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Chapter 6
Life on Earth: What do
Fossils Reveal?
Fossils
Fossils are the remains or traces of ancient
life which have been preserved by natural
causes in the Earth's crust.
Fossils include both the remains of
organisms (such as bones or shells), and
the traces of organisms (such as tracks,
trails, and burrows - called trace fossils).
Fossil Preservation
To become preserved as a fossil, an organism must:
• Have preservable parts. Bones, shells, teeth,
wood are more readily preserved than soft parts.
• Be buried by sediment to protect the organism
from scavengers and decay.
• Escape physical, chemical, and biological
destruction after burial (bioturbation, dissolution,
metamorphism, or erosion).
Fossil Preservation
Organisms do not all have an equal chance
of being preserved.
The organism must live in a suitable
environment.
Marine and transitional environments are
more favorable for fossil preservation.
Higher rate of sediment deposition .
Types of Fossil Preservation
1.
2.
3.
4.
5.
Chemical Alteration of Hard Parts
Imprints of Hard Parts in Sediment
Preservation of Unaltered Soft Parts
Trace fossils or Ichnofossils
Preservation of Unaltered Hard Parts
Preservation of Unaltered Hard Parts
The shells of invertebrates and single-celled
organisms, vertebrate bones and teeth:
a. Calcite (echinoderms and forams)
b.
c.
d.
e.
Aragonite (clams, snails, modern corals)
Phosphate (bones, teeth, conodonts, fish scales)
Silica (diatoms, radiolarians, some sponges)
Organic matter (insects, pollen, spores, wood, fur)
Chemical Alteration of Hard Parts
a. Permineralization - filling of tiny pores
b. Replacement - molecule-by-molecule
substitution of one mineral for another (silica or
pyrite replacing calcite)
c. Recrystallization - aragonite alters to calcite
d. Carbonization - soft tissues preserved as a
thin carbon film (ferns in shale)
Imprints of Hard Parts in Sediment
• Impressions
– External molds
– Internal molds
• Cast
Preservation of Unaltered Soft Parts
•
•
•
•
•
Freezing (wooly mammoths)
Desiccation (drying or mummification)
Preservation in amber
Preservation in tar (LaBrea tar pits)
Preservation in peat bogs (Lindow man –
England; Tollund man – Denmark)
Trace fossils or Ichnofossils
Markings in the sediment made by the
activities of organisms
•
•
•
•
•
Tracks
Trails
Burrows – in soft sediment
Borings – in hard material
Root marks
•
•
•
•
Nests
Eggs
Coprolites
Bite marks
Trace fossils or Ichnofossils
Trace fossils provide information about
ancient water depths, paleocurrents,
availability of food, and sediment
deposition rates.
Tracks can provide information on foot
structure, number of legs, leg length,
speed, herding behavior, and interactions.
Biological classification
A system of binomial nomenclature (i.e., two
names) is used to name organisms.
The first of the two names is the genus and
the second name is the species.
Genus and species names are underlined or
italicized.
Genus is capitalized, but species is not.
The Species
A group of organisms that have structural,
functional, and developmental similarities,
and that are able to interbreed and
produce fertile offspring.
The species is the fundamental unit of
biological classification.
The Species
Different species do not interbreed under
natural conditions. Reproductive barriers
between species prevent interbreeding.
Closely related (but different) species, such
as the horse and the donkey CAN
interbreed, but the offspring are usually
sterile (unable to reproduce).
Hybrid mammals
•
•
•
•
•
•
Mule (♂male donkey and ♀ horse)
Hinny (♂ horse and ♀ donkey)
Zedonk (♂ zebra and ♀ donkey)
Liger (♂ lion and ♀ tiger)
Tigon (♂ tiger and ♀ lion)
Wholphin (♂ false killer whale and ♀ bottlenose
dolphin; fertile)
Hybrids are usually sterile (unable to reproduce).
Cells
All organisms are composed of cells.
• Eukaryotic cells have a nucleus (or nuclei) and
organelles.
– Organisms with this type of cell are called eukaryotes
(Domain Eukarya).
• Prokaryotic cells have no nucleus or organelles.
