The Fossil Record
I.
Fossils & Ancient Life
 Paleontologists – scientists who study fossils.
 They collect and study fossils.
 They infer what past life was like.
 They group similar organisms together and arrange
them from the oldest to the youngest.
 All the information about past life is called the
Fossil Record.
 Fossil record provides evidence
about the history of life on Earth. It
also shows how organisms change
over time.
II. How Fossils Form
 For a fossil to form either the remains of the organism or some
trace of the organism must be preserved.
 Most fossils form in sedimentary rock.
 In some cases the remains of an organism’s soft parts
form an imprint in rock.
 In other cases the hard parts of the organism are
replaced or saturated with long lasting minerals.
 In some cases the remains are buried before they
decay so they are perfectly preserved.
III. Interpreting Fossil Evidence
 One of the most important pieces of information about a fossil
is its age.
 Relative Dating – the age of a fossil is determined comparing
its placement with that of fossils in other layers of rock.
 Oldest on the bottom, younger on the top.
 Index fossils are used to compare the relative age of
fossils.
 The species, to be an index fossil, must be
easily recognized, and must have existed for a
short period but have a wide geographic range.
 As a result it will be found in only a few layers
of rock, but these specific layers will be found
in different geographic locations.
 Is an estimate of a fossils age.
 Radiometric dating is the use of half-lives of radioactive
elements to determine the age of a sample.
 Radiometric decay is used to give absolute ages to
rock.
 Radioactive elements decay (breakdown) into
nonradioactive elements at a steady rate called halflife
 In radioactive dating the age of a sample is
calculated based on the amount of radioactive
isotopes it contains.
IV. Geological Time Scale
 Used to represent evolutionary time.
 First time is Precambrian Time.
 Covers 88% of Earth’s history.
 Few multi-cellular fossils exist.
 After Precambrian Time the basic divisions of the
geological time are eras and periods.
 Eras.
 Paleozoic Era.
 Began 544 mya (million years ago) and lasted
300 million years.
 Many vertebrates and invertebrates lived
then.
 Mesozoic era
 Began 245 mya and lasted 180 million
years.
 Called the age of the dinosaur.
 Mammals also began to evolve.
 Cenozoic Era
 Began 65 mya to present.
 Age of the mammal.
 Periods
 Each era is subdivided into periods.
 Last form tens of millions of years to less than 2
million years.
 For example the Triassic, Jurassic, and the
cretaceous periods are part of the Mesozoic Era.
V. Formation of the Earth
 Are hypotheses.
 The earth wasn’t born in a single event.
 Pieces of cosmic debris were attracted to one
another.
 Enough heat was produced to melt the
entire globe.
 Once melted elements rearranged
themselves according to densities.
 Most dense formed the core.
 Moderately dense ultimately formed
the solid crust after cooling.
 Least dense formed the atmosphere.
The early
atmosphere
contained CO2, CO,
N, H2O, hydrogen
sulfide, and hydrogen
cyanide.
 4 billion years ago the Earth’s surface cooled enough to allow
solid rock to form on the surface.
 3.8 billion years ago the surface cooled enough for water to
remain a liquid.
 Thunderstorms drenched the planet and oceans
filled.
VI. The First Organic Molecules.
 Atoms do not assemble themselves into complex organic
molecules, or other living cells on Earth today
 Oxygen is very reactive and would destroy them.
 Another life form would eat any organic molecules
that may form.
 Early Earth was very different.
 Miller and Urey’s experiment suggested how
mixtures of organic molecules necessary for life may
have formed on early Earth.
 Simulated the atmosphere and added
energy.
 Amino acids were produced.
 In later experiments they produced the
organic compounds cytosine and uracil (
bases found in RNA)
VII. How Did Life Begin
 200 – 300 million years after Earth cooled enough to carry
liquid water:
 Cells similar to modern bacteria were common.
 Formation of microspheres.
 Proteinoids microspheres – tiny bubbles formed
from large organic molecules.
 Aren’t cells but have some characteristics of life.
 Have selectively permeable membrane.
 Simple ways to store and release energy.
