Under present conditions, Louis Pasteur demonstrated that life can only come
from pre-existing life. Modern ideas on the origin of life follow Oparin's
suggestion that life originated in a reducing atmosphere consisting of
H2, CH4, NH3, and H2O.
Miller showed that amino acids could arise spontaneously in such an atmosphere.
Simple proteins probably arose from such amino acids. Self-perpetuating
systems were selected and perpetuated while other systems unraveled.
The origins of DNA replication and modern-style protein synthesis are
currently the subject of several competing theories.
Spontaneous generation: Theory prevailing before Pasteur, that life
could easily and spontaneously arise from nonlife.
Francesco Redi (1666): Disproved the spontaneous generation of flies;
showed that the larvae came from tiny eggs, not from rotting meat.
Redi's experiment
Invention of the microscope (around 1700):
Led to the discovery
of bacteria. Early experiments, flawed by poor sterilization, seemed to show
that bacteria could arise from nonliving matter.
Louis Pasteur (1860s): Perfected sterilization techniques and
re-enacted all earlier experiments. He proved that properly sterilized
broth would remain sterile if bacteria were excluded, but that ordinary
air contained bacteria that could contaminate the broth unless precautions
were taken. This led to the theory of biogenesis —
life can originate only from pre-existing life.
Pasteur's experiment
Alexander Oparin (1930s): Proposed that the origin of life was
impossible under present conditions, but that life had originated
spontaneously under very different conditions on the primitive Earth
(primary abiogenesis). He postulated that life could originate only
in a hydrogen-rich reducing atmosphere, which he thought contained
hydrogen (H2), methane (CH4), ammonia
(NH3), and water vapor (H2O). J.B.S. Haldane
proposed a similar theory independently, but most scientists ignored these
ideas until the 1950s.
S.L. Miller (1950s): Tested Oparin's ideas by combining
H2, CH4, NH3, and H2O in
a sterile apparatus into which he could introduce a spark to simulate
lightning. After circulating this mixture for several days, he analyzed
the products and found many amino acids, a few small peptides, and other
organic compounds.
Miller's experiment
Chemical evolution and the origin of life: Current ideas about the
origin of life are based on the Oparin-Haldane theory of chemical
evolution, in which life arose gradually in a reducing atmosphere.
- The solar system probably formed from a swirling nebula, which
formed into the sun at the center and the planets peripherally.
- Amino acids probably originated in a manner similar to the reactions
of Miller's experiment. The compounds dissolved in the primitive ponds
and oceans, forming a "hot, dilute soup."
- Proteins and DNA can form as polymers by linking smaller units
together, but not until the smaller units are concentrated. Several
concentration mechanisms (tidal pools, crystal surfaces, bubble-like
droplets, etc.) have been suggested.
- Molecules made without life are usually symmetrical or have equal
proportions of right-handed and left-handed forms, but biological
systems contain mostly asymmetrical molecules. Amino acids made by
organisms are mostly of the L- (left-handed) form, but experiments like
Miller's gave right- and left-handed amino acids in equal proportions.
Molecular asymmetry is an important property of life, but we don't know
exactly when or how it arose.
- At some point, biological systems formed tiny droplets with lipid or
protein membrane-like surfaces. Different authorities have imagined different
kinds of droplets, calling them "coacervates," "microspheres," "protobionts,"
etc.
Once these droplets formed, their contents could reach concentrations
very different from those prevailing outside or from one another (they had
individuality). Some were surely more stable than others, and were favored
by "protoselection," especially if they could increase in size and
fragment into smaller droplets, a primitive form of reproduction.
- Protein synthesis was surely much simpler originally than it is now
and was probably much less reliable in perpetuating sameness. Enzyme activity
may have originated by chance. The origins of DNA replication are obscure.
The most widely supported theory holds that RNA was
initially selected for its role in making protein synthesis more reliable,
and that an "RNA world" resulted. DNA came long afterwards, and was initially
selected for its role in copying RNA.
Exobiology: The search for life elsewhere, outside planet Earth.
- To date, much evidence exists for Miller-style synthesis of amino acids,
nitrogen bases, and other compounds elsewhere in our solar system.
- Many stars outside our solar system have planetary systems, and some of those
planets are believed to have just the right temperature and other conditions
for life to have arisen (the "Goldilocks phenomenon").
- No firm evidence has yet been found that life formed anywhere except
on Earth, but many scientists think such origins are very probable.
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