– Organisms with this type of cell are called prokaryotes
(Domain Archaea and Domain Bacteria).
Taxonomy
Organisms are grouped based on their
similarities into taxonomic groups or taxa.
Domain
Kingdom
Phylum (plural = phyla)
Class
Order
Family
Genus (plural = genera)
Species (singular and plural)
Classification of the human
Domain Eukarya
Kingdom Animalia
Phylum Chordata
Class Mammalia
Order Primates
Family Hominidae
Genus Homo
Species sapiens
There are six kingdoms of
organisms, grouped into three
domains
1. Domain Eukarya
2. Domain Bacteria
3. Domain Archaea
Domain Eukarya
Organisms with eukaryotic cells (cells with a
nucleus)
•
•
•
•
Kingdom Animalia (animals)
Kingdom Plantae (plants)
Kingdom Fungi (mushrooms, fungus)
Kingdom Protista (single-celled organisms)
Domain Bacteria
Organisms with prokaryotic cells (cells
without a nucleus)
• Kindgom Eubacteria (bacteria and
cyanobacteria or blue-green algae)
Domain Archaea
Organisms with prokaryotic cells, but which
are very unusual and quite different from
Bacteria. Archaea tend to live under
extreme conditions of heat, salinity, acidity.
• Kingdom Archaebacteria
Evolution
Evolution = change
• Organic evolution refers to changes in
populations
• In biology, evolution is the "great unifying
theory" for understanding the history of
life.
Evolution
Plants and animals living today are different
from their ancestors because of evolution.
They differ in appearance, genetic
characteristics, body chemistry, and in the
way they function.
These differences appear to be a response
to changes in the environment and
competition for food.
Lamarck's Hypothesis of Evolution
Jean Baptiste Lamarck (1744-1829)
observed lines of descent from older
fossils to more recent ones, and to living
forms.
He correctly concluded that all species are
descended from other species.
Lamarck's Hypothesis of Evolution
Lamarck assumed that new structures in an
organism appear because of the needs or
" inner want " of the organism.
Structures acquired in this way were thought
to be somehow inherited by later
generations - inheritance of acquired traits.
The idea was challenged because there was
no way to test for the presence of an
"inner want".
Lamarck's Hypothesis of Evolution
Lamarck also suggested that unused body parts
would not be inherited by succeeding
generations.
The hypothesis was tested and rejected after an
experiment in which the tails were cut from mice
for twenty generations. The offspring still had
tails.
Similarly, circumcision has been practiced for more
than 4000 years with no change among
newborn males.
Darwin's Natural Selection
Charles Darwin and Alfred Wallace were the
first scientists to assemble a large body of
convincing observational evidence in
support of evolution.
They proposed a mechanism for evolution
which Darwin called natural selection.
Darwin's Natural Selection
Natural selection is based on the following
observations:
1. More offspring are produced than can survive to
maturity.
2. Variations exist among the offspring.
3. Offspring must compete with one another for
food, habitat, and mates.
4. Offspring with the most favorable characteristics
are more likely to survive to reproduce.
5. Beneficial traits are passed on to the next
generation.
Darwin's Natural Selection
Darwin's theory was unable to explain WHY
offspring exhibited variability.
This was to come many years later, when
scientists determined that genetics is the
cause of these variations.
This principle can be stated as:
" the survival of the fittest ".
Inheritance, Genes, and DNA
Gregor Mendel (1822-1884) demonstrated
the mechanism by which traits are passed
to offspring through his experiments with
garden peas. His findings were published
in an obscure journal and not recognized
by the scientific community until 1900.
Inheritance, Genes, and DNA
Mendel discovered that heredity in plants is
determined by what we now call genes.
Genes are recombined during fertilization.
Genes are linked together to form
chromosomes.
Mendel is known as the "Father of Genetics".
Chromosomes and DNA
• Within the nucleus of each of our cells are
chromosomes.
• Chromosomes consist of long DNA
molecules (deoxyribonucleic acid).
• Genes are the parts of the DNA molecule
that transmit hereditary traits.
The DNA molecule
consists of two parallel
strands, which
resemble a twisted
ladder.
