Transcript Evolution Unit

Evolution Unit
Chapter 14 The History of Life
What do scientists think it was like on Earth billions of years ago?
Where conditions different than they are today?
What were the first life forms?
http://www.cosmic-art.co.uk/graphics/earlyearth.jpg
Early Earth and the Origin of Life
Some slides from:
http://www.eeescience.utoledo.edu/Faculty/Heckathorn/EEES%202150/Lecture%209%20%20evolution%20of%20life.ppt#48
We’ll discuss:
• Biogenesis or Spontaneous Generation
• Earth’s History
• The First Cells
(we won’t discuss natural selection or evolution until chapter 15)
I. Spontaneous Generation
• Until the 17th & 18th centuries,
spontaneous generation was used to
explain how new life started, many
cases such as simpler life forms
Spontaneous Generation
• Is the idea that non-living objects
can give rise to living organisms.
– It was common “knowledge” that worms, flies,
beetles, frogs, etc came from mud, food left out.
– Note that while it may be our first reaction to think
those earlier folks were “not so bright”,
– They drew conclusions based on their observations.
– They did not use the scientific method.
Spontaneous Generation
examples of thinking:
Don’t write thisjust read it!
• Observation: Every year in the spring, the Nile River
flooded areas of Egypt along the river, leaving behind
nutrient-rich mud that enabled the people to grow
that year’s crop of food. However, along with the
muddy soil, large numbers of frogs appeared that
weren’t around in drier times.
– Conclusion: It was perfectly obvious to people back
then that muddy soil gave rise to the frogs.
• Observation: In many parts of Europe, medieval
farmers stored grain in barns with thatched roofs
(like Shakespeare’s house). As a roof aged, it started
leaking. This could lead to spoiled or moldy grain, &
lots of mice were around.
– Conclusion: It was obvious to them that the mice
came from the moldy grain.
Biogenesis:
• The principle that states that
living things come only from
other living things.
• Remember: cell theory
How did Scientists prove that
spontaneous generation did not occur?
• Experiments by:
–Francesco Redi (1626-1697)
–Lazzaro Spallanzani (1729-1799)
–Louis Pasteur (1822-1895)
Redi’s Experiment
• IDEA: People believed maggots spontaneously
generated from rotting meat.
• Experiment- a net covered jar of meat
– it kept adult flies away & the jar had no maggots.
• Control jar- no net covering over meat- had maggots.
• Result- some people no longer supported
spontaneous generation of flies.
“Vital Force” & Microbes
• The microscope was invented about the time
Redi did his experiment.
• “Vital force” –Idea that air has a “force”
&“simple” life forms arose spontaneously.
http://www.jasa.net.au/images/peopled.htm
Spallanzani’s Experiment:
• Experiment: broth was heated & flask is sealed
• Control flask: Broth heated but flask left open.
• Result: No growth in sealed flask.
• Spallanzani concluded that growth occurred only
if microorganisms from the air contaminated it.
• Opponents- said he had
destroyed the
“vital force” with heat.
Spallanzani’s Experiment:
By mid- 1800’s- the debate about
spontaneous generation was fierce.
• Paris Academy of
Science offered prize if
an experiment could
prove one side.
• Louis Pasteur entered
the contest
Louis Pasteur
Pasteur’s Experiment
“curve neck flask”-
• Pasteur made a
-prevent microbes, but not air, from entering flask.
• Boiled flask- no growth -if neck broken=growth.
• Pasteur won the prize & ended the debate.
So Louis Pasteur
Proved
Biogenesis:
the concept
that life only
comes
from life.
Louis Pasteur
• A benefactor of humanity, his work
founded modern medicine, microbiology.
• His experiments supported the
germ theory of disease.
• Solved mysteries of rabies, anthrax, typhoid &
cholera diseases & developed the first vaccines.
• Pasteurization: killing microbes by heating food
• He hated microbes- 2 of his children died of typhoid.
