It is not known how life originated--there
are no time machines and fossils of pre-cells either do not exist or are
difficult to interpret. Even if life is generated someday in a lab through
a series of chemical reactions, there would be no proof that the first
living things evolved through the same sequence of chemical reactions.
As a result, science will never be able to prove how life started. Scientists
can, however, study what scenarios are possible given the conditions of
the early earth and determine if the characteristics of modern cells offer
support for any of the models. The question boils down to this: over the
course of a few hundred million years, could the sum of all the chemical
reactions which occurred on the early earth (in its oceans, its continually
flooded tidal zones, its subsurface, its volcanic vents, and even in the
material which was bombarding it from space) produce complex aggregates
of molecules which achieve the level of complexity of the most minimal
forms of life? Obviously, answering this question is complicated by the
facts that scientists are just beginning to appreciate the wide array
of organic molecules which can be produced in the absence of life, the
conditions of the early earth, the contribution to the chemistry of the
early earth made by molecules found in comets and meteors, etc. Although
it is easy enough to study the simplest cells alive today, these cells
are more than 3.5 billion years removed from the first cells and they
should not be considered as models for the simplest living things.
Although science can not at present answer the question
of whether life evolved (and may never be able to do so), there are simpler
questions which can be asked and possibly answered.
1) Is it possible that organic molecules (those
complex molecules of living things) arose from simple inorganic molecules
in the absence of life? Yes.
Organic molecules were once thought to exist only in living things and
to have possess an "animism" or "vitalism" which could
not arise without life (Joyce, 1998). Vitalists once argued that organic
molecules could never be generated in a lab or, for that matter, anywhere
outside a living organism. They were proven wrong. Organic molecules can
be formed without life in labs and organic molecules have been detected
in meteorites, comets, and several bodies of our solar system (such as
the Jupiter moons Callisto and Ganymede). In other words, the organic
molecules which are the "the stuff of life" can be found without
Experiments have demonstrated that the simple organic molecules which
life depends on can be synthesized using only the gases of the primitive
earth's atmosphere and a source of energy. There would have been plenty
of energy in a primitive earth-the heat of a semimolten planet, lightning,
solar radiation unfiltered by an ozone layer, etc. By simply mixing inorganic
molecules and energy, the following organic molecules have been produced:
all the amino acids found in living things (in addition to amino acids
not found in living things), all essential sugars, triphosphate nucleotide
precursors needed for the synthesis of the DNA and RNA, aldehydes, carboxylic
acids, and others. These organic molecules synthesized in the absence
of life could incorporate a large percentage of the available carbon.
2) Could these small organic building blocks have
joined to form larger biomolecules in the absence of life? Yes.
In living organisms, small organic molecules (monomers) can not only
function alone, they can bind to each other to form long chains (polymers).
Can monomers join to form polymers in the absence of life? Yes. Not only
can this occur in solution, there are a number of catalysts which can
speed these reactions. Certain mineral surfaces (feldspar, calcite, zeolites,
clays) provide sites where small organic molecules can fuse to form larger
ones. The small molecules (RNA nucleotides, amino acids) absorb onto these
surfaces and, since they are in close enough proximity to each other and
in the right orientation, they can bond to form chains. Small proteins
of over 200 amino acids have been produced and short strands of DNA and
RNA (up to 50 nucleotides long). Because minerals help to catalyze polymerization
reactions, the rocks of early earth could have been covered with chains
of at least tens of monomer units (Joyce, 1998).
3) Can molecules replicate themselves in the absence
of life? Yes, to some degree.
Short RNA and DNA molecules can serve as templates and replicate themselves.
One RNA molecule has actually shown itself not only to be a template of
its own replication, but a catalyst of the replication of other RNA molecules
as well (Green, 1992; Doudna, 1991). In 1996, a small protein (based on
a protein found in yeast) was observed to replicate itself (Lee, 1996;
Kauffman, 1996). Self-replicating peptides based on coiled motifs have
been observed (Ghosh, 2004). Amines and esters can combine to form an
amide which then serves as a template for other amines and esters to do
the same. There are a number of organic molecules which are not found
in living things which have been shown to replicate (especially vinyl
homopolymers and copolymers) (Orgel, 1992; Rebek, 1994). Yeast, fungi,
and mammals possess unrelated prion proteins which can generate fibers
under certain conditions (Chernoff, 2004).
Some experiments have tried to simulate "natural selection"
acting on self-replicating molecules. Molecules continue to replicate
themselves but mutations occur as the rounds of replication continue.
Not only can the replication continue, apparently indefinitely in the
right conditions, mutations arise which allow the molecules to replicate
faster (so that they have an advantage over the original molecules used).
Although no molecule has yet been generated which can replicate itself
well enough to serve as a model for the first genetic molecule, the search
for self-replicating molecules is a relatively recent endeavor.
4) Could organic molecules form membranous balls?
Could these precells perform some activities that cells perform? Yes.
Living cells are surrounded by lipid cell membranes. Although lipids can
form in the absence of life, could such lipids spontaneously form cell
membrane-like structures? Yes. Lipids in solution form layers that are
similar in structure and function to those of cell membranes. In solution,
lipids can form spheres known as micelles, coarcervates, and microspheres.
Simulations of the formation of organic molecules in the interstellar
ices of comets (using UV light) form organic molecules that self-assemble
into such vesicles. These vesicles can accumulate organic molecules inside
themselves, increase in size, and even split once they reach a certain
size. If enzymes (proteins which speed chemical reactions) are in these
droplets, chemical reactions can occur. If they contain the enzyme RNA
polymerase, RNA nucleotides are taken from the environment and assembled
into RNA chains (Zimmer, 1995). Organic molecules gathered from meteorites
have been found to form these membranous balls in water.