The twisted strands are
phosphate and sugar
compounds, linked
with nitrogenous
bases (adenine,
thimine, guanine, and
cytosine).
DNA
The structure of the DNA molecule was
discovered by Watson and Crick in 1953.
DNA carries chemically coded information
from generation to generation, providing
instructions for growth, development, and
functioning.
Reproduction and Cell Division
Reproduction in organisms may be:
• Sexual
• Asexual
• Alternation of sexual and asexual
generations
All reproductive methods involve cell division.
Genetic Recombination
New combinations of chromosomes result
through sexual reproduction. One of each
pair of chromosomes is inherited from
each parent.
This sexual genetic recombination leads to
variability within the species.
Asexual reproduction
• Binary fission - single-celled organisms that
divide to form two organisms
• Budding – a bud forms on the parent that may:
– Separate to grow into an isolated individual, or
– Remain attached to the parent (colonial organisms).
– Budding occurs in some unicellular and some
multicellular organisms.
• Spores shed by the parent (as in a seedless
plant like moss or ferns) that germinate and
produce male and female sex cells (leading to
alternation of sexual and asexual generations).
Diploid and Haploid Cells
In a human cell there are 23 pairs of
chromosomes. One of these pairs
determines the sex of the individual.
• Diploid cells - cells with paired
chromosomes.
• Haploid cells - sex cells (or gametes) with
only one half of a pair of chromosomes.
Example: egg cells or sperm cells
Cell division
• Mitosis - Division of body cells of sexual
organisms. Produces new diploid cells
with identical chromosomes to the parent
cells.
• Meiosis - Division of cells to form gametes
or sex cells (haploid cells), with half of
chromosomal set of the parent cell; occurs
in a two-step process, producing four
haploid gametes.
Recombination of Genes
• Fertilized egg forms when two gametes
(egg and sperm) combine. Fertilized egg
has paired chromosomes (diploid cell).
• Variation occurs because of the sexual
recombination of genes.
• Genes are recombined in each successive
generation.
Mutations
• Mutations are chemical changes to the
DNA molecule.
• Mutations can be caused by:
– Chemicals (including certain drugs),
– Radiation (including cosmic radiation,
ultraviolet light, and gamma rays).
• Mutations may also occur spontaneously
without a specific causative agent.
Mutations
Mutations may occur in any cell, but
mutations in sex cells will be passed on to
succeeding generations.
Mutations produce much of the variability on
which natural selection operates.
Causes of Evolution
Evolution may involve change from three
different sources:
• Mutations
• Gene recombination as a result of sexual
reproduction
• Natural selection
Evolution in Populations
Evolution is a process of biologic change
that occurs in populations.
• Population - A group of interbreeding
organisms that occupy a given area at a
given time.
• Gene pool - The sum of all of the genetic
components of the individuals in a
population.
Evolution in Populations
There is no exchange of genes between
different populations because they are
reproductively isolated.
Barriers keep their gene pools separate
(distance, geographic barriers,
reproductive barriers, etc.)
Geographic barriers
• Isthmus of Panama, is a barrier between oceans
and populations of marine organisms.
• Islands with isolated populations of land animals
– Galapagos Island finches
– Galapagos Island tortoises
– Hawaiian Island honeycreepers (birds)
• Grand Canyon separates different species of
animals living on opposite sides of the canyon
Reproductive barriers
• Ecological isolation - Populations inhabiting the
same geographic area, but living in different
habitats
• Temporal isolation - Populations that reproduce
at different times (such as plants that flower in
different seasons)
• Mechanical isolation - Incompatible reproductive
organs due to differences in size, shape, or
structure
• Gametic isolation - Fertilization is prevented by
incompatible gametes
Speciation
• Speciation = The process through which
new species arise.
• In the limited gene pools of an isolated
population, over many generations,
genetic differences may accumulate to the
point that the different populations are no
longer able to interbreed.
• At this point, the different populations
would be considered separate species.
Speciation
Once a new species is established,
segments of the population around the
fringes of the population may undergo
additional speciation.
With successive speciations, diverse
organisms arise with diverse living
strategies.
Adaptive Radiation
The branching of a population to produce
descendants adapted to particular
environments and living strategies
Example of adaptive radiation
Bill shapes are
adaptations to
different means of
gathering food.
