Key Organic Molecules Are Used by Living Systems

Natural selection, one of the key forces powering evolution, opens an array of improbable ecological niches to
species that can adapt biochemically. (Left) Salt pools, where the salt concentration can be greater than 1.5 M, would
seem to be highly inhospitable environments for life. Yet certain halophilic archaea, such as Haloferax mediterranei
(right), possess biochemical adaptations that enable them to thrive under these harsh conditions. [(Left) Kaj R. Svensson/
Science Photo Library/Photo Researchers; (right) Wanner/Eye of Science/Photo Researchers.]
I. The Molecular Design of Life
2. Biochemical Evolution
2.1. Key Organic Molecules Are Used by Living Systems
Approximately 1 billion years after Earth's formation, life appeared, as already mentioned. Before life could exist,
though, another major process needed to have taken place the synthesis of the organic molecules required for living
systems from simpler molecules found in the environment. The components of nucleic acids and proteins are relatively
complex organic molecules, and one might expect that only sophisticated synthetic routes could produce them. However,
this requirement appears not to have been the case. How did the building blocks of life come to be?
2.1.1. Many Components of Biochemical Macromolecules Can Be Produced in Simple,
Prebiotic Reactions
Among several competing theories about the conditions of the prebiotic world, none is completely satisfactory or
problem-free. One theory holds that Earth's early atmosphere was highly reduced, rich in methane (CH4), ammonia
(NH3), water (H2O), and hydrogen (H2), and that this atmosphere was subjected to large amounts of solar radiation and
lightning. For the sake of argument, we will assume that these conditions were indeed those of prebiotic Earth. Can
complex organic molecules be synthesized under these conditions? In the 1950s, Stanley Miller and Harold Urey set out
to answer this question. An electric discharge, simulating lightning, was passed through a mixture of methane, ammonia,
water, and hydrogen (Figure 2.1). Remarkably, these experiments yielded a highly nonrandom mixture of organic
compounds, including amino acids and other substances fundamental to biochemistry. The procedure produces the
amino acids glycine and alanine in approximately 2% yield, depending on the amount of carbon supplied as methane.
More complex amino acids such as glutamic acid and leucine are produced in smaller amounts (Figure 2.2). Hydrogen
cyanide (HCN), another likely component of the early atmosphere, will condense on exposure to heat or light to produce
adenine, one of the four nucleic acid bases (Figure 2.3). Other simple molecules combine to form the remaining bases. A
wide array of sugars, including ribose, can be formed from formaldehyde under prebiotic conditions.
2.1.2. Uncertainties Obscure the Origins of Some Key Biomolecules
The preceding observations suggest that many of the building blocks found in biology are unusually easy to synthesize
and that significant amounts could have accumulated through the action of nonbiological processes. However, it is
important to keep in mind that there are many uncertainties. For instance, ribose is just one of many sugars formed under
prebiotic conditions. In addition, ribose is rather unstable under possible prebiotic conditions. Futhermore, ribose occurs
in two mirror-image forms, only one of which occurs in modern RNA. To circumvent those problems, the first nucleic
acid-like molecules have been suggested to have been bases attached to a different backbone and only later in
evolutionary time was ribose incorporated to form nucleic acids as we know them today. Despite these uncertainties, an
assortment of prebiotic molecules did arise in some fashion, and from this assortment those with properties favorable for
the processes that we now associate with life began to interact and to form more complicated compounds. The processes
through which modern organisms synthesize molecular building blocks will be discussed in Chapters 24, 25, and 26.
I. The Molecular Design of Life
2. Biochemical Evolution
2.1. Key Organic Molecules Are Used by Living Systems
Figure 2.1. The Urey-Miller Experiment. An electric discharge (simulating lightning) passed through an atmosphere of
CH4, NH3, H2O, and H2 leads to the generation of key organic compounds such as amino acids.
I. The Molecular Design of Life
2. Biochemical Evolution
2.1. Key Organic Molecules Are Used by Living Systems
Figure 2.2. Products of Prebiotic Synthesis. Amino acids produced in the Urey-Miller experiment.
I. The Molecular Design of Life
2. Biochemical Evolution
2.1. Key Organic Molecules Are Used by Living Systems
Figure 2.3. Prebiotic Synthesis of a Nucleic Acid Component. Adenine can be generated by the condensation of HCN.