856 Encyclopedia of the Solar System
common ancestor are the thermophilic hydrogen metabo-
lizing bacteria and perhaps the sulfur metabolizing bacteria.
The arrow in Figure 3 represents the suggested position of
the last common ancestor.
It is important to note here that the last universal com-
mon ancestor is not necessarily representative of the first or-
ganisms on Earth but was merely the last organism (or group
of organisms) from which all life forms today are known to
have descended. The common ancestor may have existed
within a world of multiple lineages, none of which are in
evidence today. If all life on Earth has indeed descended
from a sulfur bacterium living in a hot springs environment,
this could be the result of at least three possibilities. First
it may be the case that hot sulfurous environments are im-
portant in the origin of life and the common ancestor may
represent this primal cell. Second, the common ancestor
may have been a survivor of a catastrophe that destroyed
all other life forms. The survival of the common ancestor
may have been the result of its ability to live deep within
a hydrothermal system. Third, the nature of the common
ancestor may be serendipitous with no implications as to
origin or evolution of the biosphere.
For over 2 Gyr after the earliest evidence for life, life
on the Earth was composed of only microorganisms. There
were certainly bacteria and possibly one-celled eukaryotes
as well. There seemed to be a major change in the envi-
ronment of the Earth with the rise of photosynthetically
produced oxygen beginning at about 2.5 Gyr ago, reaching
significant levels about 1 Gyr ago and culminating about
600 Myr ago. (Figure 4 shows a timeline of Earth’s history
with these events.) Soon after the development of high lev-
els of oxygen in the atmosphere, multicellular life forms
appeared. These rapidly radiated into the major phylum
known today (as well as many that have no known living
representatives). In time, organisms adapted to land envi-
ronments in addition to aqueous environments, and plants
and animals appeared.
4. The Origin of Life
Numerous and diverse theories for the origin of life are
currently under serious consideration within the scientific
community. A diagram and classification of current theo-
ries for the origin of life on Earth is shown in Figure 6. At
the most fundamental level, theories may be characterized
within two broad categories: theories that suggest that life
originated on Earth (Terrestrial in Fig. 6) and those that
suggest that the origin took place elsewhere (Extraterres-
trial in Fig. 6). The extraterrestrial orpanspermiatheories
suggest that life existed in outer space and was transported
by meteorites, asteroids, or comets to a receptive Earth. In
this case, the origin of life is not related to environments
possible on the early Earth. Along similar lines, life may
have been ejected by impacts from another planet in the
FIGURE 6 Diagrammatic representation and classification of
current theories for the origin of life.Solar System and jettisoned to Earth, or visa versa. Fur-
thermore, it has been suggested in the scientific literature
that life may have been purposely directed to Earth (di-
rected panspermia in Fig. 6) by an intelligent species from
another planet.
The terrestrial theories are further subdivided into or-
ganic origins (carbon-based) and inorganic origins (mineral-
based). Mineral-based theories suggest that life’s first
components were mineral substrates that organized and
synthesized clay organisms. These organisms have evolved
via natural selection into the organic based life forms visible
on Earth today. The majority of theories that do not invoke
an extraterrestrial origin require an organic origin for life
on Earth. Theories postulating an organic origin suggest
that the initial life forms were composed of the same basic
building blocks present in biochemistry today, organic ma-
terial. If life arose in organic form, then there must have
been a prebiological source of organics. TheMiller-Urey
experimentsand their successors have demonstrated how
organic material may have been produced naturally in the
primordial environment of Earth (endogenous production
in Fig. 6). An alternative to the endogenous production
of organics on early Earth is the importation of organic
material by celestial impacts and debris—comets, mete-
orites, interstellar dust particles, and comet dust particles.
A comparison of these sources is shown in Table 5. Table 6
lists the organics found in the Murchison meteorite and
compares these with the organics produced in a Miller-
Urey abiotic synthesis. Organic origins differ mainly in the
type of primal energy sources: photosynthetic, chemosyn-
thetic, or heterotrophic. The phototrophs and chemotrophs
(collectively called autotrophs) use energy sources that
are inorganic (sunlight and chemical energy respectively),
whereas heterotrophs acquire their energy by consuming
organics (see Table 2).