LIFE ORIGINS READINGS AND WORKSHEET READINGS Spontaneous Generation
It was once believed that life could come from nonliving things, such as
mice from corn, flies from bovine manure, maggots from rotting meat, and
fish from the mud of previously dry lakes. Spontaneous generation is the
incorrect hypothesis that nonliving things are capable of producing life.
Several experiments have been conducted to disprove spontaneous
generation; a few of them are covered in the sections that follow.
Redi's Experiment and Needham's Rebuttal
In 1668, Francesco Redi, an Italian scientist, designed a scientific
experiment to test the spontaneous creation of maggots by placing fresh
meat in each of two different jars. One jar was left open; the other was
covered with a cloth. Days later, the open jar contained maggots, whereas
the covered jar contained no maggots. He did note that maggots were
found on the exterior surface of the cloth that covered the jar. Redi
successfully demonstrated that the maggots came from fly eggs and
thereby helped to disprove spontaneous generation. Or so he thought.
In England, John Needham challenged Redi's findings by conducting an
experiment in which he placed a broth, or “gravy,” into a bottle, heated the
bottle to kill anything inside, then sealed it. Days later, he reported the
presence of life in the broth and announced that life had been created from
nonlife. In actuality, he did not heat it long enough to kill all the microbes.
Spallanzani's Experiment
Lazzaro Spallanzani, also an Italian scientist, reviewed both Redi's and
Needham's data and experimental design and concluded that perhaps
Needham's heating of the bottle did not kill everything inside. He
constructed his own experiment by placing broth in each of two separate
bottles, boiling the broth in both bottles, then sealing one bottle and leaving
the other open. Days later, the unsealed bottle was teeming with small
living things that he could observe more clearly with the newly invented
microscope. The sealed bottle showed no signs of life. This certainly
excluded spontaneous generation as a viable theory. Except it was noted
by scientists of the day that Spallanzani had deprived the closed bottle of
air, and it was thought that air was necessary having a “vital force” for
spontaneous generation. So although his experiment was successful, a
strong rebuttal blunted his claims.
Pasteur's Experiment
Louis Pasteur, the notable French scientist, accepted the challenge to recreate the experiment and leave the system open to air. He subsequently
designed several bottles with S-curved necks that were oriented downward
so gravity would prevent access by airborne foreign materials. He placed a
nutrient-enriched broth in one of the goose-neck bottles, boiled the broth
inside the bottle, and observed no life in the jar for one year. He then broke
off the top of the bottle, exposing it more directly to the air, and noted lifeforms in the broth within days. He noted that as long as dust and other
airborne particles were trapped in the S-shaped neck of the bottle, no life
was created until this obstacle was removed. He reasoned that the
contamination came from life-forms in the air. Pasteur finally convinced the
learned world that even if exposed to air, life did not arise from nonlife.
EARLY ABIOTIC EARTH
Based on recent computer model studies, the complex organic
molecules necessary for life may have formed in the protoplanetary
disk of dust grains surrounding the Sun before the formation of the
Earth.[19] According to the computer studies, this same process may
also occur around other stars that acquire planets.[19] (Also see
Extraterrestrial organic molecules).
The Hadean Earth is thought to have had a secondary atmosphere,
formed through degassing of the rocks that accumulated from
planetesimal impactors. At first, it was thought that the Earth's
atmosphere consisted of hydrides—methane, ammonia and water
vapour—and that life began under such reducing conditions, which
are conducive to the formation of organic molecules. During its
formation, the Earth lost a significant part of its initial mass, with a
nucleus of the heavier rocky elements of the protoplanetary disk
remaining,.[20] However, based on today's volcanic evidence, it is
now thought that the early atmosphere would have probably
contained 60% hydrogen, 20% oxygen (mostly in the form of water
vapour), 10% carbon dioxide, 5 to 7% hydrogen sulfide, and smaller
amounts of nitrogen, carbon monoxide, free hydrogen, methane and
inert gases. As Earth lacked the gravity to hold any molecular
hydrogen, this component of the atmosphere would have been
rapidly lost during the Hadean period, along with the bulk of the
original inert gases. Solution of carbon dioxide in water is thought to
have made the seas slightly acidic, with a pH of about 5.5.[21] The
atmosphere at the time has been characterized as a "gigantic,
productive outdoor chemical laboratory."[22] It is similar to the mixture
of gases released by volcanoes, which still support some abiotic
chemistry today.[22] OXYGEN? Molecular oxygen was not present as a significant fraction of Earth's
atmosphere until after photosynthetic life forms evolved and began to
produce it in large quantities as a byproduct of their metabolism. This
radical shift from an inert to an oxidizing atmosphere caused an
ecological crisis sometimes called the oxygen catastrophe. At first,
oxygen would quickly combine with other elements in Earth's crust,
primarily iron, as it was produced. After the supply of oxidizable
surfaces ran out, oxygen began to accumulate in the atmosphere,
and the modern high-oxygen atmosphere developed. Evidence for
this lies in older rocks that contain massive banded iron formations,
laid down as iron and oxygen first combined.
WORKSHEET Use each of the terms below just once to complete the passage:
microorganisms/ nonliving matter/ organisms/ spontaneously/
vital force/ S-shaped/ broth/ air
Louis Pasteur/ disproved/ microscope/
biogenesis/Francesco Redi/ spontaneous generation
Early scientists believed that life arose from (1)
____________________ through a process they called (2)
___________________________ . In 1668, the Italian physician (3)
____________________ conducted an experiment with flies that (4)
____________________ this idea. At about the same time, biologists
began to use an important new research tool, the (5)
____________________ . They soon discovered the vast world of (6)
____________________ . The number and diversity of these
organisms was so great that scientists were led to believe once again
that these organisms must have arisen (7) ____________________ .
By the mid-1800s, however, (8) ____________________ was able to
disprove this hypothesis once and for all. He set up an experiment,
using flasks with unique (9) ____________________ necks. These
flasks allowed (10) ____________________ but no organisms, to come
into contact with a broth containing nutrients. If some (11)
____________________ existed, as had been suggested, it would be
able to get into the (12) ____________________ through the open
neck of the flask. His experiment proved that organisms arise only
from other (13) ____________________ . This idea, called (14)
____________________ is one of the cornerstones of biology today.
Determine if the statement is true. If it is not, rewrite the italicized
part to make it true.
15. Biogenesis explains how life began on Earth.
16. For life to begin, simple inorganic molecules had to be formed and
then organized into complex molecules.
17. Several billion years ago, Earth’s atmosphere had no free methane.