Hawaiian honey creepers
Adaptive radiation and
Macroevolution
Adaptive radiation can occur at higher
taxonomic levels.
Example: Adaptive radiation of mammals at the
beginning of the Cenozoic Era.
Macroevolution = Evolution above the species level
Modeling how evolution occurs
The question is not whether evolution
occurs, but rather, exactly how it occurs.
What is the mechanism of evolution?
Modeling how evolution occurs
•
•
Phyletic gradualism - gradual progressive
change by means of many small steps (old
idea)
Punctuated equilibrium - sudden changes
interrupting long periods of little change
(stasis). Most change occurs over a short
period of time. (Stephen J. Gould and Niles
Eldridge, 1977).
Phyletic gradualism
vs.
Punctuated equilibrium
Fossil evidence provides
support for both models.
Allopatric Speciation
Punctuated equilibrium model suggests that
evolution occurs in isolated areas around the
periphery of the population (peripheral isolates).
Speciation may occur rapidly in these isolated
areas. This is allopatric speciation.
When the new species expands or migrates from
the isolated area into new areas, it looks like a
sudden appearance in the fossil record.
Phylogeny - The Tree of Life
Phylogeny = the sequence of organisms
placed in evolutionary order.
Diagrams called phylogenetic trees are used
to display ancestor-descendant
relationships.
Branches on the tree are called clades.
Cladograms
Diagrams drawn to show ancestordescendant relationships based on
characteristics shared by organisms.
They show how organisms are related but
do not include information about time or
geologic ranges.
Cladogram of
the horse
ancestry
Stratophenic
phylogeny arranges
organisms in treelike fashion with the
most recently
evolved individuals
on the upper
branches, and the
older, ancestral
forms on the lower
branches.
Evolutionary tree of the horses and
related organisms
Evidence for Evolution
There are many lines of evidence that show
that evolution has occurred. They come
from both the fossil record and from
biology.
Lines of evidence for evolution
cited by Darwin
•
•
•
•
Fossils provide direct evidence for changes in
life in rocks of different ages.
Homologous structures - Certain organs or
structures are present in a variety of species,
but they are modified to function differently.
Modern organisms contain vestigial organs
that appear to have little or no use. These
structures had a useful function in ancestral
species.
Animals that are very different, had similarlooking embryos.
Other lines of evidence for evolution come from:
1. Genetics - DNA molecule
2. Biochemistry – similar in closely-related
organisms, but very different in more distantly
related organisms.
3. Molecular biology - sequences of amino acids
in proteins
Evidence for Evolution from
Paleontology
Many examples of gradual or sequential evolution
in the fossil record, including:
1. Horses
2. Cephalopods and
other molluscs
3. Foraminifera and
other microfossils
Evolutionary change in
Permian ammonoid
cephalopods
Evidence for Evolution from Biology
Homologous structures - body parts with similar
origin, history and structure, but different
functions.
Evidence for Evolution from Biology
Vestigial organs suggest a common ancestry.
Vestigial organs serve no apparent purpose,
but resemble functioning organs in other
animals.
Vestigial pelvis and femur of a whale in an Eocene fossil
Evidence for Evolution from Biology
Similarity of embryos of all vertebrates suggests a
common ancestry.
Evidence for Evolution from Biology
Biochemistry - Chemicals (such as proteins,
antigen reactions of blood, digestive
enzymes, and hormone secretions) are
more similar in related organisms.
Evidence for Evolution from Biology
DNA sequencing – If organisms appear to
be similar on the basis of form, embryonic
development, or fossil record, we can
predict that they would have a greater
percentage of DNA sequences in
common, compared with less similar
organisms.
This is proven to be correct in hundreds of
analyses.
Fossils and Stratigraphy
How Do Fossils Reveal the Age of Strata?
William Smith (late 1700's) discovered that
certain rock units could be identified by the
assemblages of fossils they contained.
This knowledge led to the Principle of
Fossil Succession - Fossils occur in a
consistent vertical order in sedimentary
rocks all over the world.
The Geologic Time Scale is based on the
appearance and disappearance of fossil
species in the stratigraphic record.
