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Origin and
History of Life
Ch. 19 Outline – Part 2 – Fossils &
History of
Life on
Earth
1
Origin and
History of Life
2
Fossils
Fossils are the remains and traces of past life or
any other direct evidence of past life.
Traces:
● trails, footprints, burrows, worm casts,
animal droppings
When organisms die they usually get eaten or
decompose. Sometimes, but rarely, soft parts
might last long enough to be preserved.
Most fossils, however, consist only of hard parts.
● shells, bones, teeth, etc. don’t decay easily
Fossil Dating:
Relative
Origin and
History of Life
3
Paleontology is the study of the fossil record
Most fossils are traces of organisms embedded in
sediments
Sediment converted to rock
Becomes recognizable stratum (recognizable
layer) in sequence of rocks
Strata of the same age tend to contain the similar
fossil assemblages
Helps geologists determine relative dates of
embedded fossils despite upheavals
Strata
Strata
History of Life as a 24-hour
day
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Fossils
Trilobite
Mammoth
tusks
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Ichthyosaur
Petrified
wood
Placoderm
Ammonites
Scorpion in amber
Dinosaur footprint
Fossil Dating:
Absolute Dating
Origin and
History of Life
Relies on radioactive dating techniques & assigns an
actual date to a fossil.
Half-life:
The length of time required for half the atoms of a
radioactive isotope to change into a stable element
Unaffected by temperature, light, pressure, etc.
All radioactive isotopes have a dependable half life
- Some only fractions of a second
- Some billions of years
- Most in between
Many isotopes are used, and their combined half lives
make them useful over all periods of interest
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Origin and
History of Life
Half-Life Graph
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Origin and
History of Life
Carbon-14 Half Life Graph
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Fossil Dating:
Absolute Dating
Origin and
History of Life
Living organisms contain isotopes of some
radioactive elements in certain ratios compared
to other elements.
When an organism dies, the ratio starts to drop
because the organism no longer obtains new
elements from the world.
Scientists know what the ratio should have been
at the time the organism was alive and can
figure out how long it has been since it died
based on the ratio found in the rock sample.
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Origin and
History of Life
Radioactive Decay of C-14
10
Origin and
History of Life
Using Ratios of Isotopes to Find Age
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Origin and
History of Life
Accuracy of RadioCarbon Dating
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The Geologic Time Scale:
The Precambrian Times
Origin and
History of Life
13
Includes about 87% of the geological timescale
Little or no atmospheric oxygen
Lack of ozone shield allowed UV radiation to
bombard Earth
First cells came into existence in aquatic
environments
Oldest fossils are prokaryotes dated at 3.5 bya
Cyanobacteria left many ancient stromatolite
fossils
Added first oxygen to the atmosphere. Significant
amounts to create ozone shield by 2.7 bya.
Prokaryote Fossil of the Precambrian
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The Geologic Time Scale:
Precambrian Times
Origin and
History of Life
Eukaryotic Cells Arise
About 2.2 bya
- Mostly aerobic
- Contain nucleus as well as other membranous
organelles
Endosymbiotic Hypothesis
● Idea that a nucleated cell might have engulfed
photosynthetic or aerobic prokaryotes which later
became organelles.
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The Geologic Time Scale:
Precambrian Times
Multicellularity Arises
Origin and
History of Life
16
First multicellular protists are dated at about 1.4
bya.
Might have practiced sexual reproduction since
some cells might have become specialized to
produce gametes.
Oldest soft-bodied invertebrates appear at about
600 mya (0.6 bya).
Ediacaran fossils – many are quite bizarre
animals. Disappeared by 545 mya.
Ediacaran Fossils
Spriggina
Dickinsonia
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The Geologic Time Scale:
The Paleozoic Era
Begins with Cambrian Period
Origin and
History of Life
Thus all time previous to this is Pre-Cambrian
Lasted about 300 million years & life seemed to
“explode” at this time. Called the Cambrian
Explosion. Animals now had exoskeletons to
protect themselves.
Includes three major mass extinction events
- Disappearance of a large number of taxa
- Occurred within a relatively short time interval
(compared to geological time scale) (Just a few
million years)
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Sea Life of the Cambrian Period
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The Geologic Time Scale:
The Cambrian Period
Origin and
History of Life
20
Molecular Clock:
Based on hypothesis that
- Changes in base-pair sequences of certain DNA
segments occur at fixed rate, and
- The rate is not affected by natural selection or other
external factors
When these base-pair sequences are compared
between two species:
- Count the number of base-pair differences
- Count tells how long two species have been
evolving separately
Origin and
History of Life
21
The Geologic Time Scale:
The Invasion of Land – Paleozoic Era
Plants
Algae probably invaded damp areas of land.
