organisms and examine the evidence of cyanobacteria in Aus
 Cyanobacteria are bacteria that possess a blue pigment (that gives them their name)
 Some of these bacteria contain chlorophyll and so can photosynthesis and produce their own food
 The oldest definite evidence of fossil life is to be found in Australia in the fossilised stromatolites in
Western Australia near a town called North Pole in an area known as the Warrawoona Series
 The series = 3.5 billion years old and (amongst other rocks) contain some relatively undeformed
sedimentary rocks. Such rocks suggest they formed in a warm dry tropical volcanic environment.
 The rocks contain stromatolites which are low mounds or domes of finely laminated sediments.
 They are usually rich in carbonate but cherty layers can replace the carbonate
 These fossils can be correctly identified as there are still stromatolites forming in Shark Bay, WA
 They are fossilised microbial mats formed by photosynthetic cyanobacteria thus these stromatolites
provide the oldest, definite evidence for life on earth
 Stromatolites = Proterozoic (Archaean not suitable and after Proterozoic, life diversifies = predators)
 Formation occurs when cyanobacteria attaches itself to some sediment. As they live in tidal areas,
sediment builds up on top of the cyanobacteria killing it forming a layer of sediment, a layer of dead
cyanobacteria and then another lay of sediment. The cyanobacterium ( whilst being sedimented on) grows
through the sediment to reach the sunlight (photosynthesis) thus forming a new layer of bacteria on top.
 Stromatolites grow very, very slowly (1mm per year)
 4 billion years ago these organisms were so common and populated that they changed the atmosphere
of the world by adding oxygen as a by product of their photosynthesis
 Today stromatolites inhabit inhospitable areas where few other marine animals can survive therefore
they have little interference from predators i.e. hyper saline areas and super tidal regions
Outline the processes and environmental conditions involved in the deposition of BIFs
 Among the oldest known rocks on earth and accumulated between 3.5 – 1.9 billion years ago
 The term BIF refers to a bedding configuration in which layers of chert alternates with layers of other
minerals that are richer or poorer in iron than the chert.
 The formation of BIFs is directly linked to the rise of oxygen in the atmosphere (from cyanobacteria)
 O dissolves in water therefore O content of atmosphere is directly linked to the O content of the sea
 When oxygen concentrations in the atmosphere rise, more oxygen dissolves in sea water (and vice versa)
 In an oxygen free environment, iron also dissolves in water however when the level of oxygen rises, it
reacts with the iron dissolved in the water and causes it to precipitate rapidly and settle on the sea floor
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There are no more BIFs forming now because all the iron is out of the water
BIFs = Low oxygen; No BIFs = High oxygen
Process:
1. Iron is dissolved from rocks into sea water
2. Cyanobacteria produce O as a waste product of photosynthesis
3. O concentration in atmosphere and oceans increases
4. O concentration in sea reaches level where reacts with dissolved iron to form precipitate (iron oxide)
5. This falls to the sea floor to form a layer of iron rich sediment
6. O is now depleted and iron poor sediments are now deposited on the sea floor
7. O levels rise again to threshold where it reacts with dissolved iron
8. Precipitation of iron oxide as a layer of iron rich sediments on sea floor
9. cycle repeats itself until all dissolved iron in the ocean is used up
10. O now accumulates in large quantities in the atmosphere and oceans
Examine and explain processes involved in fossil formation and the range of fossil types
 A dead organism is usually eaten by scavengers or attacked by bacteria/fungi which break down the
soft tissue. Chemical processes such as oxidation act on any remaining tissue and alter its composition.
 Organisms that possess hard parts i.e. bones or shells will resist destruction and alteration for longer
periods of time (eventually they’ll be broken down by exposure to air, water, scavengers and chemical activity )
 In very rare occasions fossilisation can occur in naturally suitable locations for example peat bogs
(swamps with very little oxygen); tar and asphalt pits (very little oxygen); amber (tree sap stops bacteria etc); or
arctic and tundra regions (ice freezes organisms)
 Otherwise, for remains to be preserved and fossilised the agents of decay must be prevented
 Decay is prevented be rapid burial which stops scavengers/bacteria accessing the organism.
Oxidisation must be prevented so that only the soft parts decay and the organism must not be disturbed
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A fossil is the remains or trace of once living organisms – the actual remains of that organism. An
imprint or outline of the organism can be seen
A trace fossil is the remains of disruption of sediment by the normal activity of organisms i.e. foot
prints, feeding leftovers, chemical traces, burrows and faeces – an imprint of what the organism did. It
requires rapid burial so as not to be disturbed by erosion.