Fossils can be used to recognize the
approximate age of a unit and its place in
the stratigraphic column.
Fossils can also be used to correlate strata
from place to place.
Geologic range
Geologic range = The interval between the
first and last occurrence of a fossil species
in the geologic record.
The geologic range is determined by
recording the occurrence of the fossils in
numerous stratigraphic sequences from
hundreds of locations.
Using Fossils to Correlate Rock Units
Use of Cosmopolitan and Endemic
Species in Correlation
Cosmopolitan species have a widespread
distribution.
Endemic species are restricted to a specific
area or environment.
Cosmopolitan species are most useful in
correlation
Pitfalls of Correlating with Fossils
Appearances and disappearances of fossils
may indicate:
• Evolution
• Extinction
• Changing environmental conditions that
cause organisms to migrate into or out of
an area
Reworked fossils
Index Fossils
Index fossils (or guide fossils) are useful in
identifying time-rock units and in
correlation.
Characteristics of an index fossil:
1. Abundant
2. Widely distributed (cosmopolitan)
3. Short geologic range (rapid evolution)
Biostratigraphic Zones
Biozone = A body of rock deposited during the
time when a particular fossil organism
existed.
A biozone is identified only on the basis of the
fossils it contains.
Biozones are the basic unit for biostratigraphic
classification and correlation.
Fossils Indicate Past Environments
Ecology and Paleoecology
1. Ecology = Interrelationship between organisms
and their environment.
2. Paleoecology = Ancient ecology; interaction of
ancient organisms with their environment.
Depends on comparisons of ancient and living
organisms (modern analogs).
3. Ecosystem = Organisms and their environment
- the entire system of physical, chemical, and
biological factors influencing organisms.
Ecology and Paleoecology
4. Habitat = Environment in which an
organism lives.
5. Niche = Way in which the organism lives;
its role or lifestyle.
6. Community = Association of several
species of organisms in a particular
habitat (living part of ecosystem).
7. Paleocommunity = An ancient
community.
Marine Ecosystem
The ocean may be divided into two realms:
• Pelagic realm = The water mass lying
above the ocean floor.
• Benthic realm = The bottom of the sea
Marine Ecosystem
Pelagic realm
• Neritic zone = The water overlying the
continental shelves.
• Oceanic zone = The water seaward of the
continental shelves.
Marine Ecosystem
Benthic realm
• Supratidal zone = Above high tide line
• Littoral zone (or intertidal zone) = Between high
and low tide lines
• Sublittoral zone (or subtidal zone) = Low tide line
to edge of continental shelf (about 200 m deep)
• Bathyal zone - 200 - 4000 m deep
• Abyssal zone - 4000 - 6000 m deep
• Hadal zone - >6000 m deep; deep sea trenches.
Marine Ecosystem
Modes of Life of Marine Animals
Plankton - Small plants and animals that
float, drift, or swim weakly.
• Phytoplankton - Plants and plant-like
plankton, such as diatoms and
coccolithophores
• Zooplankton - Animals and animal-like
plankton, such as foraminifera and
radiolaria
Modes of Life of Marine Animals
Nekton - Swimming animals that live within the
water column
Benthic organisms or benthos - Bottom dwellers,
which may be either:
• Infaunal - Living beneath the sediment
surface; they burrow and churn and mix the
sediment, a process called bioturbation
• Epifaunal - Living on top of the sediment
surface
Marine Sediments
• Terrigenous sediment - from weathered rocks
• Biogenous sediment – of biological origin
– Calcareous oozes - foraminifera, pteropods, and
coccolithophores
– Siliceous oozes - radiolarians and diatoms
– Phosphatic material – fish bones, teeth and
scales
• Hydrogenous sediment - precipitated from sea
water - manganese nodules
Carbonate Compensation Depth
A depth in the oceans (about 4000-5000 m), which
affects where calcareous oozes can accumulate.
Above the CCD (shallower than 4000-5000 m), the
water is warmer, and CaCO3 is precipitated.
Calcareous sediments (chalk or limestone) are
deposited.
Carbonate Compensation Depth
Below the CCD (below about 4000-5000 m), water
is colder, and CaCO3 dissolves. Clay or
siliceous sediments are deposited.
