Geologic Time and Evolution
Earth Has a History
• Geologic materials record enormous changes.
• Earth is a complex, evolving system.
– Physical and biological systems continuously interact.
– Earth constantly changes and has done so through
time.
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Species arise, flourish and disappear forever.
Continents rift, drift and collide.
Ocean basins open and close.
Sea-level rises and falls.
The Hadean Eon
• Earth was inhospitable; a molten surface.
– Evidence of solidified igneous rock dates from
4.4 billion years.
• This evidence is from zircon grains, not a whole
rock.
The Hadean Eon
• Volcanic outgassing created a deadly atmosphere.
– N2, NH3, CH4, H2O, CO, CO2 and SO42- were components.
– This atmosphere had a greater density that today’s.
• Early formed crust was bombarded by meteorites.
– Meteorite impacts were abundant between 4.0 and 3.9
billion years.
• This would have destroyed early formed crust.
• Oldest evidence of crust is 4.03 billion years.
The Hadean Eon
• The first oceans formed as rain from the skies.
– Liquid water required cooling of the surface.
– First evidence of oceans from marine sediments ~
3.85 Ga.
The Archean Eon
• Time of significant change to planet Earth.
• ~3.8 billion years, Earth had cooled to form a
crust.
– Intense meteorite bombardment ceased.
– Portions of the rock record begin to survive.
The Archean Eon
• Life first appeared during
the Archean. Evidence?
– Biomarker molecules.
– Isotopic signatures.
– Preserved fossil cells.
• Clear evidence of life in
rocks dated to 3.5 billion
years.
– Life may have started earlier.
• Oldest undisputed bacteria
fossils ~ 3.2 billion years.
The Archean Eon
• Rocks after 3.2 Ga contain stromatolites.
– Layered mats of cyanobacteria (blue-green algae).
– Sediments stuck to mucous coatings on algal filaments.
• Photosynthesis changed Earth’s atmosphere.
– Converts CO2 and H2O to organic matter and free oxygen.
The Proterozoic Eon
• Protero = first; zoic = life.
– Named before Archean life was discovered.
• ~ 2 Ga (2.5 to 0.542 Ga); almost ½ of Earth history.
• The unfamiliar Archean world changed to…
– Fewer, larger lithospheric plates.
– Larger continental landmasses.
– An oxygenated atmosphere.
The Proterozoic Eon
• Atmospheric O2 permitted diversification of life.
– Aerobic respiration more efficient; allowed
multicellular life.
• Without O2, only single celled organisms possible.
• Eukaryotic (nucleated) cells evolved by at least 1.0 Ga.
– The possibility of a land-dwelling biota.
• O2 made formation of the ozone layer possible.
– Ozone absorbs deadly ultraviolet (UV) radiation.
– Prior to the ozone layer, exposed land was bathed in UV.
The Proterozoic Eon
• Over the last 500 Ma of the
Proterozoic…
– A gigantic leap in biological
complexity.
• Simple organisms gave way to
complex ones.
• Ediacaran fauna – Unusual softbodied fossils.
– Preserved in end Proterozoic
sediments.
– Multicellular invertebrates
resembling worms and jellyfish.
The Phanerozoic Eon
• Phaneros = visible; zoic = life.
• The most recent 542 Ma of Earth history.
– Began with appearance of diverse hard-shelled
organisms.
• Hard-shells vastly increased fossil preservation.
• Made possible a more complete archive of life on Earth.
The Phanerozoic Eon
• The Phanerozoic is divided into 3 Eras.
– Paleozoic – Ancient life.
– Mesozoic – Middle life.
– Cenozoic – Recent life.
• Eras emphasize changes in Earth’s biota.
The Early Paleozoic
• Cambrian and Ordovician Life Evolution:
– Earliest Cambrian – Hard shells appeared for the 1st time.
– Massive diversification followed: the “Cambrian
explosion.”
– Reflects the evolution of a complex ecosystem.
