Download Biology 4.27 Fossils

Evidence for Evolution - Fossils
• Aims:
• Must be able to outline the evidence for
evolutionary theory.
• Should be able to outline how fossils are formed
and how they can be dated.
• Could be able to explain the importance of
fossils as evidence of evolutionary theory.
Evidence for Evolution
• Evolutionary theory is now supported by many observations and
experiments.
• Although biologists do not always agree on the mechanisms by
which populations evolve, the fact that evolution has taken place is
well documented.
• Evidence for evolution comes from many sources:
• Paleontology: The identification, interpretation and dating of fossils
gives us some of the most direct evidence of evolution.
• Embryology and evolutionary developmental biology: The study of
embryonic development in different organisms and its genetic
control.
• Comparative anatomy: The study of the morphology of different
species.
Evidence for Evolution
• Biogeography: The study of geographic distributions
can indicate where species may have originally arisen.
• Artificial selection: Selective breeding of plants and
animals has shown that the phenotypic characteristics of
species can change over generations as particular traits
are selected in offspring.
• Biochemistry: Similarities and differences in the
biochemical make-up of organisms can closely parallel
similarities and differences in appearance.
• Molecular genetics: Sequencing of DNA and proteins
indicates the degree of relatedness between organisms.
Fossil Record
• The fossil record is a substantial, but incomplete, record of
evolutionary history:
– Modern species can be traced through fossil relatives to distant origins.
– Fossil species are often similar to, but usually differ from, today's
species.
– Fossil types often differ between sedimentary rock layers.
– Numerous extinct species are found as fossils.
– Fossils can be dated to establish their approximate absolute age.
– New fossil types mark changes in the past environmental conditions on
the Earth.
– Rates of evolution can vary, with bursts of species formation followed by
stable periods.
Types of Fossils
• The term fossil refers to any parts or impressions of an
organism that may survive after its death.
• Fossils form best when organisms are buried quickly in
conditions that slow the process of decay.
• Fossils are most commonly found in sedimentary rock.
• Mineral-rich hard parts (bones, teeth, shells) may remain
as fossils, or minerals dissolved in water, may seep into
tissues and replace the organic matter of the organism.
• On rare occasions, fossils retain organic material, as
when plant material is compressed between layers of
shale or sandstone.
Fossils as Evolutionary Evidence
• FOSSILS: remains of organisms or direct evidence of its
presence ie: bones, teeth, shells, eggs, footprints. Rapid
burial of dead organisms is necessary, with
decomposition by bacteria prevented.
• Types of fossils
– Mould – layers of sediment which then form sandstone or
mudstone over the organism, which later decays leaving a
cavity.
– Cast – formed similarly to a mould, however the cavity is later
filled by another material, leaving a 3-D model.
– Trace fossils- fossils of evidence of the presence of organisms
ie: footprints, nests, teeth marks etc
How Fossils Form
• Permineralisation: water
infused with minerals
passes through the shell,
replacing the chemicals in
the shell with rock-like
minerals (Calcite, Iron or
Silica).
• With the movement of the
Earth, the seabed is
moved towards the
surface. Exposure to the
elements leads to Erosion
• The fossil can be
exposed at the surface
by erosion, a cliff
collapse or by
palaeontologists
Interpretation of Fossils
• The fossil record is an orderly array in which fossils
appear in the layers, or strata, of sedimentary rocks.
• When organisms are trapped in sediments, they record
that moment in time.
• The fossils in each stratum of sedimentary rock are a
local sample of the organisms that existed at the time the
sediment was deposited.
• Because younger sediments overly older ones, it is
possible to determine the relative ages of fossils.
Aging Rocks and Fossils
• The age of fossils and rocks is determined in 2 ways:
• Relative age: determined by the sediment layers
• - using layers and fossils of known ages (index fossils)
• Absolute age: determined by RADIOMETRIC DATING
• - uses naturally-occurring radioactive isotopes.
• Radioisotopes decay at a constant rate to form stable (or daughter)
isotopes.