Use of Fossils in Reconstructing
Ancient Geography
Environmental limitations control the distribution of
modern plants and animals.
• Note locations of fossil species of the same
age on a map
• Interpret paleoenvironment for each region
using rock types, sedimentary structures, and
fossils.
• Plot the environments to produce a
paleogeographic map for that time interval.
Land Bridges, Isolation and Migration
Migration and dispersal patterns of land animals
can indicate the existence of:
• Former land bridge
(Bering Strait)
• Mountain barriers
• Former ocean
barriers between
continents
Species Diversity and Geography
Species diversity is related to geographic
location, particularly latitude.
• High latitudes have low
species diversity
• Low latitudes have high
species diversity.
As a general rule, species diversity increases
toward the equator.
Use of Fossils in the Interpretation
of Ancient Climatic Conditions
Fossils can be used to interpret paleoclimates
(ancient climates):
1. Fossil spore and pollen grains can tell about
the types of plants that lived, which is an
indication of the paleoclimate.
2. Plant fossils showing aerial roots, lack of yearly
rings, and large wood cell structure indicate
tropical climates
3. Presence of corals indicates tropical climates
Use of Fossils in the Interpretation
of Ancient Climatic Conditions
4. Marine molluscs with spines and thick shells
inhabit warm seas
5. Planktonic foraminifera vary in size and coiling
direction with temperature
6. Shells in warmer waters have higher Mg
contents
7. Oxygen isotope ratios in shells.
Overview of the History of Life
Oldest evidence of life
Remains of prokaryotic cells (blue-green
algae or cyanobacteria) more than 3.5
billion years old. Found in algal mats and
stromatolites.
Earliest Metazoan Organisms
Metazoans = multicellular organisms
• Trace fossils of metazoans about 1 billion
years ago
• First body fossils of soft-bodied metazoans
(worms, jellyfish, and arthropods) about
0.7 billion years ago
• Invertebrates with hard parts appeared in
late Proterozoic or early Paleozoic.
Geologic ranges and relative
abundances of fossil organisms
Early Paleozoic – Cambrian Period
• Most animals were deposit and suspension
feeders
• Trilobites
• Brachiopods without hinged shells (inarticulates)
• Small cap-shaped molluscs
• Soft-bodied worms
• Chitin-shelled arthropods
• Reef-building archaeocyathids
Later in the Paleozoic Era
• Trilobites
• Articulate (hinged)
brachiopods
• Nautiloids
• Crinoids
• Rugose (horn) corals
• Tabulate corals
• Branching twig-like
bryozoans (moss
animals)
• Vertebrates
– Fishes
– Amphibians
– Reptiles
Mesozoic Era
• Modern scleractinian
corals
• Bivalves
• Sea urchins
• Ammonoids
• Vertebrates
– Dinosaurs
– Primitive mammals
– Birds
Cenozoic Era
•
•
•
•
•
•
Molluscs of many types (but no ammonoids)
Planktonic foraminifera
Sea urchins
Encrusting bryozoans
Barnacles
Vertebrates
– Age of mammals
– Appearance of humans
– Many other vertebrate groups
Extinctions
Mass extinctions occurred at the ends of the
following periods:
• Ordovician
• Devonian - roughly 70% of marine invertebrates
extinct
• Permian - the greatest extinction. More than
90% of marine species disappeared or nearly
went extinct
• Triassic
• Cretaceous - affected dinosaurs, other land
animals, and marine organisms; about 25% of
all known animal families extinct
Evolutionary History of Plants
Evolutionary History of Plants
1. Earliest photosynthetic organisms were
single-celled organisms in the
Precambrian.
2. Green algae or chlorophytes may be the
ancestors of vascular land plants.
3. Plants invaded the land in the
Ordovician, reproducing with spores.
Evolutionary History of Plants –
cont’d
4. First plants with seeds appeared in the
Devonian. Gymnosperms (such as
conifers). Had pollen.
5. Carboniferous coal swamps dominated
by seedless, spore-bearing scale trees.
6. Flowering plants appeared in the
Cretaceous. Angiosperms. Dominant
plants today.