⇒ Molecular Evolution
⇒ Early Beginnings
⇒ The RNA World
⇒ The DNA World
⇒ Phylogenetic Analysis
⇒ The Evolution of Eukaryotic Cells
Molecular studies have shed light on the origin of life and its subsequent
evolution into a myriad of extinct and extant species. Theories regarding
these early events are impossible to prove conclusively with circumstantial
evidence. Molecular fossils such as introns within transcriptional
units and common biochemcial pathways shared between diverse organisms
provide additional support for current models.
Living cells possess: (1) a boundary membrane
separating the cell's contents from its external environment;
(2) one or more DNA molecules that carry genetic
information for specifying the structure of proteins
involved in replication of its own DNA , in metabolism,
in growth, and in cell division; (3) a transcriptional system
whereby RNAs are synthesized; (4) a translational system for decoding ribonucleotide sequences into amino acid sequences;
and (5) a metabolic system that provides usable forms of energy
to carry out these essential activities.
The first living system(s) were undoubtedly much simpler than any
cells alive today. The transition from nonliving to living was gradual, and
no single event led to life in all its modern complexity. Even today biologists
cannot agree on a definition of life. The following criteria are usually
included in attempts to define life. An aggregate of cells is considered
"alive" if it (1) can use chemical energy or radiant energy to drive
energy-requiring chemical reactions; (2) can increase its mass by controlled
synthesis; and (3) possesses an information coding system and a
system for translating the coded information into molecules that maintain
the system and allow it to reproduce one or more collections of molecules
with similar properties.
The best estimate for the age of the earth is 4.6 billion years. The oldest
microfossils superficially resembling bacteria have been dated at
about 3.5 billion years ago. Thus, chemical evolution (e.g., abiotic syntheses
of amino acids and their polymerization into peptides) during the
first 1.0 to 1.5 billion years of earth history probably preceded the appearance
of cellular life and its subsequent biological evolution.
The major opinion is that earth's atmosphere was nearly neutral,
nonoxidizing, and contained primarily nitrogen, carbon dioxide, hydrogen
sulfide, and water. Microfossils resembling modern cyanobacteria
("blue-green algae") have been found in limestone rocks called stromalites
dated 3.5 billion years ago. Presumably these ancient photosynthetic
bacteria produced oxygen as a by-product of splitting water, just as
cyanobacteria do today. Over more than another billion years, oxygen
slowly began to accumulate, eventually causing the primitive atmosphere
to become oxidizing.
There are two major scientific theories regarding how life came to be
on earth. It either evolved on earth from nonliving chemicals, or it
evolved elsewhere in the universe and was brought to the earth by comets
or meteorites (panspermia theory). The belief that life was created by a
supernatural force is impossible to support or refute with factual evidence
and hence is outside the realm of science.
Amino acids and other precursors of modern biomacromolecules
have been found inside meteorites, so chemical evolution of these molecules
might have been (and still may be) widespread in the cosmos. In
1953, Stanley Miller, at the suggestion of his mentor Harold Urey, used
a reflux apparatus to simulate early atmospheric conditions in an attempt
to reproduce the chemical evolution of biological precursor molecules.
He recirculated water vapor and other gases (CH4, NH3, and H2) through
a chamber where they were exposed to a continuous high voltage electrical
discharge that simulated natural lightning. After a few days, the
mixture was analyzed and found to contain at least ten different amino
acids, some aldehydes, and hydrogen cyanide. Subsequent experiments
by Miller and other researchers using different molecular mixtures and
energy sources produced a variety of other building blocks of biological
Sidney Fox and his colleagues heated amino acids under water-free
(anhydric) conditions to temperatures of 160-210°C and found amino
acids polymerized into proteinlike chains which he called proteinoids,
which are branched rather than linear. When dissolved in water, these proteinoids
exhibit several properties of biological proteins including limited
enzymatic activity and susceptibility to digestion by proteases.
Protein like peptides can also be synthesized from amino acids on
clays. Clays consist of alternating layers of inorganic ions and water molecules.
The highly ordered lattice structure of clays strongly attracts organic
molecules and promotes chemical reactions between them. Polypeptides
have been detected in laboratory simulations of these processes.
When solutions of proteinoids are heated in water and then allowed
to cool, small, spherical particles called microspheres are formed. These
microspheres are about the same size and shape as spherical bacteria.
Some are able to grow (add mass) by accretion of proteinoids and lipids
and subsequently proliferate by binary fission or budding.
Lipids can spontaneously organize into double-layered bubbles
called liposomes, which are leaky enough to absorb various substances
such as proteins from the surrounding medium. Substances trapped with in the liposome find themselves in a hydrophobic environment
that might provide more favorable conditions
for certain kinds of chemical reactions. Thus,
lipid bilayers may have promoted both aggregation
and catalysis. Vesicles composed of lipid membranes
and protein microspheres, but devoid of RNA
or DNA molecules, are hypothesized to have existed in the early stages
of life. These entities are called progenotes.
Early atmospheric conditions:
N, CO2, H2S, H2O