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Fossils
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Noadswood Science, 2012
Wednesday, May 24, 2017
Fossils
S To understand how fossils are formed, and how they provide evidence
about the past
Fossils
What are fossils?
S As soon as plants and animals die they begin to rot away – however,
sometimes the dead plants and animals can be turned into fossils (rock
copies of the original plant or animal)
S Fossils forms when dead plants or animals become
covered in a layer of sediment which initially
protects them…
Fossils
S Most of the evidence for evolution comes from the fossil record
S Fossils show how much, or how little, organisms have changed
over time
S One of the problems with the
fossil record is that it contains
gaps – not all organisms
fossilise well, and there will be
many fossils that have been
destroyed by the movements
of the Earth, or simply not yet
been discovered…
Nautilus – a “living fossil”
Fossils
S An organism dies, and settles on the sea floor
S Gradually it is covered with sediment which protects it, and over time
the layers build up
S As the layers build up the pressure increases, causing sedimentary rock to
form
S The dead organism undergoes a series of chemical changes resulting in
rock-like minerals taking the place of the original chemicals
S Over millions of years the original organism is replaced with minerals,
and a rock-like copy of the organism is left
Fossil Formation
Fossils can be formed in a couple of ways…
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Some are stone copies of the organism, formed as the organism become petrified
(turned to rock) by the deposition of minerals in the tissues as they decompose
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Other fossils consist of impressions of the organism's shape, left behind in the
surrounding stone as the tissues decompose
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Most dead organisms decay very rapidly and their tissues are recycled, leaving no
trace of their existence, but certain environmental conditions drastically slow
down the decay process, thus helping to preserve the tissues, for example: S Insufficient oxygen (organism trapped in resin, e.g. amber)
S Low temperatures (organism frozen in glacier)
S High soil acidity (organism in peat bog)
Early Life
S Why do you think most ‘early life’ has no fossil record?
S There is a lack of evidence
because scientists believe many
early organisms were soft-bodied
(soft tissue tends to decay away
completely, making the fossil
record incomplete)
Fossil Experiment
S Produce your own fossil – initially make the impression by
kneading the plasticine until it is soft, and press it flat so that it is
~2cm in depth
S Place a shell / small bone in the petroleum jelly (so that it doesn’t
stick) and push it into the plasticine – then carefully remove the
shell / small bone (leaving only the impression)
S To make the cast mix small quantities of the Plaster of Paris at a
time with water in the container – pour this into the mould –
when it is cold remove the cast (a replica of your original shell /
small bone = a fossil)!
Fossil Experiment
Make mould
Pour Plaster of
Paris
Remove ‘fossil’
Dating
Where would we find the earliest fossils?
S The earliest fossils are found in the
deepest parts of the rock (over time
more and more sediment is laid down,
meaning organisms which dies the
longest time ago are found in the
deepest parts)
Evolution
S How has our understanding of fossils over the past few hundred years led
to a better understanding of evolution?
S Most of the evidence for evolution comes from rocks and fossils – fossil
remains have been found in rocks of all ages: -
S Fossils of the simplest organisms are found in the oldest rocks
S Fossils of more complex organisms are found in the newest rocks,
S This supports the evolutionary theory that simple life forms gradually
evolved into more complex ones, and that all animals have gradually
evolved from a common ancestor
Evolution
S Is it that simple?! What problems can you identify with fossil evidence
being used solely as evidence for evolution?
S Rocks move around, so you don't always find the newest rocks near the
surface, or the oldest rocks deep down
S Evolution is not always an orderly progression from simple to evermore
complex organisms - it goes in fits and starts
S Building up the story of evolution of any one species is difficult, as in most
cases there are big gaps in fossil records (it can be like putting together a
jigsaw puzzle with half the pieces missing)
Horse Evolution
S One of the few animals for which we have a fairly complete
evolutionary record is the horse because all the main stages of the
evolution of the horse have been preserved in fossil form
S Over 60 million years, the
horse evolved from a dogsized creature that lived in
rainforests into an animal
adapted to living on the
plains and standing up to 2
metres high
Horse Evolution
S In the process its multi-toed feet, that were
adapted for walking across the forest
floor, evolved into single-toed hooves
more suited for running over open
country
Human Evolution
S Humans evolved from apes, however evidence for this transition is not
abundant when we look to fossil records
S The evolution from ape to modern man was by no
means instantaneous, yet fossil evidence of how
man gradually changed is still vastly incomplete –
we are still in search of the ‘missing link’…
S *Humans did not evolve from chimpanzees or any
other modern ape – they are instead closely related
organisms, probably all sharing a common ancestor
Human Evolution?
Time Line
S Evolution states the different species on Earth have evolved from
simple life forms which first developed more than three billion
years ago (the Earth is about 4.5 billion years old)
S The timeline below shows some of the key events in the evolution
of life forms on Earth, from the first bacteria to the first modern
humans…
Uncertainty
S Why is there still uncertainty as to how life on Earth began?
S Life forms could have come into existence in a primordial swamp
/ maybe simple organic molecules were brought to Earth on
comets which then became more complex
S These hypothesis cannot be supported or disproved because there
is a lack of valid and reliable evidence – explain what this
means…
Valid & Reliable
S Valid means that the data is reliable and answers the original
question – this data simply isn’t available for early life
S Reliable means that the data can be repeated and
reproduced by others – the early conditions of the Earth are
not categorically known, and life (to this point) has not been
artificially created (so you have no repeatable data)