Chapter 12 History of Life on Earth
Section 1: How did life begin?
I.
The Early Earth was Lifeless
a. When Earth formed about ______________________ years ago, it was a fiery ball of molten
rock. It could not have supported the development of life. Eventually, the planet’s surface
___________ and formed a rocky crust. ______________________ in the atmosphere
condensed to form vast oceans. Many scientists think life first evolved in these oceans. Scientists
think the evolution of life took hundreds of millions of years to occur. Evidence that Earth has
existed long enough for this evolution to have taken place can be found by measuring the age of
Earth.
II.
Measuring Earth’s Age
a. Scientists are able to calculate the age of Earth through ________________________________.
b. Calculation of the age on an object by measuring the proportions of the radioactive isotopes of
certain _______________________
i. Radioactive isotopes, or ___________________________, are unstable elements that
break up and give off energy in the form of charged particles (________________).
c. By measuring how many ____________________ have passed since a rock was formed,
scientists can estimate a rock’s ________________.
i. The time it takes for one-half of a given amount of a radioisotope to change is called its
_______________________________.
III.
How the Basic Chemicals of Life were Formed
a. Most scientists think that life on Earth developed through natural chemical and physical
processes. __________________________________________ is the process through which life
is thought to have developed when molecules of nonliving matter reacted chemically during the
first billion years of Earth’s history. These molecules formed many different simple, organic
molecules. Energized by the sun and volcanic heat, these molecules formed more complex
molecules that eventually became the building blocks of the first cells. This hypothesis has been
tested and confirmed using scientific methods in laboratory experiments.
IV.
The “____________________________________________________________” Model
a. In the 1920s, Russian scientist A. I. ________________________ and British scientist J.B.S.
___________________________ each suggested that the early Earth’s oceans once contained
large amounts of organic molecules.
b. They both hypothesized that these molecules formed spontaneously in chemical reactions
activated by energy from solar radiation, volcanic eruptions, and lightning.
c. Oparin hypothesized the atmosphere lacked _____________________________ which allowed
hydrogen to react with other molecules.
d. In 1953, the primordial soup model was tested by _____________________________________
by placing the proposed gases into a device and stimulating lightning by providing electrical
sparks.
e. After a few days, Miller had a complex “______________________________________” which
included life’s basic building blocks: amino acids, fatty acids, and other hydrocarbons.
V.
Reevaluating the Primordial Soup Model
a. Recent discoveries of fossils indicate life started much earlier than first thought.
b. Without the protective ____________________________________, ultraviolet radiation would
have destroyed any ammonia and methane present in the atmosphere.
c. Without these molecules, the experiment is not successful in making key biological molecules.
VI.
The ________________ Model
a. In 1986, geophysicist Louis Lerman suggested that the key processes that formed the chemicals
needed for life took place within bubbles on the ocean’s surface.
VII.
Organic Chemicals Became Complex
a. Though many scientists disagree about the details of the process that led to the origin of life,
most scientists accept that by adding energy, the basic molecules of life could have formed
spontaneously through organic molecules and living cells. How did amino acids link to form
proteins? How did nucleotides form the long chains of DNA that store the instructions for
making proteins? In the laboratory scientists have not been able to make either of these
macromolecules form spontaneously in water. However, short chains of RNA, the nucleic acid
that helps carry out DNA’s instructions, have been made form on their own in water.
VIII.
A Possible Role for ___________________________
a. 1980s, Cech and Altman found that certain RNA molecules can act like enzymes.
b. Its _________________ shape provides a surface on which chemical reactions can be catalyzed.
c. Perhaps ________________was the first self-replicating information-storage molecule.
d. After such a molecule had formed, it could also have catalyzed the assembly of the first
______________.
e. Such a molecule would have been capable of _______________________ from one generation
to the next.
IX.
Microspheres May Have Led to Cells
a. Observations show that lipids, which make up cell membranes, tend to gather together in water.
b. Lab experiments have shown that short chains of amino acids tend to gather into tiny vesicles
called microspheres.
c. Scientists think that formation of microspheres might have been the first step toward cellular
organization.
X.
Origin of _____________________
a. Most scientists agree that double-stranded DNA probably evolved after RNA and that RNA
“enzymes” catalyzed the assembly of the earliest proteins.
b. Because researchers do not yet understand how DNA, RNA, and hereditary mechanisms first
developed, how life might have originated naturally and spontaneously remains a subject of
intense interest, research, and discussion among scientists.
Section 12-2: Complex organisms developed
I.