Carbonation takes place after rapid burial in conditions where limited air is present which takes out the
nitrogen/hydrogen/oxygen leaving the carbon film enclosing the organism. Carbonised leaf:
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Petrification occurs where the original material is replaced/impregnated by substances such as calcite,
silica or iron minerals. For example porous shells, skeletons and trees. Petrified wood:
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Impressions are moulds made when sediment is pressed around a shell or bone and produce internal
and external impressions. The original material may be dissolved away. Cast of a shell:
Outline stable isotope evidence for the first presence of life in 3.8 billion year old rocks
 Rockers older than 3.5 billion years old are rare (4 billion years ago = Hadean and too hot for rocks)
 90% of the original crust was oceanic and therefore destroyed in convergence and many rocks have
been eroded out of their original igneous rock and deposited as fragments in younger rocks.
 Oldest known rocks = the Isua rocks of West Greenland (dated to about 3.8 billion years old)
 The rocks include sedimentary formations (a.k.a. water was present) and although they’ve been repeatedly
folded, faulted and reheated they can still tell us about the conditions of early earth when they formed.
 They were laid down in shallow, near shore water and included beach rounded pebbles and weathering
products from volcanic lava and temps would have been warm but not extraordinary.
 Contain a lot of carbon in the form of the mineral graphite and so produce indirect evidence of life
 When carbon is taken up by cells (in photosynthesis) the lighter isotope carbon-12 is slightly enriched
over the heavier isotope carbon-13. That is to say, once carbon dioxide is used in photosynthesis, there
will be slightly more C-12 than C-13 in the cells of the plant.
 Carbon in the Isua rocks is slightly enriched in C-12, enough to convince many scientists that it once
went through photosynthesis therefore life must have been present to produce such a skew in the ratio.
2. The environment of the Phanerozoic eon
Outline the chemical relationship between ozone and oxygen
 Oxygen that forms 21% of the atmosphere is produced as a result of the photosynthesis
 When oxygen molecules (O2) in the stratosphere absorb UV rays from the sun the energy carried in the
UV rays can sometimes break the oxygen molecule release free oxygen atoms (O)
 These free oxygen atoms (O) then recombine with the O2 to form Ozone (O3)
 O3 not particularly stable and if hit by more UV energy it may decompose again to from O and O2
 This is in equilibrium naturally i.e. as much O3 is being made that is being destroyed and there is a
constant amount of ozone in the atmosphere
 The reactions in which O and O3 molecules are being broken apart absorb UV radiation and prevent it
reaching the ground and the energy of the radiation is converted into heat and warms the stratosphere
 UV energy would other wise reach the earth could affect organism i.e. causing sunburn, skin cancer,
cataracts, inhibition of growth in plants etc
 Ozone in the upper atmosphere protects life on earth by absorbing much of this high energy radiation
Explain the relationship between changes in oxygen concentrations and the development of the ozone layer
 In order for O3 to form it was necessary to have a large amount of oxygen existing in the atmosphere
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Therefore the level of oxygen gas (O2) had to build up in the atmosphere to a point where ozone could
start to form – this took a very long time
Until about 600 million years ago (where oxygen reached about 2.1% in atmosphere) the layer of ozone
around the earth was not chemically able to shield the earth from the biologically lethal UV radiation
i.e. the ozone layer could not yet protect the earth and UV was still too dangerous
I.e. oxygen concentration of atmosphere had to increase to a certain level before ozone could begin to
form and as oxygen levels in atmosphere change, level of ozone in atmosphere changes accordingly.
(oxygen concentration and O3 = directly linked)
Describe the role of ozone in filtering UV radiation and the importance of this for life that developed during
the Phanerozoic eon
 Until the level of O3 in the atmosphere had built up enough to screen out harmful UV energy life was
confined to the oceans (water also acts as protection from UV light)
 By 2 billion years ago the atmosphere contained enough oxygen to allow for the formation of the
ozone layer and such protection permitted life to exist near ocean surfaces and ultimately on land
 This marked the beginning of complex life (hard parts) and the beginning of the Phanerozoic eon
 About 400 million years ago land plants evolved and diversified therefore atmospheric oxygen levels
soared and by 200 million years ago the concentration reached about the value it has today
3. The Cambrian event
Interpret the relative age of a fossil form a stratigraphic sequence
 Most fossils are found within sedimentary rocks and as the rocks cannot be dated (sedimentary
therefore formed from lots of other rocks) an order of events is established
 The principle of original horizontality states that due to gravity, sediment is laid down horizontally.
