Bio Review Notes #69
THE ORIGIN OF LIFE
Performance Objectives:
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 (1600s): Disproved the spontaneous generation of flies; showed that the larvae came from tiny eggs, not from rotting meat.

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.

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.

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 meahcnisms (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. A few biochemists believe that DNA replication came before protein synthesis, but most favor the "protein first" viewpoint, in which RNA and DNA were initially selected for their role in making protein synthesis more reliable.
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. 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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