The History of Life

1
The History of Life
2
The Record of Life

• You may have seen thrilling movies or read books
  that describe travel in time. The characters
  often encountering strange organisms.
• Did the differences you saw surprise you?
• After all, everything changes over time.
• It is difficult to imagine what Earth might have
  been like that long ago!

3
Early History of Earth

• Early Earth was inhospitable!
• It was probably very hot, volcanoes might have
  erupted frequently spewing gases and lava
• These gases helped form Earths early atmosphere
  which contained little oxygen but abundant carbon
  dioxide and nitrogen
• About 3.9 billion years ago, earth might have
  cooled enough for water in the atmosphere to
  condense
• This would have led to millions of years of
  rainstorms, enough to fill Earths oceans
• It is in the oceans that scientist propose that
  the first organisms appeared between 3.9 and 3.5
  billion years ago

4
History in Rocks

• Scientist cannot be sure that earth formed in
  this way. There is no direct evidence of the
  earliest years of Earths history.
• The physical processes of Earth constantly
  destroy and reform rock.
• The oldest rocks that have been found on earth
  formed only about 3.9 billion years ago
• Rocks are an important source of information
  about the diversity of life that once existed on
  the planet

5
Fossils Clues to the Past

• The millions of species that live today are
  probably only a small fraction of all the species
  that ever existed.
• About 99 of species are extinct they no longer
  live on Earth
• Fossil evidence of an organism that lived long
  ago.
• Scientists use fossils to learn about ancient
  species.

6
Fossils Cont.

• A person who studies fossils is a Paleontologist
• Fossils can form in many different ways.
• Fossils are used to understand
• Events that happened long ago
• Kinds of organisms that lived in the past
• Behavior of organisms that lived in the past
• Ancient climate and geography

7
Could ferns have lived in Antarctica?

• Scientist have discovered fossil remains of ferns
  in the rocks of Antarctica. These fern fossils
  are related to ferns that grow in temperate
  climates on Earth today.
• How did they get there?
• Pangaea link

8
How fossils form

• Fossils occur in sedimentary rocks
• What is a sedimentary rock?
• Fossils form when organisms are buried in small
  particles of mud, sand or clay soon after they
  die.
• Over time, the particles compress and harden into
  sedimentary rock
• Fossils can still form today in the bottom of
  lakes, streams and oceans.

9
The Fossilization Process

• Describe how the movements of earth might expose
  a fossil.
• Why do you think that fossils are rare in igneous
  and metamorphic rock?
• Follow this link to see the process

10
Fossils in different layers of sedimentary rock
vary in age

• Ways of dating fossils
• Relative dating The deeper the layer of rock,
  the older the fossil is. Layers closer to the
  top are younger (Geologic law)
• Relative dating only gives you an estimate of
  age, not the actual age of a fossil
• Radiometric dating Uses radioactive isotopes in
  rocks or fossils (radioactive isotopes break down
  over time giving off radiation)
• Decay rate of the isotopes can measured half
  life
• Scientist use potassium-40 to date older samples
  (1.3 billion years) and carbon-14 to date fossils
  less than 50,000 years old

11
The Geologic Time Scale

• By examining layers of sedimentary rock and
  dating the fossils that are found in the layers,
  scientists have been able to put together a
  chronology (calendar) of the Earths history.
• Draw the geologic time scale on pg 383 and 384.
  Be sure to include major life forms, major
  events, the period, era and time in years on your
  time scale. Make your time line to scale.

12
How do we know how old fossils are?

• Relative Dating dating of fossils based on
  depth
• Layers at the surface are younger than those at
  deeper layers
• Just an estimate, not really accurate

13

• Radiometric dating uses radioactive isotopes in
  rocks or fossils
• Radioactive isotopes are atoms with unstable
  nuclei that decay over time giving off radiation
• Radioactive isotopes form a new element after
  they decay
• All radioactive isotopes have a characteristic
  decay rate which scientist use
• Half life time it takes for half of the
  radioactive isotope to decay into a new element
  used to measure decay rate

14

• Example
• When a rock forms it contains radioactive
  isotopes that decay to half of its original
  amount in 1 million years. Today, if a rock
  contained equal amounts of the radioactive
  isotope and the new substance, the rocks would be
  1 million years old.

15

• Scientists use potassium-40, a radioactive
  isotope that decays to argon-40, to date rocks
  containing very old fossils.
• Chemists have determined that potassium-40 decays
  to half its original amount in 1.3 billion years
• Scientist use carbon-14 to date fossils less than
  50,000 years old

16
History of Life - Review

• What was early earth like?
• How do we think oceans were formed? When?
• When and where do we think life began?
• How do we know about organisms from the past?
• How do fossils develop?
• How can we determine the age of a fossil?
• What four eras is the geologic time scale divided
  into? What types of organisms mark each era?
• The continents were at one time joined together
  in a landmass known as Pangaea. How long ago was
  that? Refer to your geologic time scale to
  understand when that occurred.
• Summarize the trend of how organisms developed
  during Earths history.

17
The Origin of Life

• Francesco Redis experiment (1668)
• How do the results differ in the two jars?
• What might you conclude from these results?
• Louis Pasteurs experiment finally disproved
  spontaneous generation (mid 1800s)

18
Which came first, the chicken or the egg?

• Biologists have accepted the concepts of
  biogenesis for more than 100 years
• Biogenesis does not answer the question of how
  life began on Earth
• No one will ever know for certain how life began
  on earth. We can only make hypotheses about
  conditions on early Earth.

19
How does chemistry become life??

• Evolution of complex molecules

20
Primordial Soup

• Step 1 simple organic molecules must have formed
  (molecules that contain carbon)
• Step 2 Simple organic molecules must have become
  organized into complex organic molecules
  (proteins, carbohydrates, and nucleic acids)
• 1930s Oparin (Russian) hypothesized that life
  began in the oceans
• Energy from sun and lightning triggered chemical
  reactions to produce small organic molecules from
  substances in the atmosphere
• 1953 Miller and Urey (American) replicated
  conditions in the lab and produced amino acids,
  sugars, and other molecules

21
The next step

• How did we get from simple organic compounds to
  complex organic compounds?
• In the presence of heat, without oxygen, amino
  acids link to from proteins. ATP is produced in
  a similar way.
• Sidney Fox found that if you continued to heat
  the complex organic compounds protocells
  developed.

22
The Evolution of Cells

• The first true cells
• Prokaryotes that evolved from a protocell
• Anaerobic (no oxygen present in atmosphere)
• Food organic molecules abundant in oceans
• Autotrophs evolved to inhabit harsh conditions
  (ie. Archaebacteria)

23
Endosymbiont Theory

• Eukaryotes probably evolved from prokaryote cells
• Theory proposes that eukaryotes evolved through a
  symbiotic relationship between prokaryotes
• Evidence
• Chloroplast and cyanobacteria resemble
  eachother
• mitochondria and bacteria some look similar
• Chloroplast and mitochondria contain DNA that is
  similar to the DNA in prokaryotes
• Today, some prokaryotes live in close association
  with eukaryotes

24
Life Today

• Could new life originate on Earth today?
• List the specific evidence that supports each
  conclusion.
• Life comes from existing life
• Life probably originated on Earth through the
  reaction of chemicals in Earths atmosphere and
  their further reaction on Earths surface
• Cells probably evolved as the chemicals on early
  Earth became more organized.