Therefore younger layers are deposited on top of older layers.
 The principle of superposition states that layers on the bottom of a sequence are older than those above
it (provided it remains undisturbed)
 The principle of cross cutting relationships states that a rock must first exist before anything can
happen to it. (I.e. if an dyke cuts across layers of sandstone, then the dyke must be younger than the sandstone because
sandstone had to exist for the dyke to cut across it or if folding is present, folding occurred after the rock was deposited )
 Using these principles, it can be said that if a fossil at the base of an undisturbed sequence of rocks is
older than any which lie above it in sequence and vice versa
Distinguish between relative and absolute dating
 Relative age refers only to the order in which events occurred (i.e. order that a series of rock was deposited)
 Determination of relative age is based on the principles of original horizontality, superposition (most
important) and cross cutting relationships.
 Age can be determined as long as the sequence is undisturbed and has not been turned upside down by
tectonic processes
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Absolute age is the determination of the age of a rock in years or in millions/billions of years
It involves working out when the rock was formed usually by radiometric dating which involves (uses
the ratio of amounts of parent isotopes to daughter isotopes)
Sedimentary rocks cannot be radiometrically dated as they are made up of many different grains of
other rocks
Two important features of radiometric dating are:
1. The half life of a radioactive isotope is constant therefore the decay of the parent isotope is
occurring at a constant rate and its daughter isotope is accumulating in the rock. The longer the
rock exists, the more daughter isotope there will be.
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2. The half life of the isotope used has to be appropriate giving enough of both the daughter and
parent isotope so that each can be measured accurately and the relationship between them assessed.
Compare uses of relative and absolute dating methods in determining sequence in the evolution of life forms
 Relative dating can give the sequence of events but as radiometric dating techniques develop more
precise measurements of ages of rocks can be made
 The study and comparison of exposed rock layers or strata in various parts of the earth led scientists in
the early 19th century to propose that the rock layers could be correlated from place to place
 Locally, physical characteristics of rocks can be compared and correlated
 Globally, fossil evidence can help in correlating rock layers
 The principle of superposition is used to relatively date fossils in layers and therefore match layers (i.e.
fossil in Aus known to live between x and y years – same fossil found in UK so age of rock = between x and y years old )
 By correlating fossils from various parts of the world scientists are able to give relative ages to strata
 Fossils found in deepest layer of rocks in area represent the oldest life form in that particular formation
 If certain fossils are typically found only in a particular rock unit and are found many places
worldwide they may be useful as index or guide fossils in determining the age of undated strata
 By using this information from rock formations in various parts of the world and correlating the
studies scientists have been able to establish the geological time scale (it is a relative time scale)
Discuss the possible importance of the development of hardened body parts in explaining the apparent
explosion of life in the Cambrian period
 530 million years ago
 One of the most important events in the history of life was the evolution of mineralised hard parts and
this evolution defines the beginning of the Phanerozoic eon and well as the beginning of the
Palaeozoic era and the Cambrian period
 Beginning rather suddenly at the start of the Cambrian – the fossil record contains skeletons, shells,
and other pieces of mineral which were formed biochemically by animals (the evolution of hard parts means
the fossil record becomes richer as hard parts resist the destructive agents that affect soft parts of bodies )
 For a long time it was believed that life did not exists until the Cambrian as the fossil record was so
limited and this apparent burst of life is known as the Cambrian explosion
 A careful study of the rocks i.e. the Burgess Shales in Canada that date from this period, has revealed
large numbers of soft body imprints indicating a huge diversification in the types and numbers of
multicellular animals living in the seas
 Some possible explanations as to why this diversification occurred are:
- By 600 million years ago, oxygen levels were close to that of present day and this
allowed for the production of more energy for the complex life forms
- With the huge abundance that occurred in the number of eukaryotic cells in the
oceans, sexual reproduction became an alternative to asexual reproduction. Sexual
reproduction causes variations in offspring therefore evolution begins and with in the
emergence of new species thus number of species in oceans increased rapidly (Asexual
reproduction ensures continuation of the line from just one parent and does not allow for evolution or
diversification as each individual is genetically identical to its parent)
-
Initially there would have been little competition for resources and therefore a lack of
predators. When populations increased so did competition for food and space
therefore predators evolve (predators drive natural selection as only the fittest survive)
Deduce possible advantages that hard shells and armouring would have given these life forms in comparison
with the soft bodied Ediacara metazoans of the late Proterozoic in terms of predation, protection and defence
 Metazoans – multicellular animals that have their multi cells organised into tissues and organs
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In South Australia about 600km N of Adelaide in the Flinders Range is the Ediacara Gorge
Area holds traces of soft bodied animals that are much advanced over previous life forms (stromatolites)
This set of animals is called Ediacaran fauna and so far they give the best picture of late Precambrian
life – among them are forms similar to jellyfish, worms and sponges but also some like nothing else
None of the Ediacaran fauna have fossilised hard parts as they would have been broken up by water
currents however imprints of some are preserved
There were no carnivorous eaters so soft bodies did not get eaten after death
Until the Cambrian explosion animals did not need protection against one another as there was no
competition for space and food therefore no predators and no advantages for animals with shells and
armouring and they do not appear in the fossil record prior to the Phanerozoic
With the Cambrian explosion there is a burst in the number of animals with skeletons (internal and
external) and a burst in the use of amour plating and hard external shells.