______________________________ Are the Oldest Group of Organisms
a. When did the first organisms form? To find out, scientists study the best evidence of early life
that we have fossils. A fossil is the preserved or mineralized remains or imprint of an organism
that lived long ago. The oldest fossils, which are microscopic fossils of prokaryotes, come from
3.5-billion-year-old rock deposits found in Western Australia.
b. Recall that prokaryotes are single-celled organisms that lack internal membrane-bound
organelles. Prokaryotes are also known as bacteria. Among the first of the bacteria to appear
were cyanobacteria. Cyanobacteria are photosynthetic bacteria. Before cyanobacteria appeared,
oxygen gas was rare on Earth. But as ancient cyanobacteria carried out photosynthesis, they
released oxygen gas into Earth’s oceans. After hundreds of millions of years, the oxygen
produced by cyanobacteria began to escape into the air. Over the billions of years that followed,
more oxygen was added to the air. Today oxygen gas makes up 21 percent of the Earth’s
atmosphere.
II.
Two Groups of Bacteria
a. _________________________
i. Prokaryotes that contain a chemical called ______________________ in their cell walls
ii. Have the same type of __________________ in their cell membranes as eukaryotes do
iii. Include many bacteria that cause ____________________________________________
iv. E. coli
b. _________________________
i. Prokaryotes that lack peptidoglycan in their cell walls
ii. Have ________________ lipids in their cell membranes
iii. Thought to be closely related to the first bacteria to have existed on Earth
iv. Sulfolobus
III.
The First ____________________ Evolved
a. About 1.5 billion years ago, the first ___________________________ appeared. Recall that a
eukaryotic cell has a complex system of internal membranes and has DNA that is enclosed
within a ______________________. A third characteristic of almost all eukaryotes is the
presence within the cells of complex bacteria-sized mitochondria and chloroplasts. These
organelles contain their own ___________________. Except for one rare primitive form, all
eukaryotes have _________________________________. Chloroplasts, which carry out
photosynthesis, are found only in _______________________________________________.
IV.
The Evolution of Mitochondria and Chloroplasts
a. _____________________________________ was first presented in 1966 by Lynn Margulis
b. It proposes that mitochondria are the descendants of symbiotic, aerobic ____________________
c. A second successful invasion of by photosynthetic bacteria gave rise to ____________________
V.
Multicellularity Evolved _________________ Times
a. The unicellular body plan has been tremendously successful, with unicellular organisms today
constitution about half the biomass on Earth. But a single cell has limits. Distinct types of cells in
one body allow an organism to specialize in different functions including protection from
diseases, predators, and environment or movement to find prey or mates. With all these
advantages, it is not surprising that multicellularity has arisen independently many times.
VI.
_____________________________________ Have a Major Impact
a. The fossil record indicates a sudden change occurred at the end of Ordovician period about 440
million years ago. A large percentage of the organisms on Earth suddenly became extinct. This
was the first of five major mass extinctions that have occurred on Earth. A mass extinction is the
death of all members of many different species, usually caused by a large ecological disaster.
i. 1st Extinction- 440 million years ago
ii. 2nd Extinction- 360 million years ago
iii. 3rd Extinction- 245 million years ago; worst of all mass extinctions
iv. 4th Extinction- 210 million years ago
v. 5th Extinction- 65 million years ago; bye bye dinosaurs
vi. Is a 6th happening right now? Many scientists think one is taking place right now with
the destruction of many of Earth’s ecosystems.
Section 12-3: Life Invaded the Land
I.
____________________ Enables Survival on Land
a. The sun provides both life-giving light and dangerous _________________________________.
Early in Earth’s history, life formed in the seas, where early organisms were protected from UV
radiation. These organisms could not leave the water because the UV radiation made life on dry
ground _____________________________. What enabled life-forms to leave the protection of
the seas and live on the land?
II.
Formation of the _____________________________
a. About 2.5 billion years ago, photosynthesis by _________________________ began adding
___________________________ to the atmosphere.
b. As oxygen reached the upper atmosphere, the sun’s rays caused ___________ to bond and from
molecules of __________________.
c. In the upper atmosphere, ozone ____________________ the UV rays of the sun.
III.
After _________________________ of years, the Earth’s land became a safe place to live.
a. Plants and ________________________ began living on the land together
b. Arthropods crawled out of the sea
c. _____________________________ followed onto land
d. Fishes evolved
e. Amphibians were the first _______________________________ on land
f. Reptiles arose from amphibians
g. Evolution of mammals and birds