Lecture 15: The Origins of Life
During the Scienti¿ c Revolution, scientists began to test such ideas more
rigorously. In 1765, Lazzaro Spallanzani (1729–1799) claimed to have refuted
Aristotle’s idea by showing that if a meat broth was sterilized by boiling and
then placed in an airtight container no microorganisms appeared. Opponents
argued that there might be a “life force”
in the air that “animated” living things,
and Spallanzani’s containers had merely
excluded that life force.In 1862, Louis Pasteur (1822–1895)
refuted the notion of a life force in
a remarkably simple and elegant
experiment. He boiled a broth in a À ask
with a long swan-necked outlet, open to
the air. He argued that if there was a life force, it could enter, while seeds or
spores would get trapped in the bend. His retorts can still be seen today in
the Pasteur Institute in Paris, and they remain sterile. Pasteur’s experiment
seemed to prove that life could only come from previous life forms, from
eggs or spores. Spontaneous generation was impossible. If so, how were the
¿ rst living organisms created?Biologists began to get a grip on this knotty problem early in the 20th
century. Modern approaches explain the origins of life in distinct stages.
First, we must explain the creation of the simple molecules present in all
living organisms: the amino acids that make proteins, the nucleic acids that
make DNA, the carbohydrates that make sugars and starches, and the lipids
that make fats and hormones. Today, atmospheric oxygen destroys such
molecules, which is why, as Pasteur claimed, life can no longer be generated
spontaneously. However, in the 1920s, Alexander Oparin in Russia and J. B.
S. Haldane in Britain pointed out that such molecules could have thrived in
an oxygen-free atmosphere, such as that of the early Earth. How could you
test such an idea?In 1952, a graduate student, Stanley Miller, ¿ lled a glass tube with gases
such as methane, ammonia, and hydrogen that might have been present in
the early atmosphere, while carefully excluding oxygen. He added water,
because complex chemical reactions are much easier in liquids than in gasesThough we don’t yet
know all the details, we
understand enough to know
that life can be assembled
from nonliving ingredients.