Background — Spontaneous
Today, we take many things in science for granted. Many experiments have
been performed and much knowledge has been accumulated that people didn’t
always know. For centuries, people based their beliefs on their interpretations
of what they saw going on in the world around them without testing their
ideas to determine the validity of these theories — in other words, they
didn’t use the scientific method to arrive at answers to their questions.
Rather, their conclusions were based on untested observations.
Among these ideas, for centuries, since at least the time of Aristotle
(4th Century BC), people (including scientists) believed that simple living
organisms could come into being by spontaneous generation. This
was the idea that non-living objects can give rise to living organisms.
It was common “knowledge” that simple organisms like worms, beetles, frogs,
amd salamanders could come from dust, mud, etc., and food left out, quickly
“swarmed” with life. For example:
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 Shakespear’s house). As a roof aged,
it was not uncommon for it to start leaking. This could lead to spoiled
or moldy grain, and of course there were lots of mice around.
Conclusion: It was obvious to them that the mice came from the moldy
Observation: In the cities, there were no sewers nor garbage trucks.
Sewage flowed in the gutters along the streets, and the sidewalks were
raised above the streets to give people a place to walk. In the intersections,
raised stepping stones were strategically placed to allow pedestrians to
cross the intersection, yet were spaced such that carriage wheels could
pass between them. In the morning, the contents of the chamber pots were
tossed out the nearest window. When people were done eating a meal, the
bones were tossed out the window, too. A chivalrous gentleman always walked
closest to the street when escorting a woman, so if a horse and carriage
came by and splashed up this filth, it would land on him, and not the lady’s
expensive silk gown. Most of these cities also had major rat problems which
contributed to the spread of Bubonic Plague (Black Death) — hence the story
of the Pied Piper of Hamelin, Germany.
Conclusion: Obviously, all the sewage and garbage turned into the
Observation: Since there were no refrigerators, the mandatory, daily
trip to the butcher shop, especially in summer, meant battling the flies
around the carcasses. Typically, carcasses were “hung by their heels,”
and customers selected which chunk the butcher would carve off for them.
Conclusion: Obviously, the rotting meat that had been hanging in
the sun all day was the source of the flies.
From this came a number of interesting recipes, such as:
Recipe for bees:
Kill a young bull, and bury it in an upright position so that its horns
protrude from the ground. After a month, a swarm of bees will fly out of
Jan Baptista van Helmont’s recipe for mice:
Place a dirty shirt or some rags in an open pot or barrel containing a
few grains of wheat or some wheat bran, and in 21 days, mice will appear.
There will be adult males and females present, and they will be capable
of mating and reproducing more mice.
In 1668, Francesco Redi, an Italian physician, did an experiment
with flies and wide-mouth jars containing meat. This was a true scientific
experiment — many people say this was the first real experiment
— containing the following elements:
After this experiment, people were willing to acknowledge that “larger”
organisms didn’t arise by spontaneous generation, but had to have parents.
With the development and refinement of the microscope in the 1600s, people
began seeing all sorts of new life forms such as yeast and other fungi,
bacteria, and various protists. No one knew from where these organisms
came, but people figured out they were associated with things like spoiled
broth. This seemed to add new evidence to the idea of spontaneous generation
— it seemed perfectly logical that these minute organisms should arise
spontaneously. When Jean Baptiste Lamarck proposed his theory of evolution,
to reconcile his ideas with Aristotle’s Scala naturae, he proposed
that as creatures strive for greater perfection, thus move up the “ladder,”
new organisms arise by spontaneous generation to fill the vacated places
on the lower rungs.
Observation: There are flies around meat carcasses at the butcher
Question: Where do the flies come from? Does rotting meat turn into
or produce the flies?
Hypothesis: Rotten meat does not turn into flies. Only flies can
make more flies.
Prediction: If meat cannot turn into flies, rotting meat in a sealed
(fly-proof) container should not produce flies or maggots.
