Spontaneous generation
World of Scientific Discovery, 2007
Spontaneous generation refers to the idea that living creatures can arise from nonliving matter. Before
microscopes enabled people to view forms of life too small to see with the naked eye, the origin of
many organisms was a mystery. People often wondered why, for example, maggots would appear in
decaying meat. Aristotle first expressed the idea of spontaneous generation in the fourth century BC,
but the scientific community of seventeenth-century Europe raised doubts about Aristotle's idea.
Francesco Redi (1626-1697), an Italian physician and poet, was the first to demonstrate the
improbability of spontaneous generation by setting up an experiment in which he analyzed the
reproduction of flies. Redi set out to test the possibility that life might arise from tiny seeds or eggs that
were too small to be seen. He prepared eight different containers of meat; four were sealed and four
were left open to the air. After a few days, Redi noticed that maggots appeared only in the meat that
was exposed to air; flies had regularly landed on this exposed meat. He repeated the experiment,
putting meat in gauze-covered containers to determine if it was the exposure to air or the fact that flies
landed on the meat that caused the maggots. Indeed, the gauze-covered meat did not show any
maggot development. His practical experiment demonstrated that maggots were not formed by
spontaneous generation, but from eggs laid by flies.
After improvements in the microscope, people could see microscopic cells and organisms. Many
scientists still speculated about the origin of bacteria, and the mystery of spontaneous generation
lingered a while longer, as different experiments with bacteria yielded contradictory results. In 1748,
John Turberville Needham, an English naturalist and Roman Catholic priest, collaborated with
Georges Buffon to further confuse the issue of spontaneous generation. Needham boiled mutton broth
and sealed it in glass containers for a few days. When the containers were opened, there were
numerous microorganisms present. Needham concluded that his experiments showed that life could
arise from nonliving matter. Needham received great acclaim for his work and was elected to the
Royal Society, the first Roman Catholic to earn such a distinction. He also became the director of the
Academy of Sciences at Brussels.
Twenty years passed before someone noticed the flaw in Needham's experiment. Lazzaro
Spallanzani, an Italian biologist, pointed out that Needham had failed to boil the mutton long enough to
kill off all microorganisms. Spallanzani conducted the same experiment; he boiled the mutton broth for
nearly an hour and found that no maggots appeared. Alas, the debate was still far from over; many
scientists still clung to the theory of spontaneous generation. Some thought that Spallanzani had
destroyed a suspected "vital" component in the air and that the boiling time was not significant.
It was another century before Louis Pasteur (1822-1895) would prove conclusively that sterilization
could hinder the growth of microorganisms. In the mid-nineteenth century, John Tyndall, an Irish
physicist, showed that some of the dust in the air consists of microorganisms. This explained why
broth so easily became affected by microorganisms. Pasteur made the greatest breakthroughs in
refuting spontaneous generation. The French chemist made many of his observations by using the
microscope and working with heat. For example, he discovered two types of yeast in fermented wine,
one of which could cause the wine to spoil. In order to ensure that only the other, harmless type of
yeast remained in the wine, the wine was heated. French winemakers initially strongly opposed this
idea. After Pasteur showed that his heating method worked, the process of pasteurization was widely
used to kill undesirable microscopic organisms in many liquids. Like Tyndall, Pasteur showed that the
dust in air included spores of living organisms that could cause them to spread and reproduce. In
1864, Pasteur conducted an important experiment that showed that dust could spread organisms that
spoil meat. He reaffirmed these results in other experiments, in which he showed that no bacteria
appeared in boiled sugar solutions that were in contact with air, as long as bacteria were first filtered
out of the air. Once and for all, Pasteur had disproved the theory of spontaneous generation that had
occupied scientists for centuries.
Rudolph Carl Virchow, a German pathologist, challenged Pasteur's ideas. Virchow thought that all
cells arose from cells. He refused to accept the possibility that disease could travel in microscopic
form, but thought that it had to be passed by direct contact from one cell to another.
Lately, the scientific community has subscribed to what might be called a revised version of the theory
of spontaneous generation. This modern version of spontaneous generation deals with the origins of
life on Earth. The origin of earthly life is a mystery because the elements that currently comprise the
Earth and the planets were made in the interior of stars--environments where the existence of life is
impossible. Yet, somehow, life on Earth arose and evolved. One theory holds that life did indeed erupt
spontaneously from nonliving matter, but this process happened over a period of perhaps a billion
years. Some experiments have demonstrated the strong possibility of the spontaneous generation of
Earth's first organisms. Before the planet was filled with living material, it was called prebiotic Earth. In
a famous experiment, water containing the gases and molecules that were present on the newborn
Earth were bombarded with lightning to simulate the intense and nearly continual electric storms that
occurred on the early planet. The lightning caused a chemical reaction in the molecules that led to the
formation of amino acids, the building blocks of life. This and similar experiments led to the notion that
it was inevitable that, given the conditions on the young planet, life would have developed
spontaneously from the compounds on the prebiotic Earth.
In 2004, scientists found evidence that may force their peers to revise their notions of the evolution of
life on Earth. Scientists from the University of Alberta (Canada) analyzed 3.5-billion-year-old volcanic
rock and found that it contained signs of bacterial life. Scientists had been looking for bacteria from
Earth's infancy for more than 50 years, yet this is the first clear and solid evidence that such life forms
existed on the early Earth. The researchers discovered minute mineralized tubes in ancient pillow lava
that strongly support the presence of microbes, which colonized the basaltic glass of the early oceanic
crust (the source of this rock). In addition to the telltale tubes, the investigators identified traces of
elements associated with organisms and with DNA, including carbon, nitrogen, phosphorus, and
potassium.
A new and highly controversial theory of the origins of life on Earth was proposed by European
scientists in 2002. The researchers' theory--which has more in common with spontaneous generation
than other modern ideas--states that earthly life arose from inorganic "incubators," or tiny
compartments in iron sulfide rocks. The new theory turns the popular idea--that the building blocks of
life formed before cells--on its head. The new theory claims that cells came first, but claims that they
were inorganic, nonliving cells made of iron sulfide. These inorganic cells formed on the lightless
ocean floor, not at the surface where the atmosphere and the oceans met and interacted. According to
these scientists, life arose as a chemical response to convection currents running through the Earth's
ancient crust. In this view, the hot, rich hydrothermal fluids, similar to those found today at deep-sea
vents, initiated reactions within the minute cavities in iron sulfide rocks to create iron sulfide cells.
Thus, all life arose from these iron sulfide ancestors. The theory has generated debate and interest
because it implies that life could evolve on other planets or moons in the same way, as long as similar
conditions prevail.
Source Citation
"Spontaneous generation." World of Scientific Discovery. Gale, 2007. Science in
Context. Web. 6 July 2015.
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