• Plankton, deposit feeders, giant predators.
The Early Paleozoic
• Cambrian and Ordovician Life Evolution:
– Life during the Ordovician included several “firsts.”
• The 1st vertebrates were jawless fish (agnathans).
• The 1st crinoids (flower-like echinoderms).
• The 1st green algae and primitive land plants.
– The end of the Ordovician witnessed a mass extinction.
– The seas roiled with life, but there was no life on land yet.
The Middle Paleozoic
• Silurian and Devonian Life Evolution:
– New species replaced those lost to extinction.
– Vascular land plants evolved and spread across Earth.
• Internal water transport systems.
• Woody tissues.
• Seeds.
– Land plants changed Earth.
The Middle Paleozoic
• Silurian and Devonian Life Evolution:
– Fish rapidly evolve and proliferate.
• Jawed fish.
• Boney fish.
– The 1st land animals
followed plants.
• Scorpions.
• Spiders.
• Insects.
• Crustaceans.
The Middle Paleozoic
• Silurian and Devonian Life Evolution:
– At the end Devonian, the 1st amphibians appeared.
• Walked on legs.
• Breathed air with lungs.
The Late Paleozoic
• Carboniferous and Permian Life Evolution:
– Life continued to evolve.
• Dense tropical wetlands hosted vegetation and giant insects.
• Amphibians diversified.
• Reptiles appeared for the 1st time.
– The amniote egg permitted reproduction away from water.
– Reptiles populated previously inhospitable environments.
The Late Paleozoic
• Carboniferous and Permian Life Evolution:
– The Paleozoic ended with the Permian extinction.
• 90% of all marine species disappeared.
• Some evidence links the extinction to a bolide impact.
Early and Mid-Mesozoic Era
• Triassic and Jurassic Life Evolution:
– New species filled niches vacated by extinction.
• Corals became dominant reef builders.
• Gymnosperms proliferated.
• Reptiles diversified.
– Plesiosaurs – Swimming reptiles.
– Pterosaurs – Flying reptiles.
– Turtles appear.
Early and Mid-Mesozoic Era
• Triassic and Jurassic Life Evolution:
– By end of the Triassic the first true at dinosaurs appeared.
– Dinosaurs differ from other reptiles in significant ways.
• Legs are positioned beneath their bodies.
• They bear evidence of warm bloodedness.
– By the end Jurassic giant sauropods were abundant.
Early and Mid-Mesozoic Era
• Triassic and Jurassic
Life Evolution:
– The 1st feathered
birds
(archaeopteryx).
– The 1st ancestors of
mammals appeared
at Triassic.
• They resembled
small rat like
creatures.
The Late Mesozoic
• Cretaceous Life Evolution:
– Teleost fish appeared and became dominant.
• Symmetrical tails, specialized fins, short jaws, rounded
scales.
– Swimming reptiles and gigantic turtles swam the seas.
– Angiosperms (flowering plants) appeared and spread.
• Produce seeds rapidly.
• Insects facilitate pollination.
• Hardwood trees proliferated.
The Late Mesozoic
• Cretaceous Life Evolution:
– Dinosaurs reached their evolutionary peak.
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Inhabited almost all environments on earth.
Social herds of grazing dinosaurs roamed the plains.
Large carnivores fed upon the herbivores.
Pterosaurs soared overhead, birds began to diversify.
– Mammals developed larger brains and specialized teeth.
The Late Mesozoic
• The K-T (Cretaceous–
Tertiary) boundary
event.
– There is abundant
evidence of catastrophic
change.
• Instantaneous global
change in fossil
assemblages.
• Sudden mass extinction of
most species on earth.
– The dinosaurs which had
ruled the planet for 150
Ma vanished.
– 90 percent of plankton
disappeared.
– 75% of plant species
vanished.
The Late Mesozoic
• The K-T (Cretaceous–Tertiary) boundary event.
