850 Encyclopedia of the Solar System
2. What is Life?
Our understanding of life as a phenomenon is currently
based only on our study of life on Earth. One of the pro-
found results of biology is the realization that all life forms
on Earth share a common physical and genetic makeup. The
impression of vast diversity that we experience in nature is
a result of manifold variations on a single fundamental bio-
chemistry. The biochemistry of life is based on 20 amino
acids and 5 nucleotide bases. Added to this are the few sug-
ars, from which are made thepolysaccharides, and the
simple alcohols and fatty acids that are the building blocks
of lipids. This simple collection of primordial biomolecules
(Fig. 1) represents the set from which the rest of biochem-
istry derives.
Except for glycine, the amino acids in Figure 1 can have
either left handed (L-) or right handed (D-) symmetry.
FIGURE 1 The basic molecules of life.
FIGURE 2 The L and D form of the amino acid alanine.Figure 2 shows the two versions, known as enantiomers
(from the Greekenantiosmeaning opposite), for alanine.
Life uses only the L-enantiomer to make proteins although
there are some bacteria that use certain D-forms in their
cell walls, and many others have enzymes that can con-
vert the D-form to the L-form. In addition, L-amino acids
other than the 20 listed in Figure 1 are occasionally used
in proteins and are sometimes used directly, for example as
toxins by fungi and plants. We do not yet understand how
and why life acquired a preference for the L-amino acids
over the D-amino acids; this is one of the key observations
that theories for the origins of life seek to explain.
The genetic material of life—DNA (deoxyribonucleic
acid) and RNA (ribonucleic acid) —are both constructed
from nucleotide bases that form the alphabet of life’s ge-
netic code. In DNA, these are adenine (A), thymine (T),
cytosine (C), and guanine (G). In RNA, thymine is replaced
by uracil (U). The nucleic acids each provide a four-letter
alphabet in which the codes for the construction of proteins
are based. This information recording system is found in all
living systems.
The biochemical unity of life, in particular the genetic
unity, strongly suggests that all living things on Earth de-
scend from a common ancestor. This is the phylogenic unity
of life as shown in Figure 3. These genetic trees are ob-
tained by comparing the ribosomal RNA within each or-
ganism. Sections within the RNA are remarkably similar
within all life forms. These conserved sections show only
random point changes and not evolutionary trends. Thus
the similarity between the genetic sequences of any two
organisms is a measure of their evolutionary distance, or
more precisely the time elapsed since they shared a com-
mon ancestor. When viewed in this way, life on Earth is
divided into three main groups: the eucarya, the bacteria,
and the archaea. The eucarya include the multicellular life
forms encompassing all plants and animals. The bacteria are
the familiar bacteria including intestinal bacteria, common