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Chapter 3

What the rocks say: how geology and

paleontology reveal the history of lifeGeological Time Line

◼ Time hierarchy

Eons

Eras

Periods

Epochs

Stages

◼ The Geologic Time Line - see handouts

Geological Time Line

◼ Geologic time is divided into

Precambrian Time

◼ Before Cambrian (<570 mya)

◼ Life is mostly microscopic single celled organisms

Phanerozoic Time

◼ Cambrian era and after

◼ Visible life (>570 mya)

eraera era

eon

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Geological Time Line

◼ Divisions of geologic time are based on

the fossil content of rocks

Formation of earth ~ 4.6 bya

Origin of life ~ 3.6 bya

Early Precambrian rocks contain few fossils –

so decay rates of radioactive isotopes are

used to age rocks

Coal forests in Illinois

This layer is missing

in GLB

Glaciers carved

out GL basins

Limestone bedrock

forms

1st Mass Extinction

Archaea

Methanogenic

Key Concepts

◼ Early estimates of the earth’s age were

based on flawed assumptions:

◼ Darwin recognized that evolution required

an old earth

◼ Lord Kelvin argued that earth was younger than

Darwin assumed

Proposed earth was no more than 20 million years old

Calculations later proven to be flawed

Kelvin – young earth Key Concepts

◼ Many elements have stable and unstable

isotopes

◼ Unstable isotopes have a fixed probability

of decay

◼ Isotopes with high decay probabilities

decay rapidly; isotopes with low decay

probabilities decay slowly

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Radioactive elements decay, releasing particles and energy.

High energy particles may damage living cells or DNA.

Radiometric dating indicates that

the earth is 4.6 billion years old

Radiometric dating allows for

precise estimates of the age of

geological formations

Radioactive decay occurs at a constant exponential or geometric rate.

The rate of decay is proportional to the number of parent atoms present

Most minerals which contain radioactive isotopes are in igneous rocks. The

dates they give indicate the time elapsed since the magma cooled.

Uranium and Phosphorus most common.

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Radiometric dating estimates the age of

the earth at 4.56 billion years old

Anatomy of Planet Earth

◼ Earth made of layers of varying densities

◼ Inner core makes one more rotation than the crust every 400 years

◼ Spins like a poorly balanced top – wobbles on its axis

Tectonic Plates

◼ Lithosphere and crust

broke into large irregular

chunks

◼ Float on sluggish molten

rock of asthenosphere

and drift about freely

◼ Collided and moved

apart many times

◼ Process continues today

Diverging Plates

◼ Where plates pull apart, hot molten rock (fluid

magma) emerges as lava

New matter is added to the plates

New oceanic plates are formed

◼ The place where this happens is known as a

mid-ocean ridge.

Beneath each of the world's great oceans there is a

mid-ocean ridge.

Mid-ocean ridges are areas of much volcanic and

seismic activity.

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The Growing AtlanticConverging Plates

◼ Huge plates of the earth's surface are

slowly moving together

Edge of one plate is gradually destroyed by

the force of collision

sometimes the impact simply crimps the

plates' edges, thereby creating great

mountain ranges: process = orogeny.

When one tectonic plate bends beneath the

other, it is called subduction.

Animals Diversify

The Cambrian Explosion

570-505 million years ago

Paleozoic

◼ The Paleozoic era witnessed the

proliferation of life in diverse forms across

the planet.

Animals and plants began to move out of the

water and populate terrestrial environments.

◼ As the continents shifted around the globe,

they had, by the end of the Paleozoic,

combined to form the supercontinent

Pangaea.

The Paleozoic Periods

◼ Cambrian Cambrian Explosion!

◼ Ordovician

◼ Silurian

◼ Devonian

◼ Carboniferous – Coal forests

◼ Permian – ended in Mass Extinction event

Largest mass extinction until the one going on now!

Cause? - maybe glaciation (Snowball Earth);

asteroid?

Shallow seas covered GLB

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Precambrian Cambrian

Ordovician Silurian

Devonian Carboniferous

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Permian Triassic

Jurassic Fossils allow us to learn about

extinct species

◼ Morphology

◼ Behavior

◼ Development

Scanning electron microscopy provides

evidence of cellular structure

◼ Structure of melanosomes

suggests striking plumage

Cat scans help determine function

of hadrosaur crest

◼ Crest connected to nasal

cavity

Sound generated by

blowing air

◼ Ears tuned to this

frequency

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Fossils provide clues about behavior Fossils provide clues about