 Several hypotheses suggest these may have acquired
more and more characteristics of living cells.
 Formation of RNA and DNA.
 Remember DNA stores information which is
transcribed in RNA and then translated into
proteins.
 Under the right conditions some RNA can help DNA
replicate, other RNA process mRNA after
transcription, still others catalyze chemical
reactions.
 Some RNA can grow and duplicate themselves.
 RNA may have existed before DNA.
 From this, DNA system to direct protein
synthesis may have evolved.
VIII. Free Oxygen
 Microfossils of single celled prokaryotes were found in rock 3.5
billion years old.
 These evolved in the absence of oxygen.
 By 2.2 billion years ago in the Precambian seas photosynthetic
organisms became common.
 These produced a by-product of oxygen.
 This combined with iron in the water and
formed iron oxide (rust).
 The iron oxide fell to the
bottom of the ocean floor
changing the color of the
oceans from brownish to blue
green.lso formed bands of iron
ore that are still mined today.
 Oxygen next began to accumulate in the
atmosphere.
 The ozone layer began to form.
 Rise in oxygen caused some of the life
forms on Earth to become extinct.
 Other life forms evolved more efficient
pathways that used oxygen for
respiration.
 Other organisms that didn’t
evolve were forced into airless
habitats where their anaerobic
descendents live today.
IX. Origin of Eukaryotic Cells
 About 2 billion years ago prokaryotic cells (cells without a
nucleus and membrane bound organelles) began evolving
internal cell membranes.
 What resulted was the ancestor of all eukaryotic
cells.
 Other prokaryotic organisms entered the
ancestral eukaryotes.
 A symbiotic relationship
developed.
 Endosymbiotic theory – eukaryotic cells
formed from symbiosis among several
different prokaryotic organisms.
 One had the ability to use
oxygen to make ATP.
Became
mitochondria.
 Others had the ability to
photosynthesize to make food.
These became
chloroplasts.
X. Sexual reproduction and Multicellularity
 After eukaryotic cells arose they began to reproduce sexually.
 This method of reproduction increases genetic
variation and greatly increases the chances of
evolution.
 After a few million years of sexual reproduction the
development of multi-cellular organisms from single celled
organisms occurred.
 These first multi-cellular organisms experienced a
great increase in diversity and an increase in
evolution follows.
XI. Evolution of Multi-cellular life
 Precambrian time.
 During this time simple anaerobic forms of life
appeared followed by photosynthetic forms.
 Aerobic forms of life evolved and eukaryotes
appeared.
 From these, multi-cellular life arose.
 Life existed only in the sea.
 Few fossils exist.
 Paleozoic Era
 The fossil record became rich with the evidence of
many types of marine life. Especially in the
Cambrian period.
 Cambrian period
 Diversification of life is called the
“Cambrian Explosion”.
 First time organisms had hard body
parts.
 Here the first known representative of
most animal phyla arose.
 Invertebrates – jellies and
worms,
 Brachiopods – small animals
with 2 shells (resemble clams
but not related).
 Trilobites were also common.
 Ordovician and Silurian Periods
 Ancestors of modern octopi and squid
appeared.
 Aquatic arthropods arose.
 The first vertebrates – jawless fish
appeared.
 The first insects.
 The first plants evolved from their
aquatic ancestors.
 Devonian Period
 Some plants (ferns) had adapted to
drier areas.
 In seas both invertebrates and
vertebrates thrived.
 Invertebrates were far
more numerous.
 This period is called the “Age of the
Fishes” because many groups of fish
were present.
 Had jaws, skeletons, and
scales.
 Sharks first appeared.
 Animals began to invade the land.
 Some fish evolved into the
first amphibians.
 Carboniferous and Permian Periods
 Life expanded over the continents.
 Other vertebrates like
reptiles evolved from
amphibians.
 Winged insects evolved.
 Giant ferns and other
plants formed vast swampy
forests.
Their remains
formed deposits that
changed into coal
over millions of
years.
 At the end of the Paleozoic Era there
was a mass extinction.
 Mass extinction – many types of
organisms become extinct at the
same time.
