Evolution introduction

Biogenesis vs. Spontaneous Generation
Earth’s History

Biogenesis vs. Spontaneous
Generation
 Definition: All living  Definition: Living things
(biotic) things come can arise (come
from other living things. from)nonliving (abiotic)
 Common sense today, things.
not believed in the 17th  Observations that
century. supported the belief:
 Experiments conducted Maggots appeared on
to prove biogenesis. rotting meat, fish
appeared in seasonal
ponds.

Francesco Redi - 1668
 Addressed maggots appearing on meat.
 Initial observation: Maggots appear on meat after flies
have landed.
 Hypothesis: Maggots come from flies NOT meat.
 Experiment: Placed meat in an open jar and in a jar that
was covered with cheese cloth.

 Control Group

 Experimental Group

More Proof Needed…
 Microscope was invented around same time as Redi’s
experiment.
 Scientists began observing ‘tiny creatures’.
 Accepted Redi’s conclusion but not for microscopic
organisms.
 Believed microorganisms arise spontaneously from a
“vital force” in the air.

Lazzaro Spallanzani(1729 -1799)
 Initial observation: Microorganisms grow easily in
food.
 Hypothesis: Microorganisms formed not from air but
from other microorganisms.
 Experiment: Placed broth in two flasks, boiled broth,
one flask left open, the other sealed closed.
 Control Group:

 Experimental Group:

Still not convinced…..
 Spallanzani concluded that the boiled broth became
contaminated only when microorganisms from the air
entered the flask.
 Opponents objected to his method and disregarded
his conclusion.
 Opponents claimed that Spallanzani had heated the
flasks too long and destroyed the “vital force” in the air
inside the flasks.
 Belief in spontaneous generation continued for
another 100 years. Until………

Controversy grows Fierce!
 By mid-1800’s the arguments over S.G. were fierce.
 Paris Academy of Science offered a prize (equivalent to
$1 million today) to anyone who could clear up the
issue once and for all.
 The winner: Louis Pasteur.
 Why is that name familiar to you?

Pasteur’s Experiment
 Observations and hypothesis were the same as
Spallanzani’s.
 To answer the objections to Spallanzani’s experiment,
Pasteur made a curve-necked flask.
 Air from outside can mix with inside.
 Curve in the neck prevented solid particles, such as
microorganisms, from entering the body of the flask
with the broth.
 Finally, BIOGENESIS became a cornerstone of biology.

Formation of Earth
 Earth is believed to be 4.56 billion years old.
 Earth took 400 million years to form from gas, dust
and debris circling the sun.
 Earth grew as it was bombarded with debris.
 Each collision released enough energy to melt the
surface of the forming planet.
 Estimates of Earth’s age made from studying layers of
sediment in the crust.
 Accurate estimate through radioactive dating.

Radioactive (Radiometric) Dating
 Isotopes: atoms of the same element (same atomic
number or number of protons) but differ in number of
neutrons.
 Mass number = protons + neutrons
 Is0topes are designated by chemical name followed by
their mass number.
 Example: carbon -12 and carbon- 14.
 Radioactive Decay: isotopes with unstable nuclei
(protons and neutrons in center) tend to release
particles or radiate energy (decay)

Radioactive Dating cont.
 Rates of decay of many radioactive isotopes have been
determined.
 Half-life: the length of time it takes for ½ of any size
sample of an isotope to decay.
 Can range from fraction of a second to billions of years
but is specific to each isotope.
 Age of material is determined by measuring amount of
a particular radioactive isotope it contains and
comparing it to the amount of some other substance in
the sample that remains constant.

Radioactive Dating cont.
 http://science.discovery.com/videos/100-greatest-
discoveries-shorts-radiometric-dating.html
 If the history of the earth were condensed to a 24 hour
clock, how long have humans been on the planet?

First Organic Compounds
 All elements found in organic compounds are thought
to have existed on Earth and in the rest of the Solar
System when Earth formed.
 How and where did these elements assemble into
organic compounds found in life?
 Oparin hypothesized in 1923, Urey and Miller
experimented in 1953 to test Oparin’s hypothesis.
 http://science.discovery.com/videos/100-greatest-
discoveries-shorts-origin-of-life.html

The first cells.
 Little to no oxygen gas when they first arose so…….
 The first cells were anaerobic.
 Small size of the oldest microfossils indicate the first
cells were prokaryotes (no nucleus).
 First cells were heterotrophs, consuming organic
molecules from their environment.
 Autotrophs evolved due to strong pressure in the
environment (competition for food).
 First autotrophs performed chemosynthesis.

Photosynthesis and Aerobic
Respiration
 Photosynthesizing autotrophs evolved and put oxygen
into the atmosphere (around 3.5 billion years ago).
 Took billions of years for oxygen gas to reach today’s
levels.
 Oxygen was dangerous to many early organisms
because it could destroy coenzymes essential to cell
function.
 Oxygen bonded to other compounds in some
organisms, preventing damage, this was the first step
to aerobic respiration.

The first eukaryotes.
 2 to 1.5 billion years ago.
 Small aerobic prokaryote entered and began to live and
reproduce inside larger, anaerobic prokaryotes.
 Mutually beneficial relationship – endosymbiosis
 Small aerobic prokaryote = today’s mitochondria
(remember they have their own DNA)
 Similar situation for chloroplast in photosynthetic cells.
 http://videos.howstuffworks.com/discovery/29535-
assignment-discovery-prokaryotes-the-first-cells-
video.htm

Evolution introduction

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Evolution introduction