Chapter 3
What the rocks say: how geology and
paleontology reveal the history of life
Assigned reading
• Assigned reading: Chapter 3 of your text book
Darwin recognized that evolution
required an old earth
• Darwin argued that earth could be billions of
years old based on geological observations he
made.
• Disputed by Lord Kelvin
– Proposed earth was no more than 20 million years
old based on the rates of cooling of rocks
– Kelvin’s calculations later proven to be flawed
because they ignored heat generated by
radioactive decay.
Radioactive decay
• Chemical elements are defined by the number of
protons they possess in their nucleus. They also
possess neutrons in their nucleus, but the
number of neutrons may differ between different
versions of an element, which are referred to as
isotopes.
• For example, three isotopes of carbon:
– Carbon-12 has 6 protons and 6 neutrons.
– Carbon-13 has 6 protons and 7 neutrons.
– Carbon-14 has 6 protons and 8 neutrons.
Radioactive decay
• Some elements are unstable and
spontaneously shed protons and neutrons
(releasing energy as heat in the process).
These are called radioactive elements.
• E.g. Uranium-238 breaks down through a
series of [also unstable] intermediate
elements to Lead-206.
Half-lifes
• Each radioactive isotope has a characteristic
rate of decay called it’s half-life.
• Half-life is the time taken for half of the mass
of an element to decay.
• For example, Uranium-238 has a half life of
4.47 billion years.
Radioactive clocks
• By examining the ratios of a radioactive
element and its decay product in a piece of
rock it is possible for scientists to estimate
that rock’s age.
• Thanks to radioactive clocks we can estimate
the age of the earth as about 4.6 billion years.
Radiometric dating indicates that the
earth is 4.6 billion years old
See text chapter 3 page 62 for detailed explanation of this slide
The fossil record is incomplete
• Fossils provide insight into early life forms, but
the fossil record preserves evidence of only a
tiny fraction of living things.
• Most organisms do not fossilize because they
are consumed (by scavengers and
microorganisms) before they can be fossilized.
Fossilization
• Fossilization generally requires rapid covering
of the organism by sediment (e.g. in a lake)
where over time the bones are replaced by
minerals as water percolates through them.
• In cases where no oxygen is present, microbes
do not destroy soft tissue and sometimes it is
preserved too.
Process of fossilization
Fossils allow us to learn about extinct
species
• It is obviously possible to determine a lot of
information about the morphology of extinct
species from an examination of fossils, but
less obvious information can be inferred also.
For example, about behavior and diet.
Fossil evidence shows that some dinosaurs nested
in groups, brooded their eggs and young and
protected them from predators.
Fossils provide clues about behavior
Live birth in plesiosaurs
Plesiosaur being born.
Predation
Trackways provide information about
social behavior in sauropods.
Scanning electron microscopy provides
evidence of cellular structure
• Structure of melanosomes
suggests striking plumage
in dinosaurs.
Cat scans help determine function of
hadrosaur crests
• Crest was connected to nasal cavity and sound
generated by blowing air through the cavity
where it resonated.
• Based on the shape of different species ear
bones we can tell that individual species were
tuned to the same frequency sounds as were
produced by the crest.
Biomarkers
• Some evidence of life found in rocks is not fossils,
but the presence of molecules (biomarkers) that
can only have been produced by living organisms.
• For example, 1.64 bya rocks from Australia
contain okenane, a molecule derived from
okenone, whose only known source is purple
sulfur bacteria which use the molecule in
photosynthesis.
Biomarkers reveal traces of life
• Biomarker: distinctive molecules only
produced through biological activity
Enzymatic reaction
only carried out by
purple sulphur
bacteria.
These bacteria live
only in low oxygen,
high sulfur
environments.
Carbon isotopic signatures used to
infer diet of early hominins
• C4 plants (grasses) have lower C13 levels than C3
plants (e.g. bushes, trees).
– C13/C14 ratio can be used to infer types of plants
eaten by looking at the ratio of carbon isotopes in
tooth enamel of animals.
– Hominins have ratios intermediate between grazers
and browsers suggesting they ate a mix of plants
(and animals that ate those plants).
Occasionally soft tissues fossilize
• The Burgess shale in
British Colombia
preserves a snapshot of
a community of
organisms 505 million
years ago.
~65,000 specimens of ~93
species have been
collected.
Burgess Shale fossils
• The Burgess Shale fossils date from the so-called
“Cambrian Explosion” and include a diverse array in
invertebrate animals.