 This affected both plants and
animals.
 As much as 95% of complex
life in oceans disappeared.
 Trilobites that had been
around since the early
Paleozoic Era were gone.
 Many amphibians were gone.
 Many fish and reptiles however
survived.
 Mesozoic Era
 Lasted approx. 180 million years.
 Increasing dominance of dinosaurs.
 Flowering plants appeared.
 Triassic Period
 Organisms that survived the mass
extinction became the main forms of
early life.
 Fish, reptiles, insects,
cone-bearing plants.
 Reptiles were so
successful during this
time period it is known as
the “Age of the Reptiles”.
 225 million years ago the
first dinosaur appeared.
 Mammals also appeared
during the late Triassic
Period.
Probably evolved
from mammal like
reptiles.
They were small.
 Jurassic Period
 Dinosaurs ruled for about 150
million years.
 One of the first birds appeared.
 Cretaceous Period
 Reptiles still the dominant
vertebrate.
 Dinosaurs dominate the land.
 Flying reptiles lived, but did become
extinct during this period.
 Birds survived.
 In the seas turtle, crocodiles, fish,
invertebrates, and now extinct
reptiles thrived.
 New forms of life arose: trees,
shrubs, and small flowering plants
like the ones we see today.
 At the end of the cretaceous period
was another mass extinction.
 ½ of all plants and
animals were gone,
including the dinosaurs.
 Cenozoic Era
 Mammals evolved adaptations that allowed
them to live on the land, in the water, and in
the air.
 “Age of the Mammals”.
 Tertiary Period
 There was a warm mild climate.
 Whales and dolphins evolved.
 Flowering plants and insects
flourished.
 Grasses provided a food source for
grazing animals.
 Ancestors of today’s deer,
cattle and sheep
appeared.
 Quaternary Period
 During this period the climate
cooled causing a series of ice ages.
 Glaciers advanced and
retreated over parts of
Europe and North
America.
 So much water was frozen
that the level of the oceans
fell by 100m.
 20,000 years ago the climate warmed
and the glaciers began to melt
causing the sea levels to rise again.
 In the oceans algae, coral,
mollusks, fish, and
mammals thrived.
 In the skies birds and
insect thrived.
 On land mammals like
cats, dogs and mammoths
were common.
 About 4.5 million years
ago our earliest ancestors
evolved.
 Modern humans, Homo
sapiens, evolved as early
as 100,000 years ago in
Africa.
XII. Patterns of Evolution
 Macroevolution – large scale evolutionary changes that take
place over long periods of time.
 Macroevolution has six important patterns
 Mass Extinction
 97% of all species that have ever
lived are now extinct.
 Usually extinction happens at a
constant rate.
 Mass extinction is when a huge
number of organisms die off at the
same time.
 May be caused by a single
major event.
 Many paleontologists
believe most mass
extinctions are caused by
several factors.
 Mass extinction leaves many
habitats open.
 This opens up lots of
opportunity for survivors.
 This often results in a
burst of evolution.
 Adaptive Radiation
 A single species or a small group of
species have evolved into several
different forms that live differently.
 For example Darwin’s finches.
 Can occur on a large scale.
 Dinosaurs from ancient
reptiles.
 Convergent Evolution
 Side effect of adaptive radiation.
 A process by which unrelated organisms
resemble one another.
 Have analogous
structures.
 This is due to
environmental demands
on unrelated organisms.
 Punctuated Equilibrium
 Describes a pattern of long stable
periods interrupted by brief periods
of rapid change.
 Developmental Genes and Body Parts
 A result of Hox genes
 “Master control gene” that controls
growth as embryos develop.
 1st Homologous hox genes establish
body plans.
 2nd major evolutionary changes such
as differences in the number of wing
and legs may be based on hox genes.
 The inactivation may
change the number of
wings on body parts.
 Small changes in timing of genetic
control during embryonic
development can contribute to
variation involved in natural
selection.
 Coevolution
 Process by which two species evolve
in response to changes in each other
over time.
 Coevolution of flowering plants and
insects.