Ciencias Para el Mundo Contemporáneo
UNIT 3. THE ORIGIN OF LIFE AND EVOLUTION
Humans have always wondered about the origin and variety of life. Many theories have been
proposed, some with a scientific base, others based on religious terms.
1) SPONTANEOUS GENERATION.
Spontaneous generation is the belief that living things can arise suddenly, from inanimate matter,
without the need of a living progenitor. It was settled in the fourth century B.C. by greek philosophers
and scientists, as Aristotle, and has been maintained for centuries. The appearance of maggots on
decaying meat was an especially strong evidence, for many people, that spontaneous generation did
occur.
In 1668, the Italian physician Francisco Redi began a series of experiments that showed that dead
meat does not give rise spontaneously to maggots, but scientists still continued to believe that
microorganisms were small and simple enough to be generated from nonliving material.
The matter remained unresolved until 1864, when Louis Pasteur, in a series of classic experiments,
disproved Spontaneous Generation:
Pasteur placed broth in flasks that had open-ended, long necks. After bending the necks of the flasks
into S-shaped curves he boiled the contents. The contents of these uncapped flasks remained
uncontaminated even months later. Pasteur explained that the S-shaped curve allowed air to pass into
the flask but trapped microorganisms at the bottom of the curve, preventing them from traveling into
the broth.
2) PANSPERMIA.
Panspermia is a Greek word that means "seeds everywhere". The panspermia hypothesis states that the
"seeds" of life exist all over the Universe and can be propagated through space from one location to
another. Mechanisms for panspermia include the deflection of interstellar dust by solar radiation, and
extremophile microorganisms traveling through space within an asteroid, meteorite or comet.
3) THE PRIMORDIAL SOUP.
"Primordial" means happening first in a sequence of time. The Primordial Soup Theory states that
about 3.5-3.8 billion years ago, the Earth's atmosphere was made up of nitrogen, ammonia, methane,
and hydrogen. There was either very little or no oxygen in the Earth's atmosphere at this time. Energy
from either the sun, volcanoes, or lightning, fueled the chemical reactions between these gases.
When it would rain, the droplets would carry these molecules to a body of water (pond, lake, or
possibly an ocean), forming a primordial soup of amino acids. Amino acids, which are the building
blocks of proteins, would later evolve into organisms.
In 1953, two American chemists, named Stanley Miller and Harold Urey finally came up with an
experiment to test the Primordial Soup Theory:
The Urey-Miller experiment used water, ammonia, nitrogen, and methane. The chemicals were sealed
inside an array of glass tubes and flasks connected in a loop, with one flask full of water, and the other
containing electrodes. They heated the water, until the water began to evaporate. Then, sparks were
fired through the synthetic atmosphere of water vapor to simulate lightning. After that, they cooled the
atmosphere so that the water could condense and then trickle down into the flask for a continuous
cycle. They let the experiment run continuously for a week; heating and condensing the vapor. After
that week, Miller and Urey observed that 10-15% of the carbon in the flasks had become simple
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Marta García Dpto. Biología y Geología

Ciencias Para el Mundo Contemporáneo
organic compounds. Out of that, 2% of the carbon formed amino acids which are the building blocks
of life. Within the flasks they discovered 13 of the 21 amino acids that are used to make proteins in
living cells.
PROTOCELLS (COACERVATES THEORY)
Oparin’s Coacevates Theory suggests that those macromolecules could have organized into
membranous bodies with definite shape, unity, and properties which resemble those of living things
(they reproduce by dividing by fission, they grow by accumulating additional molecules and they
undergo simple metabolism). So, Coacervates are theorized to have been the early ancestors of cells.
The next phase is marked by the appearance of nucleic acids, which began to control the reproductive
process and directed the internal activities of the coacervates. Life has arisen!
These early cells would have been autotrophs, but some of them would have evolved into
heterotrophs.
THE ORIGIN OF BIODIVERSITY
There are two types of theories about the diversity of species:
Fixism: suggests that species don’t change, they keep inmutable since creation, and came out already
adapted to the environment without undergoing changes. These theories are not accepted nowadays.
Evolutionism: affirms that species change along time, even generating new species.
1) FIXISM.
Fixist ideas were based on a literal interpretation of Genesis and other sacred books, affirming that
actual species were the same in the past, keeping unchanging along time. Creationism is the religious
version of fixism, suggesting that living things had been created by God. Creationist thought the Earth
was formed 6.000 years ago, according to biblical references.
But…what about fossils?
The fossil record means a difficulty for fixist ideas, as it demonstrates the existence of species which
have disappeared. To explain this, Cuvier (s.XVIII) advocated the theory of CATASTROPHISM:
“There have been violent and sudden natural catastrophes such as great floods and the rapid formation
of major mountain chains. Plants and animals living in those parts of the world were often killed off,
and then, new life forms moved in from other areas. As a result, the fossil record shows abrupt
changes.”
2) EVOLUTIONISM:
There are four main evolutionary theories:
2a. Lamackism. 2b. Darwinism. 2c. Neodarwinism. 2d. Punctual Equilibrium.
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Marta García Dpto. Biología y Geología