• This rate of decay is measured by half-life (how long it takes for onehalf of the parent radioactive material to decay to a daughter
product).
• The ratio of parent isotope to daughter isotope in the rock reveals
the number of half-lives, or length of time in years, that has elapsed.
Think of radioactive elements as "geologic clocks."
Relative Age
• The Law of Superposition states that in a layered, depositional
sequence (such as a series of sedimentary beds or lava flows), the
material on which any layer is deposited is older than the layer itself.
Thus, the layers are successively younger, going from bottom to top.
Radiometric (Absolute) Dating
• Half Lives for Radioactive Elements
• Although this is one of the most commonly used techniques for
dating, some inaccuracies have been recorded.
• Not all rocks can be dated absolutely, so a combination of
techniques are used.
Fossils in a Rock Profile
•
Layers of sedimentary rock
arranged in the order they
Most recent
sediments
were deposited - most
Numerous
extinct species
recent layers nearer the
Fossil types
differ in each
sedimentary
rock layer
surface.
•
Recent fossils are
found in recent
sediments
Sedimentary layers can be
disturbed by subsequent
tectonic activity.
•
New fossil types
mark changes in
environment
Allows arrangement of
fossils in chronological
order –Doesn’t give
absolute date.
Oldest
sediments
Only primitive
fossils are found in
older sediments
Dating Fossils
•
The relative age of
fossils is useful, but
fossils provide reliable
Age Range
(years)
Material
Dated
Electron Spin
Resonance
500 000 – 1000
Bone, tooth
enamel, cave
deposits
Fission Track
1 million – 100 000
Volcanic rock
Obsidian Hydration
800 000 – present
Obsidian
(volcanic glass)
Amino acid
racemization
1 million – 2000
Bone
Dating Method
historical data only if
we can determine
their absolute age.
•
A number of methods
are used to date
Thermoluminescence
fossils.
A fossil trilobite, a primitive arthropod
that dwelled in the seas of the
Devonian period 370 million years ago
Pottery, fired clay,
less than 200 000
bricks, burned
rock
Uranium/Thorium
Less than 350 000
Bone, tooth
dentine
Carbon 14
1000 – 50 000+
Bone, shell,
charcoal
Potassium/Argon
10 000 – 100
million
Volcanic rocks
Activity
• Complete the tasks from pages 343/4 in
the Biozone books.
Geological Time Scale
The History of Life on Earth
•
The history of life is divided up into eons, eras, periods,
and epochs:
Formation of
the earth
4600 mya
Oldest known microfossils
Oxygen produced by
found in 3500 million year
plants accumulates in
old chert in Western
the atmosphere
Australia
Precambrian Eon
Millions of years ago
Quaternary
Eras
Millions of years ago
Evolutionary History
Bacteria and algae
Protists
• Based on fossil evidence and radiohistory of plants, fungi, bacteria,
protists, and non-chordate animals
Land plants
isotope dating, the evolutionary
Fungi
can be compiled.
Sphenophytes (ferns etc)
Conifers
Cycads
Angiosperms
Cnidarians
• Bacteria, protists, and fungi have an
evolutionary history extending back
to the Precambrian.
Invertebrates
Flatworms
Molluscks
Annelid worms
Insecta
Crustacea
Diplopoda
Arachnids
Echinoderms
• Some invertebrate groups extend
back to the Cambrian Period, but
land plants only as far back as the
Devonian Period.
Millions of years ago
Evolutionary History
Tunicates
Agnatha (jawless fishes)
Sharks and rays
Ray finned fishes
•
Similarly, the evolutionary
history of chordates can be
Fish
Lungfish
Amphibians
Amphibians
Chelonia (turtles a& tortoises)
Reptiles
traced back to the
Crocodilia
Rhyncocephalia (tuatara)
Squamata (lizards & snakes)
Cambrian, but most animal
groups are much more
recent than this.
Birds
Birds
Monotremes
Mammals
Marsupials
Placentals
Millions of years ago