– Catastrophic impact by a 10 km comet or meteorite.
– The Chicxulub crater lies beneath the northern
Yucatan.
• Radiometric dating indicates the crater formed at ~ 65
million years.
• 100 km wide; 16 km deep.
• Periodic impacts are
likely contributors to
other extinctions.
The Cenozoic Era
• Cenozoic (65 Ma-present) – The most recent history.
• Cenozoic Paleogeography:
– During the last 65 million years, Earth has continued to change.
– The final stages of the breakup of Pangaea.
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Australia detached from Antarctica.
Greenland separated from North America
The North Sea formed between Britain and Europe.
Sea-floor spreading continued to open the Atlantic Ocean.
The Cenozoic Era
• Cenozoic Life Evolution:
– After the K-T boundary, plant life recovered.
– Forests of angiosperms and gymnosperms
reappeared.
– The 1st grasses appeared in the middle Cenozoic.
– Dinosaur descendants (birds) diversified and spread.
The Cenozoic Era
• Cenozoic Life Evolution:
– The Cenozoic is known as the age of mammals.
– Mammals rapidly diversified to fill vacated niches.
– By mid-Cenozoic huge mammals appeared.
• Woolly mammoths.
• Giant beavers.
• Ground sloths.
– Late Cenozoic, human ancestors 1st appeared.
The Cenozoic Era
• Cenozoic Life Evolution:
– Ape-like primates diversified in
the Miocene (~ 20 million
years).
– The 1st human-like primate
appeared about 4 million
years.
– The first members of the
genus Homo appeared to 2.4
million years.
The Cenozoic Era
• Cenozoic Life Evolution:
– What sparked the evolution of the genus
Homo?
• Climate changes that led to the spread of
grasslands?
– Permitted departure from the trees.
– Life on the ground allowed more time for infant
development.
– This permitted growth of larger brains.
– Homo erectus appeared ~ 1.6 million
years.
• Made stone axes.
– Homo sapiens appeared ~ 500 thousand
years.
– Modern humans appeared ~150 thousand
years.
– Many giant mammals died off 10 thousand
years.
• Climatic change?
• Hunting pressure by humans?
The Geologic Time Scale
Pangea
• Comes from Latin words
– Pan = whole
– Gaia = Mother Earth
• Proposed by German scientist Alfred Wegener
– Idea was based on:
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“Fit” of the continents.
Location of glaciations.
Fossil evidence.
Rock type and structural similarities.
Paleoclimatic evidence.
Pangea
Glacial Evidence
Paleoclimatic Evidence
Fossil Evidence
• Identical fossils found
on widely separated
land.
– Mesosaurus – A
freshwater reptile.
– Glossopteris –
Subpolar plant with
heavy seeds.
– Lystrosaurus – A nonswimming, landdwelling reptile.
– Cynognathus – A nonswimming, landdwelling reptile.
Geologic Time
• Discovering the magnitude of the Earth’s past was a
momentous discovery in the history of humanity.
• This discovery forever altered our perception of
ourselves within nature and the Universe.
Geologic Time
• Before the lat 17th century, geologic time was
thought to be the same as historical time
– Believed Earth formed on October 23, 4004 BCE
• Scientists began to find clues to an ancient
Earth
– Observed marine fossils high in the mountains
• Ancient animals in loose sediment
• Suggested long periods of time
Geologic Time
• There are two ways of dating geological
materials.
– Relative ages – Based upon order of formation.
• Qualitative method developed 100s of years ago.
• Permit determination of older vs. younger
relationships.
– Numerical ages – Actual number of years since an
event.
• Quantitative method developed recently.
• Age is given a number.
Relative vs. Absolute
• Relative ages
assign order
to events.
• Numerical ages
assign exact
dates to events.
Relative Age
• Uniformitarianism – The present is key to the past.
– Physical processes that we observe today operated in the
same way in the geological past.
– Modern processes help us understand ancient events.
