Origin of life on Earth

¶ … Origin of Life

When Watson and Crick discovered DNA, double nucleotide chains containing the "language" of life, scientists had new information on which to base further research about the way living cells function and, in turn, the origins of life. In a 2007 article for Scientific American, Robert Shapiro explains why Watson and Crick's discovery did not provide the definitive answer to one of science's most provocative questions, as scientists had hoped.

One of the major problems to be resolved in discovering the origin of life is akin to the age-old question about which came first, the chicken or the egg. The discovery of RNA seemed to provide a plausible explanation. RNA, a class of molecules, carries information from DNA to cell structures. Scientists were poised to accept that RNA was the self-replicating molecule providing the catalyst for life. The question still remained: how did the first RNA arise? The spontaneity required for self-replication to suddenly take place goes well beyond scientific inquiry, some argue, and into the realm of improbability. The group of scientists who believe in the replicator model have yet to show how a complex molecule could have formed initially. There are theories that certain compounds joined together by chance to form a molecule that could somehow self-replicate. Through an evolutionary process, the molecules mutated and adapted, eventually forming more complex organisms in which different parts performed different functions. The diagram provided by Shapiro shows, disorganized molecules organize into chains that mutate, surviving a selection process, and eventually organizing into organisms with compartments. As stated, this scenario is still considered theoretical; scientists have not established proof.

Likewise, proof does not exist the alternative theory of metabolism first, which holds that "compartmentalized" organisms formed spontaneously; particular compartments were capable of the kind of functions necessary to generate reaction networks leading to growth and evolution.

Shapiro outlines five truths that must be accommodated in any explanation about the origin of life: a boundary separating life from non-life; an energy source for the organization process; a mechanism by which energy is provided for the organization process; a chemical network supporting adaption and evolution; and a system of reproduction. In order for the metabolism-first scenario to be plausible, certain conditions had to have existed: an energy source driving a chemical reaction; an increasingly complex chain of chemical reactions; a net gain of material produced by the reactions; and the storage of information in the chemical network as a whole, not in designated molecules such as RNA or DNA. Shapiro shows this in a diagram that starts with the same disorganized state as in the replicator-first model. Molecules organize, not in straight chains, but in pentagonal chains that link together with shared sides. Arms of molecules attach themselves. The end result is the same as with the replicator-first model, a complex organism that has components capable of different functions. Scientists do not dispute this final model but the mechanism by which it was formed.