Background — Spontaneous Generation
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 Shakespeare'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
- Observation: In the cities, there were no sewers, no garbage
trucks, no electricity, and no refrigeration. 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. Food was
purchased and prepared on a daily basis, and when people were done eating a
meal, the bones and left-overs 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 the filth flowing in the gutters,
it would land on him, and not the lady's expensive silk gown (many of these
gowns were so ornately embroidered that they were not easily washable, and
neither washing machines nor dry cleaners existed). Many cities also had major
rat problems. People back then may or may not have not connected the presence
of rats with the spread of Bubonic Plague (Black Death, a dreaded and fatal
disease), but they were probably bothered by the rats chewing on things and by
the rat fleas biting them (just as cat/dog owners, even now, are bitten by the
offspring of their pet's fleas). People may not have realized that the Plague
was spread by the bites of those fleas, but I imagine they knew that if only
they could get rid of the rats, the pesky fleas would soon disappear, too —
hence the story of the Pied Piper of Hamelin, Germany, leading all the rats
out of town.
- 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 the
- 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.
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:
- 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 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 butcher shop.
group(s) — One group of jars were sealed with lids, and another group of
jars had gauze placed over them.
— Several jars were included in each group.
- Data: Presence or absence of flies and maggots observed 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.
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.
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 there.
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
- 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 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."
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.
- 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.
- The Slow Death
of Spontaneous Generation (1668-1859)