Testing: Wide-mouth jars each containing a piece of meat were subjected
to several variations of “openness” while all other variables were kept
control group — These jars of meat were set out without lids
so the meat would be exposed to whatever it might be in the butcher shop.
experimental group(s) — One group of jars were sealed with lids,
and another group of jars had gauze placed over them.
replication — Several jars were included in each group.
Data: Presence or absence of flies and maggots seen in each jar
was recorded. In the control group of jars, flies were seen entering the
jars. Later, maggots, then more flies were seen on the meat. In the gauze-covered
jars, no flies were seen in the jars, but were observed around and on the
gauze, and later a few maggots were seen on the meat. In the sealed jars,
no maggots or flies were ever seen on the meat.
Conclusion(s): Only flies can make more flies. In the uncovered
jars, flies entered and laid eggs on the meat. Maggots hatched from these
eggs and grew into more adult flies. Adult flies laid eggs on the gauze
on the gauze-covered jars. These eggs or the maggots from them dropped
through the gauze onto the meat. In the sealed jars, no flies, maggots,
nor eggs could enter, thus none were seen in those jars. Maggots arose
only where flies were able to lay eggs. This experiment disproved the idea
of spontaneous generation for larger organisms.
In 1745 - 1748, John Needham, a Scottish clergyman and naturalist showed
that microorganisms flourished in various soups that had been exposed to
the air. He claimed that there was a “life force” present in the molecules
of all inorganic matter, including air and the oxygen in it, that could
cause spontaneous generation to occur, thus accounting for the presence
of bacteria in his soups. He even briefly boiled some of his soup and poured
it into “clean” flasks with cork lids, and microorganisms still
A few years later (1765 - 1767), Lazzaro Spallanzani, an Italian abbot
and biologist, tried several variations on Needham’s soup experiments.
First, he boiled soup for one hour, then sealed the glass flasks that contained
it by melting the mouths of the flasks shut. Soup in those flasks stayed
sterile. He then boiled another batch of soup for only a few minutes before
sealing the flasks, and found that microorganisms grew in that soup. In
a third batch, soup was boiled for an hour, but the flasks were sealed
with real-cork corks (which, thus, were loose-fitting enough to let some
air in), and microorganisms grew in that soup. Spallanzani concluded that
while one hour of boiling would sterilize the soup, only a few minutes
of boiling was not enough to kill any bacteria initially present, and the
microorganisms in the flasks of spoiled soup had entered from the air.
This initiated a heated argument between Needham and Spallanzani over
sterilization (boiled broth in closed vs. open containers) as a way of
refuting spontaneous generation. Needham claimed that Spallanzani’s “over-extensive”
boiling used to sterilize the containers had killed the “life force.” He
felt that bacteria could not develop (by spontaneous generation) in the
sealed containers because the life force could not get in, but in the open
container, the broth rotted because it had access to fresh air, hence the
life force inherent in its molecules, which contained and replenished the
life force needed to trigger spontaneous generation. In the minimally-boiled
flasks, he felt the boiling was not severe enough to destroy the life force,
so bacteria were still able to develop.
By 1860, the debate had become so heated that the Paris Academy of Sciences
offered a prize for any experiments that would help resolve this conflict.
The prize was claimed in 1864 by Louis Pasteur, as he published the results
of an experiment he did to disproved spontaneous generation in these microscopic
One very important point to note here is that Pasteur did not seek to find
an answer to the broad question, “Has spontaneous generation ever
occurred?” Rather, as any good scientist, he limited his scope to a very
narrow piece of the picture: “Is it possible for spontaneous generation
to occur given the specific conditions under which Needham (and
others) claims it will occur,” i.e. the “life force?” Interestingly,
in 1936, when Alexander Ivanovich Oparin, a Russian scientist, published
The Origins of Life, in which he described hypothetical conditions
which he felt would have been necessary for life to first come into existence
on early Earth, some scientists found it difficult to acknowledge that
under the very different conditions which Oparin was proposing for early
Earth, some form of “spontaneous generation” might indeed have taken place.