First plants were nonvascular (did not have
tissues to conduct water). Limits height of plants.
Seedless vascular plants came a little bit later
- Club mosses, horsetails, seed ferns were tree
sizes at this time.
Later flourished in warm swamps in
Carboniferous period. First non-flowering seed
plants appear.
Origin and
History of Life
22
The Geologic Time Scale:
The Invasion of Land – Paleozoic Era
Invertebrates
Arthropods were first animals on land
- spiders, centipedes, mites, millipedes
Exoskeletons and jointed appendages preadapted them to live on land
Evolution of wings (dragonflies) allowed insects to
move into many diverse groups over time.
Swamp Forests of the Carboniferous Period 23
Dragonfly had wingspan of
over 1 meter
Origin and
History of Life
24
The Geologic Time Scale:
The Invasion of Land – Paleozoic Era
Vertebrates
Animals with a vertebrate column.
Jawless fishes first appeared
- Fish are ectothermic (cold-blooded) aquatic
vertebrates with gills, scales and fins.
Jawed fish appeared a little later
- Many were giant predatory fish covered with
armor
Origin and
History of Life
25
The Geologic Time Scale:
The Invasion of Land – Paleozoic Era
Vertebrates (cont’d)
Fleshy fins helped smaller fishes hold their places
in strong currents. May have allowed them to
venture onto land.
Lobe-finned fishes are believed to be ancestors of
amphibians.
 Amphibians are thin-skinned vertebrates that
must return to the water to reproduce.
- Some were up to 6 m long.
The Geologic Time Scale:
The Mesozoic Era
Origin and
History of Life
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Divided into three Periods:
1. Triassic Period
Non-flowering seed plants became dominant
- Cycads: short, palm-like leaves, large cones
Reptiles appear. Have scaly skin, lay shelled
eggs that can hatch on land.
- Adaptive radiation occurs producing forms that
live in air, in sea, and on land
2. Jurassic Period
Some dinosaurs achieved enormous size
Mammals remained small and insignificant
The Geologic Time Scale:
The Mesozoic Era
Origin and
History of Life
3. Cretaceous Period
Great herds of dinosaurs roamed plains
At end of Cretaceous dinosaurs underwent a
mass extinction
Theropods, bi-pedal dinosaurs, most likely gave
rise to the birds (Archaeopteryx)
Mammals:
- Began an adaptive radiation
- Moved into habitats left vacated by dinosaurs
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Dinosaurs of the Mesozoic Era
Parasaurolophus walkeri
Triceratops
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Theropods
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The Geologic Time Scale:
The Cenozoic Era
Origin and
History of Life
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Mammals
- Endothermic, have hair & mammary glands
- At beginning of era, mammals were small;
resembled rats
- Continued adaptive radiation into new
environments:
Bats - took to air. Whales, dolphins,
manatees, etc. returned to sea
Primates - first ones small & squirrel-like.
Apes appeared about 34 mya.
First hominids about 5 mya (group
that includes humans)
Mammals of the Oligocene Epoch
Many browsing animals lived at this time
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The Geologic Time Scale:
The Cenozoic Era
Origin and
History of Life
32
Pleistocene Epoch
- Multiple ice ages occurred in Northern
Hemisphere
- Era of giant ground sloths, beavers, wolves,
bison, mastodons, and mammoths
- These giant mammals may have been
hunted to extinction by humans
Woolly Mammoth of the Pleistocene Epoch 33
Origin and
History of Life
34
Factors That Influence Evolution
Plate Tectonics
- Earth’s crust consists of slab-like plates
- Tectonic plates float on a lower hot mantle layer
Positions of continents and oceans are not fixed
Movements of plates result in continental drift
- 225 mya there was a giant continent called
Pangaea
- 200 mya it had split into two continents known
as Laurasia and Gondwana
- By 65 mya modern continents were taking
shape
Continental Drift
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Plate Tectonics
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Origin and
History of Life
37
Mass Extinctions
Mass Extinctions
There have been five great episodes of mass
extinction on Earth
Lesser events seem to have happened every 26
million years. May be due to movements through
Milky Way Galaxy.
Mass extinctions have often been followed by
great explosions of the diversity of life forms on
the planet.
Origin and
History of Life
Mass Extinctions
Permian
Occurred at the end of the
following periods:
Ordovician
438 mya
75% of species
disappeared
Devonian
360 mya
70% of marine
invertebrates
disappeared
245 mya
90% of ocean species
disappeared; 70% on
land
Triassic
208 mya
60% of species
disappeared
Cretaceous
66 mya
75% of species
disappeared ;including
dinosaurs
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Mass Extinctions
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