Astrobiology 851
FIGURE 3 A phylogenetic tree showing the
relatedness of all life on Earth. The last universal
common ancestor is shown by the arrow. This is the
root of the tree.soil bacteria, and the pathogens. The archaea are a differ-
ent class of microorganisms that are found in unusual and
often harsh environments such as hypersaline ponds and
H 2 -rich anaerobic sediments. All methane-producing mi-
crobes are archaea. Archaea are also found in soils and
grow on and in humans, producing methane in the gut.
Archaea are not known to be human pathogens or to pro-
duce substances that are toxic to humans. Why some bac-
teria but no archaea are pathogenic is not yet understood.
2.1 The Ecology of Life: Liquid Water
In addition to describing the building blocks of life, it is
instructive to consider what life does. In this regard it is
possible to define a set of ecological or functional require-
ments for life. There are four fundamental requirements
for life on Earth: energy, carbon, liquid water, and a few
other elements. These are listed in Table 1 along with
the occurrence of these environmental factors in the Solar
System.
TABLE 1 Ecological Requirements for LifeOccurrence in the
Requirement Solar SystemEnergy Common
Predominately sunlight Photosynthesis at 100 AU
light levelse.g.,
Chemical energy H 2 +CO 2 →CH 4 +H 2 O
Carbon Common as CO 2 and CH 4
Liquid water Rare, only on Earth for
certain
N, P, S and other elements CommonEnergy is required for life from basic thermodynamic
considerations. Typically on the Earth this energy is pro-
vided by sunlight, which is a thermodynamically efficient
(low entropy) energy source. Some limited systems on Earth
are capable of deriving their energy from chemical reactions
(e.g., methanogenesis, CO 2 +4H 2 →CH 4 +2H 2 O) and
do not depend on photosynthesis. On Earth these systems
are confined to locations where the more typical photosyn-
thetic organisms are not able to grow, and it is not known
if an ecosystem that was planetary in scale or survived over
billions of years could be based solely on chemical energy.
There are no known organisms on Earth that make use of
temperature gradients to derive energy. These organisms
would be analogous to a Carnot heat engine. Table 2 lists
some of the most important metabolic reactions by which
living systems generate energy. This list includes autotrophs
(which derive energy from nonbiological sources) as well as
heterotrophs (which derive energy by the consumption of
organic material, usually other life forms).
Elemental material is required for life, and on Earth
carbon has the dominant role as the backbone molecule
of biochemistry. Life almost certainly requires other ele-
ments as well. Life on Earth utilizes a vast array of the
elements available on the surface. However, this does not
prove that these elements are absolute requirements for
life. Other than H 2 O and C, the elements N, S, and P are
probably the leading candidates for the status of required
elements. Table 3 lists the distribution of elements in the
cosmos and on the Earth and compares these with the com-
mon elements in life.
As indicated in Table 1, sunlight and the elements
required for life are common in the Solar System. What
appears to be the ecologically limiting factor for life in the
Solar System is the stability of liquid water. Liquid water is a
necessary requirement for life on Earth. Liquid water is key