Transcript Spontaneous Generation vs Biogenesis

The Birth of
Experimental Biology
Spontaneous Generation
vs.
Biogenesis
Spontaneous generation
Spontaneous generation was a widely-held
idea by both scientists and non-scientists
for a very long time.
 Historical documents contain “recipes” for
creating creatures such as: bees, frogs,
mice, etc.


These “recipes” were based on observations.


When the Nile river flooded, soon many frogs
appeared in the mud.
If damp rags were left in a pile of wheat grain, many
mice appeared 21 days later.
What did you see and how did it get
there? Talk to your partner and decide.
Spontaneous generation vs. Biogenesis

Spontaneous Generation: The belief
that non-living material(s) give rise to
living organisms.

Biogenesis: The principle that states
that all living things come from other
living things.
What do our experiments suggest?

What conclusion could be supported by
our data?
Spontaneous Generation is put to the test

Francesco Redi (1668):


The first scientist to design and
carry out a controlled
experiment to test the idea of
spontaneous generation.
Redi’s hypothesis (informal):
 Flies appear near rotting meat,
not because the meat gives rise
to flies, but because flies lay eggs
on rotting meat.
Redi’s Experiment

Control Group

Meat left in jars with no
covering.

Experimental Group

Meat left in jars that
were covered.
Identify the Variables of Redi’s Experiment

Independent Variable:



Dependent Variable:



Remember, it’s what differs between the control and
experimental groups.
Covering on Jar
Remember, it’s what will be measured or observed to
see if the hypothesis is supported or disproved.
Appearance of flies inside the jar
Controlled Variables:


Things that are kept the same in both groups.
Type and size of jar, type and amount of meat,
location of jars, time period, etc.
Redi’s Results and Conclusion

The results of Redi’s experiment supported his
hypothesis. No flies appeared on the meat in
the covered jars; if flies did not land on and
lay eggs on the meat, no flies were
produced.

People were able to accept the idea that “large”
organisms, could not arise spontaneously.



such as flies, mice, and frogs
Some still wondered about the spontaneous generation
of microbes (microscopic organisms).
Score:
Spontaneous Generation
Biogenesis
0
1
Microbes




Microbes are microscopic (very small), typically
unicellular, organisms.
Examples: yeast, bacteria, etc.
The invention of the microscope allowed
scientists their first glimpse at a world of living
creatures that surround us every day, yet go
unnoticed.
Many people, including scientists, thought that
these simple organisms could arise from pond
water, air, etc.
 In fact, if you place a bit of hay in water and
look at the water under the microscope,
initially it will be free of microorganims. 2-3
days later it will be teeming with them.
Another try, another hypothesis
We will put hay in this water. When we
learn to use microscopes, we will look to
see what happens.
 What should be our control group?
 What should be our experimental group?
 What do you think we will find at the end
of a couple of days?

The Spontaneous Generation debate
“heats up”!

Lazzaro Spallanzani (1767):


a scientist that set up an experiment to
test whether a “vital force” in the air
could give rise to microorganisms.
Spallanzani’s hypothesis (informal):

Beef broth becomes cloudy due to bacteria
present in the air. The bacteria from the air
enter the broth and contaminate it, rather
than the bacteria arising from the air or
broth itself.
Spallanzani’s Experiment

Control Group:


Flask with boiled broth
left open
Experimental
Group:

Flask with boiled broth
sealed tightly.
Spallanzani’s Conclusion


The result of Spallanzani’s experiment supported his
hypothesis. The beef broth only became contaminated
with bacteria (cloudy)if the broth was exposed to air
that contained living microorganisms.
 For many people, this experiment confirmed that simple
organisms do not spontaneously arise either.
 But the debate wasn’t settled yet! A fellow scientist,
and others, said that the boiling killed the “life
force” of the air in the flask. Supposedly the
“altered” air inside the sealed flask lacked the
“life force” and was not able to give rise to living
bacteria.
Score:
Spontaneous Generation 0
Biogenesis
2
The Big Prize



The Paris Academy of Science offered a
prize to the person who could best
resolve the spontaneous generation
debate once and for all.
And the winner was…
Louis Pasteur (1864):

A scientist who designed an experiment
using a flask that would allow air
containing the “vital force” to enter the
flask, but would keep microorganisms
out.
Pasteur’s Experiment

Designed a modified flask.

The flask had a long neck that was curved
like a side-ways “S”. (Control Group)



The curved-neck flask allowed outside air to move
into the flask containing boiled broth.
The curve in the neck forced solid particles, such as
microorganisms, to become trapped; they could not
enter the flask.
More than a year later, for comparison,
Pasteur broke the neck off of a flask.
(Experimental Group)
Pasteur’s Experiment
Pasteur’s Conclusion
The results of his experiment supported Pasteur’s
hypothesis. His curved-neck flask remained
sterile for a year. This proved that air had no
force with which to create life from non-living
material. If the flask was opened, it quickly
became contaminated by bacteria already
present in the air.
 Score:
Spontaneous Generation 0
Biogenesis
3
The debate was finally settled! Biogenesis remains
a key principle of Biology.