One of the theories for this is the introduction of a predator/prey ecosystem – because the seas became
more heavily stocked and competition for food and space became much greater
Animals that had the developed hard parts had an advantage
Due to new predators, prey may have evolved large size, powerful toxins or changes in lifestyle and
behaviour in order to become more predator proof.
As new predators in turn evolved more sophisticated ways of attacking prey so the burst of
evolutionary change would have flourished
The advantages of being a Post Cambrian, hard part organism:
Feature
Advantage
PROTECTION
- exoskeleton
- protected against sand/gravel
(against the environment)
- shells
that may get washed onto it
- protected against desiccation
(drying out) i.e. a jellyfish
would dry out if caught in the
sun however a crab would
always have a moist body
DEFENCE
- exoskeleton
- exoskeleton/shell are hard for
(against predators)
- shell
predators to pierce/break into
- skeleton
- skeletons allow for muscle
- spine
attachment therefore fast
movement to get away
- spins deter predators (too hard
to get to the flesh therefore
predators give up)
PREDATION
(help in hunting)
-
claws
pincers/nippers
teeth/jaws
skeleton
-
catching grasping prey
efficiently and effectively
allows for larger prey to be
eaten (large prey can be caught
and chewed up – not just one bite)
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skeleton allows for muscle
attachment = fast movement
Cambrian Explosion overview:
- heaps more species, heaps more diversity, heaps more animals
- Precambrian = soft bodies (no fossils); Post Cambrian = hard parts (lots of fossils)
- May have been heaps of species and huge diversity in Precambrian animals but they
didn’t leave traces of themselves so that information remains unknown
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Explosion due to respiration (= oxygen to use to build bigger bodies); sexual reproduction
(resulting in more mutations and therefore more species); environment changes (= new organisms
that suit it better than the old ones therefore become more abundant )
4. Exploiting new environments
Outline the theory of evolution by natural selection
 Evolutionary theory states that all organisms developed from common ancestors/pre-existing
organisms – i.e. all organisms have a common origin
 All living things are fundamentally similar because their basic chemistry has been inherited from this
common ancestor/pre-existing organism
 The Theory of Evolution (by Charles Darwin) refuted earlier ideas of Lamarck (giraffes necks) and stated
that natural selection is the process in which the environment selects favourable characteristics and
eliminates harmful ones. After many generations of selection, characteristics of a population may
change due to the environment selecting those adaptations (NB NOT adaption – use adaptation) that provide
the best chance of survival
 This theory has four main points:
1. In any population there are naturally occurring variations; all the members of one species
are not identical. Members of a species may be born with a variation that makes them more
suited to living in the particular environment in which they dwell
2. In any generation there are offspring that do not reach maturity and reproduce; the
characteristics of these organisms are eliminated from the population
3. Those organisms that survive and reproduced are well adapted to that environment; they
have favourable characteristics/variations
4. favourable characteristics/variations are passed onto offspring; they become more and more
common in the population
 Thus nature is effectively selecting certain characteristics for survival – known as Natural Selection
(survival of the fittest – fittest meaning the most fit for the conditions)
 An example is the peppered moth:
Outline evidence that present day organisms have developed from different organisms in the distant past
 Direct evidence:
 Fossils are the only direct evidence that evolution has taken place
 By putting fossils into a relative time order, palaeontologists can see the changes that have happened
to a particular species as a result of evolution
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Transitional fossils are fossils that show characteristics of two major groups i.e. the Archaeopteryx
(bird-like reptile) and the Lobe-fin Fish (amphibian-like fish)
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Indirect evidence:
Based on the study of living plants and animals
If the theory of evolution is used, it explains the distribution of living things, their comparative
anatomy, how they grow from embryos and the chemicals they contain
Distribution
- There are very few animals that occur naturally in all countries and in most cases
flora and fauna are unique to their area even though climate and other conditions may
be similar.