• Rather than lose his Christian faith, as Charles Darwin did when
his child died, Pasteur spent his life working to help others.
http://www.accessexcellence.org/RC/AB/BC/Louis_Pasteur.php
Wikipedia.com
II. Earth’s History
1. The earth was formed ~4 billion years ago
2. It took ~500 million years for the crust to solidify.
3. The oldest fossils of microorganisms
• 3.5 billion years old,
• embedded in rocks in western Australia
Prokaryotes dominated from 3.5 to 2 billion years ago.
Earth’s History
1. The Formation of the Earth
–
About 5 billion years ago, solar
system was probably a swirling mass
of gas & dust.
2. Earth’s Age
– Estimates are: about
4 billion years old.
–
1.
2.
How did scientists come up with this
age since it was so long ago?
Sedimentary rock layers
Radiometric dating
http://www.aerospaceweb.org/question/astronomy/q0247.shtml
Radiometric dating
• is the process by which the
approximate age of an object is
determined based on the mass of
certain radioactive isotopes
present.
Radiometric Dating
• Isotopes are atoms with varying numbers of
neutrons.
• So- the ages of rocks & other materials can be
determined by measuring the amount of
radioactive isotopes found in samples
• An isotopes Half-life is the time that one-half
of a sample of the isotope takes to decay.
Some isotopes used for radiometric dating
Isotope:
Half-life:
•
•
•
•
5730 years
704,000,000
1,250,000,000
4,500,000,000
Carbon-14
Uranium-235
Potassium-40
Uranium-238
What is the half life of strontium -90?
How much strontium-90 is left after 2 half lives?
Number of
Half-lives
Time (years)
% Strontium90 remaining
% Strontium90 that has
decayed
0
1
2
0
28
56
100
50
25
0
50
75
3
4
5
84
112
140
12.5
6.25
3.125
87.5
93.75
96.875
6
168
1.5625
98.4375
http://www.ausetute.com.au/halflife.html
http://www.ausetute.com.au/halflife.html
3. First Organic Compounds
• Early Earth’s atmosphere: Probably had an
atmosphere with CO2 (similar to Venus) and
compounds like ammonia( NH3) hydrogen gas (H2),
water vapor (H20) and methane (CH4)
“ Reducing atmosphere”
• Today’s Atmosphere: Contains 21% oxygen –
probably from photosynthesis.
“Oxidizing Atmosphere”
1. Alexander Oparin(1894-1980)- hypothesis that
First Simple Organic Compounds- may have been able
to form in oxygen free atmosphere.
2. Miller/Urey Experiment
• By the 1950s, scientists were in “hot pursuit” of the
origin of life.
• In 1953, Stanley L. Miller & Harold C. Urey,
working at the University of Chicago, conducted an
experiment that tested Oparin’s hypothesis
• They took molecules which were believed to represent
the major components of the early Earth's atmosphere
and put them into a closed system, added “lightening”
• Results: could synthesize organic compounds like
AMINO ACIDS
• Gases used were: 1. methane (CH4), 2. ammonia(NH3)
3. hydrogen (H2), and water (H2O).
• Ran electric current through to simulate lightning
storms (believed to be common on the early earth.)
• Observed that as much as 10-15% of the carbon was
now in the form of organic compounds. 2% of carbon
formed some of the amino acids which make proteins.
http://www.chem.duke.edu/~jds/cruise_chem/Exobiology/miller.html
Other theories
• There is a lot of controversy about whether the
early earth atmosphere was really as “reducing”
(enough that amino acids could be produced) as
Miller/Urey made their experiment. Probably too
extreme.
• Some scientists hypothesize that organic compounds
could have been carried to Earth by debris from
space on Meteorites
( the Earth was frequently impacted and organic
compounds could have accumulated this way)
This 4.5 billion-year-old rock, labeled meteorite ALH84001, is believed to have once been
a part of Mars and to contain fossil evidence that primitive life may have existed on Mars
more than 3.6 billion years ago. The rock is a portion of a meteorite that was dislodged
from Mars by a huge impact about 16 million years ago and that fell to Earth in Antarctica
13,000 years ago. The meteorite was found in Allan Hills ice field, Antarctica, by an annual
expedition of the National Science Foundation's Antarctic Meteorite Program in 1984. It is
preserved at the Johnson Space Center's Meteorite Processing Laboratory in Houston.
