Chapter 8: History of Life on Earth
Objectives for the Chapter:
1. Summarize how radioisotopes
Earth's age.
2. Compare 2 models that descri
organized.
3. Describe how cellular organiza
4. Recognize the importance tha
to the development of life. 5. Distinguish between 2 groups
6. Describe the evolution of euka
7. Recognize an evolutionary adv
8. Summarize how mass extincti
of life on Earth.
9. Relate the development of ozo
land.
10. Identify the first multicellular
11. Name the first animals to live
12. List the first vertebrates it lea
s can be used to determine
ibe how the chemicals of life
ation might have begun.
at a mechanism for heredity has
s of prokaryotes.
aryotes.
vance first seen in protists.
ions have affected the evolution
one to the adaptation of life on
r organisms to live on land.
e on land.
ave the oceans.
Vocabulary
Radiometric dating
Radioisotope
Half-life
Fossil
Cyanobacteria
Eubacteria
Archaebacteria
Endosymbiosis
Protist
Extinction
Mass extinction
Mycorrhizae
Mutualism
Arthropod
Vertebrate
Continental drift
Assignment:
Using the picture on the
front cover, hypothesize
what you think the Earth
conditions were like 4.5
billion years ago.
15 points
The Age of the Earth
When the Earth formed, about 4.5 billion years
ago, it was a fiery ball of molten rock.
Eventually, the planet's surface cooled and
formed a rocky crust. Water vapor in the
atmosphere condensed to form the vast
oceans. Most scientists think life first evolved
in these oceans and all life evolved from there.
By measuring the age of objects, we can get a
measurement of Earth's age. This is called
radiometric dating (the measurement of an
object's age by measuring its content of
radioactive isotopes). An isotope is a form of
an element whose atomic mass differs from
that of other atoms of the same element.
Radioactive isotopes, or radioisotopes, are
unstable isotopes that break down and give off
energy in the form of radiation. The radiation is
measured and compared to the amount of time
it takes for that element to decay completely.
The time it takes for 1/2 of a given amout of a
radioisotope to decay is called its half-life.
Scientists can then compute how many halflives have past since the rock was formed.
Formation of the Basic
Chemicals of Life
Most scientists think life on Earth developed
through natural chemical and physical
processes. Discussion of 2 models, on the next
page, will help us better understand this.
The "Primordial Soup" Model
In the 1920s, A.I. Oparin and J.B.S. Haldane
suggested that the Earth's oceans contained
large amounts of organic molecules, like soup
is filled with many different vegetables and
meat. They hypothesized that these molecules
formed spontaneously in chemical reactions
activated by the sun, volcanic eruptions, and
lightning.
Along with Harold Urey, they also proposed the
Earth's atmosphere lacked oxygen. Instead, it
was rich in nitrogen gas, hydrogen gas, water
vapor, ammonia, and methane. In 1953,
scientist Stanley Miller tested their hypothesis
by placing all the above gases in an apparatus
(on the next page) and used electricity sparks
to mimic lightning. He found simple organic
molecules could be formed this way.
Reevaluating the Miller-Urey Model
Recent discoveries have caused scientists to
reevaluate the Miller-Urey experiment. We now
know that the reductant molecules used in their
experiment could have not existed in abundance
on early Earth. 4 billion years ago the Earth did
not have a protective layer of gas, ozone.
Without ozone, UV radiation would have
destroyed any ammonia or methane present in
the atmosphere. When these gases are absent
from the Miller-Urey experiment, organic
molecules cannot be made. So where did they
come from?
The Bubble Model
In 1986, Louis Lerman suggested that they key
processes that formed the chemicals needed for
life took place within the bubbles on the ocean's
surface. Find the steps to the process below.
Precursors of the First Cells
Scientists disagree about the details of the
process that led to the origin of life, but most
have accepted under certain conditions, the basic
molecules of life form spontaneously. The
question is, since the process can not be
duplicated in a laboratory setting, where did the
first chain of RNA, eventually making DNA, come
from?
Thomas Cech believed
RNA molecules can act as
enzymes which provides
a surface in which
chemical reactions can
occur on. He
hypothesized RNA was
the first self-replicating
information-storage
molecule and it catalyzed
the assembly of the first
proteins. In 1989, he
received a Nobel Prize
Award for his work.
Although many scientists disagree about the
details of the origin of heredity, many agree
double-stranded DNA evolved after RNA. Each
day more and more experiments and evidence
collection is taking place and a single, new
discovery could lead scientists to the answer.
Assignment:
Answer the following
questions and email your
responses to
[email protected].
The subject of the email
should be Chapter 8,
Assignment 1.
1. Explain how radioisotopes
are used to determine the age
of a rock.
2. Critique 2 scientific models
that explain the origin of life.
3. Describe the first step that
may have lead to cellular
organization.
10 points
The Evolution of Prokaryotes
When did the first organisms form? To find out,
scientists study the best evidence of early life
forms: fossils. Fossils are the preserved or
mineralized remains or imprints of and organism
that lived long ago. Recall, prokaryotes are
single-celled organisms that lack internal
organelles. Among the first to appear were
marine Cyanobacteria. Cyanobacteria are
photosynthetic prokaryotes that can produce
their own energy. The beginnings of oxygen on
Earth is credited to them. Through
photosynthesis, oxygen was produced as a byproduct. Eventually, enough oxygen was
produced to form our ozone and to allow
organisms to revolve outside of water.