- This is explained by evolution and the theory of Continental Drift – separation of
landmasses isolated groups of animals/plants on climates where they evolved with
different environmental pressures
Comparative anatomy
- It has long been noted that the limbs of many vertebrates have a common basic
structure called the Pentadactyl Limb (5 fingers)
- the front limbs of whales, horses, bats, crocodiles, birds and humans all have the same
basic bone structure but with size, shape and position depending on the limbs function
(swimming, flying, walking, digging) i.e. a common ancestor but adaptation to environment
Comparative embryology
- All vertebrates start life as a fertilised ovum and the early stages of growth in the
embryo look very similar and it is very difficult to identify each individual organism
in the embryo.
- Evolution explains these similarities by a common ancestor – as vertebrates evolved,
the latter stages were modified by evolution to produce final forms but a similar gene
still controls early development
Chemicals in living things
- Tests show that blood of humans and chimpanzees are chemically very similar but
both quite different from a dog. Evolution suggests they all evolved from a common
ancestor but the dog’s evolutionary path separated earlier.
- Although many chemicals show differences, others are almost the same for most
living things.
- They all consist primarily of organic compounds (carbon, proteins, sugars); share a
common genetic code of DNA/RNA; rely on enzymes to control chemical reactions;
rely on respiration to make energy available for cellular processes
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Summarise the main evolutionary changes resulting from the selection of living things exhibiting features that
allowed them to survive in terrestrial environment
 Plants, invertebrates and finally vertebrates evolved to live on land in the middle of the Palaeozoic era
 There were major problems in doing so in relation to exposure to air for example:
- Air provides little buoyancy so support is needed (stronger muscles, bones, limbs developed)
- There is a great danger of desiccation as water is harder to obtain
- There are greater variations in temperature (daily and seasonally)
- Gases (needed for respiration and photosynthesis) behave differently in air to the way they
behave in water and therefore have to be obtained in a different way (not gills - lungs)
- No nutrients in the air like there is in the sea (new food source must be found)
- New methods of reproduction are required (internal fertilisation required)
- Light travels differently though air compared to water (eyes need to change)
- Sound travels differently though air compared to water ( hearing needs to change)
- A more chambered heart needs to develop to transport blood more efficiently
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Outline the major steps in the expansion to the terrestrial environments by plants, amphibians and reptiles
 Plants
 First plants to move on land were mosses and liverworts (closely related to algae, small flat moss like plants
with no roots, xylem, phloem with simple leaves and stem and still required moist environments ) during the Silurian
 The first fossils of land plants occur in the mid Devonian, although there are fossils of spores that
probably came from land plants, in rocks of late Ordovician
 Almost all the major characteristics of land plants are solutions to the problems associated with air
 Air has little buoyancy so a support system that enables vertical standing needed i.e. stems, trunks, as
well as turgidity (amount of water filled in the cell i.e. beanbag – full beanbag is more sturdy) of a plant is used
 Land plants cannot afford evaporation from moist surfaces they have developed a kind of water
proofing called a cuticle - a waxy layer that protects the plant from either too much or too little water.
 The development of the cuticle prevented water and nutrients from entering the plant over the whole
surface so roots gradually developed
 In order to gather gases necessary for photosynthesis and respiration, stomata evolved so CO2 and O2
could be obtained/released – if too hot/dry stomata closed off by guard cells to control loss of water
 Plants developed broad leaves to gather the maximum amount of light to allow photosynthesis
 The xylem developed to transport water and minerals from roots to throughout the plant; the phloem
developed to transport the products of photosynthesis from the leaves to the rest of the plant. (ph = f)
 Mechanism for reproduction in air was also required - spores were first method used (i.e. ferns) spores
are airtight and allow for efficient reproduction (however fertilisation of gametes in ferns still requires water)
 The first fossils of macroscopic land plants are best found in Ireland and are about 425 myo
 The best known is Cooksonia which:
- was bifurcated a few times and topped with small spores
- had no leaves, flowers or seeds
- probably had roots that were horizontally growing stems connected to the soil
- has xylem vessels recorded in fossils
- became extinct in the early Devonian
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Insects – first appeared in the Devonian and become more common by the mid-Carboniferous
As plants extended their habitats they would have provided a food base for animal life evolving i.e.