Relative Age
• Superposition.
– In an undeformed sequence of layered rocks…
• Each bed is older than the one above and…
• Younger than the one below.
– Younger strata are on top; older strata below.
Relative Age
• Horizontality and continuity.
– Strata often form laterally extensive horizontal sheets.
– Subsequent erosion dissects once-continuous layers.
– Flat-lying rock layers are unlikely to have been
disturbed.
Relative Age
• Crosscutting relations
– Geologic features intrude on preexisting strata
• Igneous intrusions (dikes and sills)
• Faults
• Plutons
Relative Age
• Inclusions – A rock fragment within another.
– Igneous xenoliths – Country rock that fell into magma.
– Weathering rubble – Debris from pre-existing rocks.
• The inclusion is older than the material enclosing it.
Relative Age
• Fossil (faunal) succession
– Fossil remnants, or traces of once living organisms,
are often preserved in sedimentary rocks.
– Fossil are useful for relative age determination.
• Several types of fossils will occur as an assemblage.
• Fossils are time markers.
Fossil Succession
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Species evolve, exist for a time, and then go extinct.
First appearance, range, and extinction dates rocks.
Fossils succeed one another in a known order.
A time period is recognized by its fossil content.
Fossil Succession
• Fossil range – First and last appearance.
– Each fossil has a unique range.
– Overlapping ranges provide
distinctive time markers.
• Permit correlation of strata.
– Locally.
– Regionally.
– Globally.
Fossil Succession
• Evolution – Fossils preserve changes in Earth’s
life.
– Sedimentary rocks preserve fossil ecosystems.
– Organisms inhabiting Earth have obviously changed.
• Over geologic time, most species have exhibited both…
– Trends toward specialization.
– Catastrophic extinctions.
FOSSILS
• Most common fossils are bones, shells,
pollen grains, and seeds.
• Only the hard parts of dead plants or
animals become fossils.
• The soft tissues either decay or are eaten
before the fossils can form.
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Fossils
• Palentologist
– A scientist who searches for clues to biological
change by studying fossils
• Fossils
– They are used to form a picture of the Earth’s past
– They are the remains or evidence of a living thing
FOSSILS
• For fossils to form, the remains of
organisms usually have to be buried in
sediments soon after the organism dies.
• Plants and animals that lived in or near
water were preserved more often because
sediment in the form of mud and sand
could easily bury plants and animals that
died in or along the sides of the water.
• When sediments slowly hardened and
changed to sedimentary rocks, the
organism was trapped in the rocks.
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Three Types of Rocks:
• Sedimentary rocks are formed from layers of
sediments. Most fossils are found here.
• Igneous rocks are formed by cooling and
hardening of hot molten rock or magma.
• Metamorphic rocks are formed when
sedimentary or igneous rocks are changed by
heat, pressure, or chemical reactions.
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Many Kinds of Fossils
• Petrification: turning an organism into stone
by replacing some or all of the animal or plant
material with minerals.
• Molds and Casts: There are two types of
fossils formed when an animal or a plant is
buried in sediments that harden into rock.
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Many Kinds of Fossils
• Molds: The empty space left in a rock
when the soft parts of an organism decay
and the hard parts are dissolved by
chemicals. It has the same shape.
• Casts: If the mold is filled in by minerals in
the sediment it hardens to form a cast. It
has the same shape.
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Many Kinds of Fossils
• Imprints: made by thin objects – such as
leaves and feathers. They leave an
impression in soft sediment like mud and
then it hardens.
• Trace fossils: Marks or evidence of animal
activities such as, tracks, trails, footprints,
and burrows
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Many Kinds of Fossils
• Preserving entire organism: 3 ways
• Freezing: whole body or complete sections
of organism is preserved.
• Amber: Sap from trees hardens. If an
insect is trapped in it, it is preserved.
• Tar Pits: Animals become stuck in it and
eventually sink, preserving the organism.
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