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 grain.
- 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,
- Conclusion: Obviously, all the sewage and garbage
turned into the rats.
- 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
- 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 the corpse.
- 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
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 shop.
- 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
- Testing: Wide-mouth jars each containing a piece of
meat were subjected to several variations of “openness” while
all other variables were kept the same.
control group — These jars of meat were set out without
lids so the meat would be exposed to whatever it might be in the
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 grew
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 organisms.
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
- 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 it.
- 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
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
- 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.
- Lewis, Ricki. 1992. Life. Wm.C. Brown. Dubuque, IA. (p. 59)
- Schroeder, Gerald L. 1990. Genesis and the Big Bang. Bantam
Books. NY. (pp. 107-110)
Death of Spontaneous Generation (1668-1859)
of Pasteur’s Experiment
- 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
- 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 cool.
- For the next several lab periods, observe the flasks and
record any changes in color, turbidity, smell, etc.