there was a perfect environment ready to be explored with much food and no predators
Marine animals best pre-adapted (i.e. with adaptations suited to land) were the arthropods (jointed limbs and an
exoskeleton i.e. insects, millipedes, crustaceans, arachnids)
Already had an almost water proof cover and were very strong for their size with sturdy walking legs
They lived on and in plant debris which provided food and shelter
Their hard exoskeleton saved them from becoming desiccated and tiny holes in the exoskeleton
allowed them to take in oxygen without needing a specific organ for breathing
Original surface dwelling arthropods were prey for larger animals and so were major part of ecosystem
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Lungfish
A group of fishes which lived solely in the water had developed fleshy lobed-fins and a pair of
openings in the roof of the mouth that led to clearly visible external nostrils
Such fish were able to rise to the surface and take in air which was passed onto internal sacs that
functioned as lungs (this combination of gills and lungs does not occur now but was not uncommon in the Palaeozoic )
The lobefins are sturdy and contain a series of strong bones which were arranged much like a
quadraped – this may have allowed the animal to leave a body of water that was drying out or
stagnating and move to another body of water that offered a better chance of survival
The first invertebrates to venture on land are descended from the lungfish Crossopterygii and moved in
the middle to late Devonian - tens of millions of generations of Crossopterygii were required to evolve
into animals that could live comfortably on land
Amphibians (frogs, salamanders) – fossil records begin in the late Devonian
A three chambered heart developed to route the blood more efficiently to and from the lungs
The limb and girdle (shoulders, hips) bones were modified to become stronger to overcome the constant
drag of gravity and better support the body in air
The spinal column was transformed into a sturdy but flexible bridge of interlocking elements
Improved hearing took place
Amphibians have continued to return to the water to lay their fishlike eggs (tadpoles gills for respiration)
Identify the advantages the terrestrial environment offered the first land plants and animals
 After overcoming the difficulties of a new environment the first land dwellers had many advantages
over their ancestors in the marine environment
 INITIALLY the terrestrial environment had:
- Unlimited space, very few predators, unlimited food source
- A variety of climates into which plants could move with little competition
- Gases and light necessary were more readily available than in water
5. Past extinction and mass extinction events
Compare models of explosive and gradual adaptations and radiations of new genera and species following
mass extinction events
 It is evident from fossil records that evolution has occurred gradually and quickly (geologically speaking)
 Gradual evolution (phyletic gradualism) – relatively slow adaptation and evolution of an organism
 In gradual evolution changes occur by slow degrees and it takes many millions of years i.e.
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Punctuated evolution (punctuated equilibrium) – rapid evolution where organisms evolve rapidly from
others after long episodes of little evolutionary progress
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Evolutionary radiations – groups of plants of animals undergo remarkably rapid evolutionary
expansion, that is, one or more phyla, classes, order or families have produce many new genera or
species during brief intervals of time. (radiation refers to the pattern of expansion from ancestral organisms to a
range of new organisms which have many features similar to their ancestor but are specially adapted to their environment )
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Evolutionary radiation often occurs in groups within just a few million years of their origin
Since the old and new groups occupy different niches ecological competition does not restrain the
diversification of the new group
When a new group has formed there may also not be predators which have developed efficient
predation methods. Until they do the new group is free to form new species in a short period of time
Initially early rates of rapid expansion lead to a range of new species descended from the original
ancestor and after this rates of evolution slow dramatically indicating as groups of organisms
expanded and quickly exploited any adaptations that its body plan allows it to develop.