How did science explain molecule
to cell jump? Some ideas:
Spheres of proteins organized as membrane
• Microspheres
Droplets which contain organic molecules
• Coacervates
• These do not have the properties of life.
What came
st
1 -
The cell, the DNA or
the RNA?
• RNA may have been first.
• RNA may act as enzymes, templates,
& can self- replicate.
• Early cells may have been the basis
for early cell genetics.
III. The first cells
How were conditions on Earth billions of
years ago different than they are today?
Early Earth conditions:
- little or no Oxygen available
- many organic molecules
EARLY EARTH:
Iron-containing rocks recovered from
period strata contain no rust, indicating no
oxygen.
The earth was very hot, evaporating the
liquid water into the atmosphere.
As the earth cooled, water became lakes and
oceans.
It was also believed that tectonic activity
caused many volcanic eruptions at that time
Banded iron formations are evidence of the age of oxygenic
photosynthesis – approximately 2 BYA in photo
Early Earth Atmosphere
compared to today:
• Then (more than 4 billion years ago):
– nearly no oxygen
– Carbon dioxide
– Nitrogen
• Now:
–78% nitrogen
–21% oxygen
KNOW THIS!
– Trace amounts of other gases & water vapor.
First Cells Characteristics:
• No direct evidence - but there are fossils
that seem to be bacteria & support the
idea that the first cells were probably:
• ANAEROBIC
• HETEROTROPHIC
• PROKARYOTIC
Early and modern prokaryotes
Beginnings of PHOTOSYNTHESIS
• Some life (cyanobacteria) may have become
photosynthetic due to competition for
nutrients about 3 billion years ago.
• Oxygen is a waste product of
photosynthesis
• So: almost all the oxygen
In today’s atmosphere is a
product of photosynthesis.
(It may have taken a billion years for oxygen
to reach current atmospheric levels)
http://www.abc.net.au/science/news/img/environment/cyanobacteria22080
Cyanobacteria
Formula for photosynthesis:
6CO2 + 6H2O + light energy
C6H12O6 + 6O2
Carbon dioxide + water + sunlight
produces
glucose
+ oxygen
http://user.uni-frankfurt.de/~schauder/cyanos/image9.jpg
Fossilized evidence of bacteria
Stromatolites are fossilized
bacterial mats. Many fossils of
prokaryotes are found in layers
that make up the prokaryotic
mats.

Bacterial mats and
stromatolites
Autotrophs (review)
• Organisms that make their own food
• Autotrophic organisms may be:
– photosynthetic (using sunlight)
– chemosynthetic (using chemicals
like sulfur, iron , etc)
Accumulation of Oxygen:
• As oxygen accumulated in the atmosphere,
the ozone layer (O3) appeared:
• Ozone protects living organisms
from UV (ultraviolet) radiation
from the sun.
The First Eukaryotic Cells
• Endosymbiosis- the theory that a
small aerobic prokaryotic cell was
engulfed by a larger anaerobic
prokaryote and began to live &
reproduce inside it.
• Evolved into modern mitochondria, or
chloroplasts & other organelles.
Some major
episodes in the
history of life.
Clock
analogy for
some key
events in
evolutionary
history
Timeline earth development
www.geology.wisc.edu/.../Earliest.html
http://earth.geology.yale.edu/~avf5/teaching/Files_pdf/Timeline.jpg
Killer Volcanic Eruptions
Ushered in Dinosaurs
• Dinosaurs first came to be
200 million years ago after
volcanic eruptions killed off
their rivals, long before the
giant creatures were wiped
from the face of the Earth by a meteorite.
• A climatic catastrophe that saw an abrupt rise in
atmospheric gases, coupled with powerful volcanic
eruptions decimated crurotarsans, creatures closely
related to today's crocodiles, according to a study led
by Brown University paleobiologist Jessica Whiteside.