• The Cambrian Explosion was called an “explosion”
because it appeared at first that a diversity of complex
multicellular animals appeared suddenly in the fossil
record.
• However, there is now plenty of evidence of life on
earth and of multicellular organisms well before this
period.
Earliest signs of life
• Oldest evidence of life dates to
3.7 bya
– Carbon contained in rocks is
alleged to be biological in origin,
but that claim is controversial
• Oldest stromatolite (bacteria)
fossils date to 3.45 bya and
microorganisms still dominate
the planet today.
Top: Living stromatolites
Bottom: Fossil stromatolites
How do early organisms fit in the tree
of life?
Earliest fossil
Eukarya: ~1.8 bya
Earliest fossil
bacteria: potentially
3.45 byo; abundant
by ~2.6 bya,
corresponding to
rise in oxygen
Earliest fossil
Archaea: ~3.5 bya
Timeline of life on Earth
• Earliest fossils of Domain Archaea ~ 3.5 bya.
These are the Archaebacteria and are
extremophiles living today in e.g. hot springs, salt
lakes and other challenging environments.
• Earliest fossils of Domain Bacteria ~ 3.45 bya.
The true bacteria. Ubiquitous on Earth. Their
production of oxygen altered the planet.
• Earliest fossils of Domain Eukarya ~ 1.8 bya.
Eukaryotes. Much bigger more complex cells
than the bacteria and archaea. Cell has a nucleus
and membrane-bound organelles.
Oldest fossils of multicellular life date
back 2.1 billion years
• Unclear where they fit in the tree of life as they bear
little resemblance to anything living today. Not clear
if they were bacteria, archaea or eukarya.
Origins of multicellularity
Clearly there were multiple independent origins
of multicellularity because fungi, plants and
animals all include both multicellular and
unicellular representatives.
Eukaryotic multicelluar life
• Earliest fossils of filamentous
algae date to 1.6 bya
– Red algae: 1.2 bya
– Green algae: 750 mya
Red algae fossil; 1.2 bya
The dawn of animals
• Earliest animal life strongly resembles sponges
– Oldest fossils 650 myo
– Biomarkers (a cholesterol-like molecule only made by
sponges) has been identified in 635mya old rocks
Ediacaran fauna
• A diverse and unique collection of animals dominated
the oceans from 575 – 535 mya. Most are hard to place
taxonomically as they look very strange.
Evolution of Ediacaran fauna
Diversification of animals
• Only a fraction of Ediacaran fauna share traits
with existing lineages and almost all were
extinct within 40 million years
• Most existing lineages are found for the first
time in the fossil record during the Cambrian
period (~540-485 mya), including our own
lineage, the chordates.
Chordates emerged during early
Cambrian
Dunkleosteus (380 mya) a 6m
long predator.
Haikouichthys (above left) early chordate from the Cambrian
Transition from ocean to land was a
major event in evolution
• Prokaryotes colonized terrestrial environments
first
– Fossils date to 2.6 bya. For about 2 billion years only
prokaryotes were found on land.
• Terrestrial animals, plants, and fungi, appeared
much later.
• All faced similar challenges surviving on land.
Had to avoid drying out, had to support
themselves out of water, had to be able to
reproduce out of water. And, for animals, had to
be able to move and breathe on land.
First terrestrial plant and fungal life
• Oldest terrestrial plant fossils are
475 myo
– Early plants resembled mosses and
liverworts
• Large forest ecosystems
developed within 100 million
years (after the evolution of the
vascular system).
• Fungi appear ~ 400 myo
– Associated with plants
First terrestrial animal life
• Invertebrate trackways date to 480 mya
– Probably relatives of insects and spiders
– Not clear whether they lived on land permanently.
Many modern invertebrates (e.g. crabs) live in and
out of water.
• Oldest fossil of a fully terrestrial animal (a
millipede) dates to 428 mya.
First terrestrial vertebrates
• Oldest track ways of terrestrial vertebrates 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:
• Mammals (~ 150 mya)
• 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
• Teleost fish (early Triassic ~ 240 mya)
Evolution of mammals
• Mammals evolved from a
group called the synapsids.
These included the sailbacked pelycosaurs (right)
• They were the dominant
vertebrates from about
300 mya to 210 mya when
the Permian extinction
wiped most of them out.
– First mammals emerged 150
mya during the age of the
dinosaurs.
Diversification of mammals
• Mammals diversified after dinosaurs went
extinct (~65 mya) in the Cretaceous extinction.
• Whales, bats, and primates all emerged around
50 mya
Oldest modern human fossils are ~200,000 years
old.