Later evolution is restricted to the development of variations on the basic adaptive themes that evolved
earlier on. In time, new families evolve and eventually only new genera or species evolve
Extinctions and evolutionary radiation are often linked – if rate of extinction is low then it is likely that
rate of evolutionary radiation is low (and vice versa) due to the emptiness/fullness of an environment
Emptiness of an environment means there is a lack of competition for space/food and few predators
Fullness of an environment indicates the environment will not be bale to sustain more new species
Distinguish between mass extinctions and smaller extinctions
 Changes resulting from more than one the limiting factors that usually hold an animal in check (food,
space, predators, competition, disease, changes in environment) have led to the extinction of most of the species
that have inhabited earth (less than 5% of all the species that have existed in the earths history are alive today)
 Extinction goes on in low rates all the time however there are times when the extinction of species
have been clustered within brief intervals of time
Mass Extinction
Small Extinction
Great than 40% of all species
Few species lost
Often associated with catastrophic events (i.e. bolides) Often associated with limiting factors (food, space)
Global
Local
Large taxonomic groups lost (families and groups)
Small taxonomic groups lost (species)
Loss of entire ecosystems
No loss of entire ecosystems
I.e. Permian-Triassic (P-Tr)
I.e. Australian Megafauna

After every mass extinction the number of species on earth increases again and some taxa that were
not previously very diverse have come to flourish in the after math of such an event (i.e. mammals
remained small and inconspicuous until dinosaurs suffered extinction ) however some who were a part of the
extinction fail to ever regain their diversity levels again
Explain the recent extinction of the marsupial, bird and reptile megafauna in Australia, as an example of
small extinction events involving several large species
 Not extinctions are caused by natural factors and humans are often responsible for altering an
environment and thus affecting organisms
 Megafauna means large animals and included large marsupials, birds and reptiles
 The megafauna most probably got large at around 6mya during an ice age when having a bigger
surface to volume ration meant that more heat was conserved and due to the poor quality grass the
megafauna had the large stomachs to ferment it for longer thus retracting the nutrients out of it. Bigger
herbivores naturally leads to bigger carnivores
 Big animals need more food, more water and more time to grow and reproduce this populations are
typically small
 This extinction is considered relatively small as a significant amount of marsupials survived
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There are two theories to explain why megafauna became extinct
Theory one is due to climate change:
- During the last ice age in the Pleistocene (1.8 mya) many extinctions took place
- The Australian climate changed from cold and dry to warm and dry
- Ecosystems changed and surface water became a scare commodity
- Many browsing animals lost their habitat and these pressures were too much for the
megafauna – loss of food, water and habitat
Theory two is due to human intervention:
- The size of the megafauna made them slow and cumbersome
- This made them ideal to hunt
- The arrival of humans to Australia coincides with the beginning of the extinction
Theory two is less likely for a number of reasons and so the climatic, first theory is more accepted
however human intervention is no discredited, as it probably assisted the climatic extinction through
hunting of the large animals
Assess a variety of hypotheses proposed for the mass extinctions at the end of the Permian and at the end of
the Cretaceous
 250 million years ago – P-Tr extinction is the largest of all time
 57% of all families and 95% of all species of marine animals became extinct and many more were
nearly eliminated (i.e. more than half of all terrestrial/marine families and 95% of everything in the ocean gone )
 Rapid and took place in less than 1 million years, possibly much faster
 Coal beds stop abruptly at the P-Tr boundary and no coal was laid down anywhere in the world for
another 6 million years (i.e. fewer plants and cold dry environments all unsuitable for coal production )
 Pangaea had completed its development = more severe climatic conditions existed across the continent
 Shallow inland seas had been drained or were very limited and this reduced the amount of favourable
sites for shallow marine invertebrates; frigid polar regions existed at the poles so many organisms had
to shift towards lower latitudes or many did not survive; etc (Trilobites go extinct)
 The late Permian was also a time of extraordinary volcanic activity that lasted for a million year
(Siberian Ice Traps – huge basalt flood plains)
 Initially the gases would have caused global climate cooling (from the ash and sulphur dioxide) and a drop
in sea levels. I.e. an environment that was previously hot and humid turns freezing cold and organisms
that cannot adjust to this die off.