2 Types of Prokaryotes
1. Eubacteria - prokaryotes that contain
peptidoglycan in their cell walls. Many
eubacteria cause disease and decay.
2. Archaebacteria - prokaryotes that lack
peptidoglycan in their cell walls. They have
uniques lipids in their cell membranes.
Th Evolution of Eukaryotes
Eukaryotes began appearing about 1.5 billion
years ago. Recall, eukaryotes are organisms
that have organelles in their cells, like a
nucleus and mitochondria. Other eukaryotes,
like plants and protists, have chloroplasts.
Many biologists think that those organelles
(mitochondria and chloroplasts) originated
through a theory called endosymbiosis. This
theory was proposed by Lynn Margulis in 1966
and states mitochondria are the descendants of
symbolic, aerobic eubacteria and chloroplasts
are the descendants of symbiotic,
photosynthetic eubacteria.
Multicellularity
Many biologists group all living things into 6
broad categories called kingoms. The 2 oldest
kingdoms are eubacteria and archaeabacteria
that are sometimes grouped together in a
kingdom called Monera. The first eukaryotic
kingdom was the kingdom Protista, which
includes all protists. This kingdom includes
both single-celled and multicellular organisms.
The development of multicellular organisms
was an important milestone in the evolution of
life. The other 3 kingdoms, Plantae, Animalia,
and Fugi, evolved later.
Origins of Modern Organsisms
Most animal phyla that exists today originated
during a relatively short time during the late
Precambrian and early Cambrian periods. This
rapid diversification of animals is sometimes
known as the "Cambrian Explosion.". In 1909,
scientists uncovered a multitude of fossils in the
Burgess Shale in Canada. Many of the fossils
found were from strange organisms not alive
today. For example, a Trilobitie, which is a
marine organism.
Mass Extinctions
The fossil record indicates throughout Earth's
history there has been periods where all of a
single species dies (extinction) and also other
times when many species die at the same time
(mass extinction). 5 total mass extinctions have
occurred in the past and scientists think we are
currently in our 6th mass extinction. This is
because the Earth's ecosystems are being
destroyed hand over fist for human development
and advancement.
Assignment: Answer the following questions
and email responses to
[email protected]. The
subject of the email should be
Chapter 8, Assignment 2.
1. Contrast 2 major groups of
prokaryotes.
2. Explain Margulis's theory of
endosymbiosis.
3. Compare bacteria with
eukaryotes.
4. Justify the argument that
today's organisms would not
exist if mass extinctions had not
occurred. 10 points
Life Invaded the Land
The first multicellular organisms to live on land
may have been fungi living together with plants
or algae. Such paired organisms were able to
live on land because each group possessed a
quality needed by the other.
Early plants and
fungi formed a
biological
partnership
called
mycorrhizae
which enabled
them to live in
rocky
environments.
Fungus
provides
minerals and
plants provide
energy.
This kind of relationship is called mutualism. In
a mutualistic relationship, both species benefit.
Plants and fungi began living together on land
about 430 million years ago.
Arthropods
After 100 million years of fungi and plant union,
the Earth was covered in dense forests. Theses
forests were able to provide a food source not
yet seen on our planet, paving the way for landdwelling animals. The first of those animals were
the Arthropods. An arthropod is an animal with a
hard outer skeleton, a segmented body, and
paired, jointed limbs. Examples include lobsters,
crabs, insects, and spiders.
A unique
kind of
terrestrial
arthropod the insectevolved the
ability to fly.
They have
grown to
become the
most
plentiful
and diverse
group of
animals in
Earth's
history.
Fishes
Fishes were among the first of the vertebrates
(organisms with a backbone) on our planet.
The first of their kind were the jawless fishes
which appeared 530 million years ago. Jawed
fishes followed about 430 million years ago. In
the beginning, vertebrates stayed in the oceans,
but eventually evolved the ability to live outside
of water. Fishes are the most abundant and
successful living vertebrates today. They make
up over 1/2 of all living vertebrates.
Amphibians
Amphibians appeared 370 million years ago as
the first vertebrates to live out of the sea.
Their characteristics included smooth skin, 4
legs, skeleton for walking, and lungs. Examples
are frogs, newts, salamanders, and toads. Even
though they adapted to lie outside the water,
amphibians need a moist environment to live in.
Reptiles
Reptiles evolved from amphibians about 340
million years ago. Examples include snakes,
lizards, turtles, and crocodiles. Reptiles are
better suited for dry environment because they
have scaled skin that traps moisture and they
lay water-tight eggs.
Mammals and Birds
Birds evolved from featured dinosaurs during or
after the Jurassic period. Therapsids gave raise
to mammals about the same time dinosaurs
evolved. Birds have sophisticated flight, eggs
with hard shells, and feathers. Mammals have
mammary glands, hairs, and teeth. Humans
evolved later with growing intelligence.
Assignment: Answer the following questions
and email responses to
[email protected]. The
subject of the email should be
Chapter 8, Assignment 3.
1. Summarize why ozone was
important in enabling life on
land.
2. Name the first multicellular
organisms that colonized land.
3. Identify the first kinds of
animals to live in land.
4. Describe the first kinds if
vertebrates that inhabited land. 10 points
End of Chapter 8