Evolution and extinction
Key points to consider:
• How are fossils formed and what can we learn
from the fossil record?
• How have organisms changed over time?
• Why can scientists not be certain about how life
began on Earth?
• How and why have some organisms become
extinct?
Exploring the fossil record
• Fossils provide evidence of how much (or how little)
different organisms have changed since life developed on
Earth.
Exploring the fossil record
Fossils may be formed in various ways, including:
• from the hard parts of animals, e.g, bones, shells,
exoskeletons, which do not decay easily;
• as preserved traces of animals or plants, for
example, footprints, burrows or rootlet traces
• from parts of animals and plants which have not
decayed because
Fossil fish. As the fish
decayed the flesh
became replaced by
minerals which then
formed rock
Footprints of
Australopithecus, an
ancester of humans,
preserved in hardened
volcanic ash
1.it was too cold e.g. trapped in ice;
2.there was too little oxygen, or
3.toxic substances killed the bacteria that would
cause decay
• when parts of the animal or plant are replaced by
other materials (minerals) as they decay – these
are the most common fossils;
Fossilised fern
fronds. As they
decayed they
became
mineralised
CASTS OF ANIMALS OR PLANTS
Formed when the hard parts of animals
are replaced by minerals
Human casts from Pompeii
IMPRESSIONS
- These fossils are not actually
animals or plants but traces
they have left behind
Ardley Quarry in Oxfordshire
- Dinosaur tracks?
ICE FOSSILS
- Some animals/plants do not decay after death but are preserved in ice
mammoth fossil found
• rare fossils in Australia
Exploring the fossil record
The fossil record is incomplete, because:
• Most organisms were not fossilised when they died, as the right conditions for fossil
formation are rare to find.
• Soft body parts in particular may not fossilise – most early organisms were softbodied and left little trace of their existence.
• Many fossils have not yet been discovered
• Geological activity has worn away and destroyed rock in which fossils may have been
found.
fossil formation
Trilobites
preserved in
sedimentary
rock
Fossil of seed fern
Fossils in a Rock Profile
Layers of sedimentary rock are
arranged in the order in which
they were deposited, with the
most recent layers nearer the
surface.
Most recent
sediments
Recent fossils are
found in recent
sediments
Numerous
extinct species
Fossil types
differ in each
sedimentary
rock layer
The interpretation of rock layers
containing fossils allows us to
arrange the fossils in
chronological order (order of
occurrence)
Sedimentary layers can be
disturbed by tectonic activity.
New fossil types
mark changes in
environment
Oldest
sediments
Only primitive
fossils are found in
older sediments
What do fossils tell us?
Most of the organisms that survived millions of years ago are now extinct.
Organisms have changed over time – we can find fossils of ancient fish, fossils of the same
fish 10,000 years ago and the modern form of that same species now.
The changes represent evolution.
(Accidental changes to the appearance or physiology of organisms by (mutation) meant that
they survived while others did not.)
Evolution is still going on!
Hairy
Mammoth
What do these
animals have in
common?
They are all
EXTINCT
Quagga
T-rex
Extinction may be caused by:
1. Changes to the environment
2. New predators
3. New diseases
4. New competitors
5. Catastrophies e.g. volcano/asteroid
Dodo
True or false?
1. The oldest rocks contain the youngest fossils.
2. Animals and plants have changed over time and this is called evolution.
3. Complete animals can become fossilised in ice.
4. Fossilisation is very common.
5. The fossil record is complete.
6. Not everyone has the same view of the fossil record.
7. We have found every fossil that exists on Earth.
8. Extinction is the permanent loss of all the members of a species.
9. A single catastrophic event can cause a species to become extinct.
Can you answer these questions now?
Key points to review:
• How are fossils formed and what can we learn
from the fossil record?
• How have organisms changed over time?
• Why can scientists not be certain about how life
began on Earth?
• How and why have some organisms become
extinct?