 As the eruptions die off, volcanic gases like carbon dioxide take over and trigger global warming so
the freezing world becomes a hothouse and the organisms that cannot adjust die off
 This extinction was climatic
 Some marine organisms regain their numbers and come back looking exactly the same (Lazarus species)
for example calcareous sponges however bivalves and gastropods become dominant
 On land, Lazarus species include conifers and ginkgos however tetrapods are dominated by the
mammal-like reptile, Lystrosaurus and eventually the first dinosaur develops
 Evidence for P-Tr extinction includes:
- Salinity in sea fell sharply for the first time
- Atmosphere went from very high oxygen content (30%) to very low (15%)
- Evidence shows global warming and glaciations near P-Tr
- Extreme erosion on land suggests ground cover disappeared
- Geomagnetic reversal occurred near P-Tr
- Siberian Traps (huge body of basalt)
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65 million years ago – K-T extinction of Dinosaurs
Almost all large vertebrates become extinct – 40% of all known species become extinct (global)
Considered “sudden” in geological terms but how short the event was is still under conjecture
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Despite the scale most groups of organisms survived i.e. insects, mammals, birds, plants, corals,
molluscs and fishes went on to diversify tremendously soon after the end of the Cretaceous i.e. many
species died out but a representative of their groups survived and later diversified
Two main possible causes for the extinction– volcanism on a huge scale or bolide impact (meteorites)
Bolide impact is favoured and it is thought that high pressure impact occurred at the K-T boundary
which forced tonnes of material (i.e. ash, dust, water) into the atmosphere blocking out the sun and
causing a cooling effect (the bolide is considered to be about 10km across)
Evidence to support this theory includes a layer of sediment rich in an extremely rare metal called
iridium has been found – iridium only occurs in high concentrations on meteorites. As well as a large
depression off the coast of Mexico may be the original impact crater.
Vendian Organism (Ediacaran)
 Spriggina

Dickinsonia
Cambrian Organism
 Trilobites
 Trilobite eyes were apparently designed to
give optimal upward facing vision - so as to
detect something swimming at them from
above. They would not have evolved such
organs were there not a pressing biological
need for them
 Extinct marine arthropods that had
segmented, oval exoskeletons divided by
grooves into head, thorax, and tail
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Anomalocaris
Marine
Large head, big eyes, round mouth
Reached up to 2m in length
Top predator from the Cambrian era
Mouth suitable for slicing and dicing
Had a pair of long, spiky grasping
appendages
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Time Period
Permian (250mya)
95% of all marine species
57% of families
Climatic
(Hypothesis 1 most likely)
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Cretaceous (65mya)
75-80% of all organisms
dinosaurs and ammonites
extinct
Bolide example
(Hypothesis 1 most likely)
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Pleistocene (45000 years
ago)
Australia Megafauna
Hypothesis 1 most likely
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Hypothesis 1
Volcanism cause volcanic
winter (sulphur dioxide and ash)
This is followed by increased
greenhouse effect and global
warming (due to carbon dioxide)
and temp rises 5 degrees
Warming triggers melting of
subsurface frozen methane
reservoirs in sea floor
Added methane enhances
greenhouse effect pushing
temp up a further 5 degrees
Evidence includes presence of
massive flood basalts in
Siberia and increased level of
C12 in sedimentary rocks from
time of extinction (C12 present in
frozen methane)
Bolide impact from a 10km
wide meteorite causing
shockwaves and generating a
nuclear winter (due to amount of
dust and water in atmosphere)
Caused the collapse of food
chains from lack of sunlight
Evidence includes carter in
Mexico (Yucatan Peninsular);
corresponding iridium layer
around the world; shocked
quartz; glass spherules that
form from impacted molten
rock; presence of layers
deposited by tsunamis in
Caribbean and S. Dakota,
presence of soot layers (fires)
Climate change
Plate tectonics cause Australia
to move northwards changing
it from cool/dry to hot/dry
Water resources and food
become scarce thus big
organisms with big resource
demands more likely to die
Leads to dwarfism of
megafauna i.e. the marsupials
we have today
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Hypothesis 2
Bolide impact from a meteorite
that would be about 10-15km wide
Impact released massive shock
wave and causes nuclear winter.
May cause volcanism as crust
rebounds (i.e. hits and pushes
magma out on the other side)
Evidence includes shocked quartz,
Siberian flood basalts, layers that
show extinction event was rapid
Flaws include lack of crater, lack
of iridium layer, large stratigraphic
sequences in Greenland show
extinction took up to 100 thousand
years
Massive volcanism causing a
volcanic winter
Evidence includes flood basalt
deposits of the same age (Deccan
traps); layer of iridium which is
present in mafic mantle material;
layer of soot from forest fires set
alight by lava
Human impact
As humans arrive in Aus they prey
on the megafauna who were easy,
slow, large targets
Use of fire by humans contributed
to destruction of environment and
direct death of megafauna
Evidence includes human remains;
soot from fires; similar scenarios
on other continents (blitzkrieg)
Flaws include tools for hunting
large organisms did not develop
until approx 6000 years ago; no
bite, spearhead marks on remains
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