development

Key Concept

◼ The fossil record can never be complete

because most organisms don’t fossilize

Most organisms don’t fossilize

Age of fossils can be estimatedOccasionally soft tissues fossilize

◼ Burgess shale

505 million years ago

~65,000 specimens

~93 species

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Key Concept

◼ Technology allows scientists to gain

insights into the natural history, behavior,

and appearance of extinct organisms

Biomarkers reveal traces of life

◼ Biomarker: distinctive molecules only

produced through biological activity

Presence of okenane reveals of purple sulfur

bacteria 1.64 billion years ago

Carbon isotopic signatures used to

infer diet of early hominins

◼ C4 plants have lower C13 than C3 plants

C13/C14 ratio used to infer types of plants eaten

Key concepts

◼ Isotopes and biomarkers carry information

about the history of life

History of life on earth revealed by

fossil record

Scientists search for evidence of life

in old rocks

◼ Earliest life unlikely to be

preserved in fossils

◼ Presence of carbon in early

rocks suggests life

Isotopic signature

distinguishes from lifeless

carbon sources

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Earliest signs of life

◼ Oldest evidence of life dates

to 3.7 bya

Carbon contained in rocks

Claim is controversial

◼ Oldest stromatolite (bacteria)

fossils date to 3.45 bya

Top: Living stromatolites

Bottom: Fossil stromatolites

Key Concepts

◼ Scientist use diverse methods to

reconstruct the history of life on earth

◼ Potential signs of life date to 3.7 bya but

claim is controversial

◼ Earliest accepted fossils of bacteria date

to 3.45 bya

How do early organisms fit in the

tree of life?

Earliest fossils:

potentially 3.45 byo;

abundant by ~2.6

bya, corresponding

to rise in oxygen Earliest fossils:

~3.5 bya

Earliest fossils:

~1.8 bya

Key concept

◼ Earliest signs of life are microbial, and

microbes still constitute most of the world’s

biomass and genetic diversity

Origin of multicellularity a major

transition in history of life

◼ Evolved independently in different lineages

◼ Extant organisms provide clues about

origin of multicellularity

Oldest fossils of multicellular life

date back 2.1 billion years

◼ Unclear where they fit in the tree of life

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Eukaryotic multicelluar life

◼ Earliest fossils of algae date

to 1.6 bya

Red algae: 1.2 bya

Green algae: 750 mya

Red algae fossil; 1.2 bya

Key concept

◼ The transition to multicellular life began at

least 2.1 billion years ago

◼ Multicellularity evolved independently in a

number of lineages

The dawn of animals◼ Early animal life resemble sponges

Oldest fossils 650 myo

Biomarkers also demonstrate existence of sponges during this time

Pinacoderm made of

pinacocytes

Ostia lined by porocytes

Mesohyle with

amoebocytes

Choanoderm with

choanocytes

Cell Layers

Earliest animal tracks date to 585

million years ago

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Ediacaran fauna◼ Diverse and unique animals dominated the

oceans from 575 – 535 mya

Many hard to place taxonomically

Currently existing lineages

recognizable during the early Cambrian

Early Cambrian: 542 – 511 mya

Chordates first appear ~515 million years ago

Key Concepts

◼ Only a fraction of Ediacaran fauna share

traits with existing lineages

Almost all extinct within 40 million years

◼ Most existing lineages are found in the

fossil record during the Cambrian period

Includes our own lineage, the chordates

Transition from ocean to land a

major event in evolution

◼ Prokaryotes colonized terrestrial

environments first

Fossils date to 2.6 bya

◼ Terrestrial animals, plants, and fungi,

appeared much later

First terrestrial plant and fungal life◼ Oldest terrestrial plant

fossils are 475 myo

◼ Large forest

ecosystems within 100

million years

◼ Fungi appear ~ 400

myo

Associated with plants

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First terrestrial animal life

◼ Invertebrate trackways date to 480 mya

Probably relatives of insects and spiders

Not clear whether they lived on land

permanently

◼ Oldest fossil of fully terrestrial animal

dates to 428 mya

First terrestrial vertebrates

◼ Oldest trackways date to 390 mya

◼ Oldest fossils of tetrapods date to 370 mya

Familiar forms of life did not

emerge until recently

◼ 350 million years ago many currently

existing lineages had yet to evolve

Teleost fish

Mammals

Birds

Flowering plants

Evolution of mammals

◼ Mammals evolved from

therapsids (a group

within synapsids)

Dominant vertebrates

around 280 myo

First mammals

emerged 150 mya

Diversification of mammals◼ Mammals diversified after dinosaurs went

extinct (~65 mya)

◼ Whales, bats, and primates all emerged around 50 mya

Human Evolution

◼ Split from ape line

Between 6-7 mya

◼ Ape line led to

chimpanzees and

bonobos

◼ Sahelanthropus is

thought to be the

oldest hominin

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Oldest human fossils are ~200,000

years old

Evolution of other major lineages

◼ Birds: ~150 mya

Descendants of dinosaurs

◼ Flowering plants: ~132 mya

Grasses did not diversify until ~20 mya

◼ Insects: emerged ~400 mya but most

current lineages appear much later

Flying insects radiate dramatically at the

same time as flowering plants (What’s the

link?)

Cretaceous Period

Key Concepts

◼ Many of the most diverse existing plant

and animal lineages evolved relatively

recently