Observation(s): From Needham’s and Spallanzani’s experiments, it
was known that soup that was exposed to the air spoiled — bacteria grew
in it. Containers of soup that had been boiled for one hour, and then were
sealed, remained sterile. Boiling for only a few minutes was not enough
to sterilize the soup. Pasteur had previously demonstrated that the dust
collected by drawing air through a cotton ball contained large numbers
of bacteria, hence he knew that bacteria were present in the air and could
be filtered out by using a cotton ball. He also knew that bacteria would
settle out on the walls of a long, bent, glass tube as air was passed through
Question: Is there indeed a “life force” present in air (or oxygen)
that can cause bacteria to develop by spontaneous generation? Is there
a means of allowing air to enter a container, thus any life force, if such
does exist, but not the bacteria that are present in that air?
Hypothesis: There is no such life force in air, and a container
of sterilized broth will remain sterile, even if exposed to the air, as
long as bacteria cannot enter the flask.
Prediction: If there is no life force, broth in swan-neck flasks
should remain sterile, even if exposed to air, because any bacteria in
the air will settle on the walls of the initial portion of the neck. Broth
in flasks plugged with cotton should remain sterile because the cotton
is able to filter bacteria out of the air.
Testing: Pasteur boiled broth in various-shaped flasks to sterilize
it, then let it cool. As the broth and air in the containers cooled, fresh
room air was drawn into the containers. None of the flasks were sealed
— all were exposed to the outside air in one way or another.
control group — Some flasks opened straight up, so not only
air, but any bacteria present in that air, could get into them.
experimental group(s) — Pasteur used some flasks with long,
S-shaped necks (swan-neck flasks) and closed others with cotton plugs.
This allowed air to enter these flasks, but the long, swan neck or the
cotton balls filtered out any bacteria present in that air. He subsequently
broke the long necks off some of the swan-neck flasks.
replication — Pasteur used several flasks in each of his groups.
According to one freshman biology text, some of his original flasks, on
display (in France), still are sterile.
Data: Broth in flasks with necks opening straight up spoiled (as
evidenced by a bad odor, cloudiness in previously clear broth, and microscopic
examination of the broth confirming the presence of bacteria), while broth
in swan-neck flasks did not, even though fresh air could get it. Broth
in flasks with cotton plugs did not spoil, even though air could get through
the cotton. If the neck of a swan-neck flask was broken off short, allowing
bacteria to enter, then the broth became contaminated.
Conclusion(s): There is no such life force in air, and organisms
do not arise by spontaneous generation in this manner. To quote Louis Pasteur,
“Life is a germ, and a germ is Life. Never will the doctrine of spontaneous
generation recover from the mortal blow of this simple experiment.”
Alcamo, I. Edward. 1997. Fundamentals of Microbiology, 5th Ed. Benjamin
Cummings Publ. Co., Menlo Park, CA. (pp. 7-9)
Curtis, Helena. 1983. Biology, 4th Ed. Worth Publ. NY. (pp. 77-78, 238)
Lewis, Ricki. 1992. Life. Wm.C. Brown. Dubuque, IA. (p. 59)
Schroeder, Gerald L. 1990. Genesis and the Big Bang. Bantam Books. NY.
Slow Death of Spontaneous Generation (1668-1859)
Activity-Recreation of Pasteur’s
low-salt broth (chicken or beef, home-made or purchased)
2 250-mL Erlenmeyer flasks
2 1-hole rubber stoppers with bent glass tubing inserted (see diagram)
Students should work in teams of 2 to 3 people. Each team should perform
the following steps.
Mark Erlenmeyer flasks accordingly:
flask with stopper and glass tube going straight up
flask with stopper and glass tube bent in S-curve
Place about 50 mL of broth in each Erlenmeyer flask.
Place appropriate lids on flasks.
Boil broth in flasks with appropriate lids on them for 30 min., then let
For the next several lab periods, observe the flasks and record any changes
in color, turbidity, smell, etc.
Copyright © 1997 by D. B.
Fankhauser and J. Stein Carter. All rights reserved.