Astrobiology 855
the temperature does not fall anywhere below about 200◦C,
which is the temperature required for heat sterilization of
dry, dormant organisms. This is a difficult requirement be-
cause the time it takes heat to diffuse down a given distance
scales as the square of the distance. Thus, heat must be ap-
plied a million times longer to sterilize to a depth of 1 km
compared to a depth of 1 m.
It is not known when the last life-threatening impact oc-
curred on Earth. As shown schematically in Figure 4, the
rate of impact, extrapolated from the record on the Moon,
rises steeply before 3.8 Gyr ago. It is therefore likely that
the Earth was not continuously suitable for life much be-
fore 3.8 Gyr ago. There is persuasive evidence, including
microbial fossils andstromatolites, that microbial life was
present on the Earth as early as 3.4 Gyr ago. Stromato-
lites are large features—often many meters in size—that
can be formed by the lithification of laminated microbial
mats, (Fig. 5) although physical processes can result in sim-
ilar forms. Phototactic microorganisms living on the bottom
of a shallow lake or ocean shore may be periodically cov-
ered with sediment carried in by spring runoff, for exam-
ple. To retain access to sunlight, the organism must move
up through this sediment layer and establish a new micro-
bial zone. After repeated cycles, a layered series of mats
are formed by lamination of the sediments containing the
organic material. One characteristic of these biogenic mats
that distinguishes them from nonbiologically caused layer-
ing is that the response is phototactic, not gravitational, so
that the layered structure is not usually flat but is more often
dome-shaped because covered microorganisms in a lower
layer on the periphery of the structure would move more
toward the side to reach light. In this way, stromatolites can
be distinguished sometimes from similar but nonbiological
laminae. Often stromatolites contain microfossils—further
testimony to their biological origin.
Microbial life, which is possibly capable of photosyn-
thesis and mobility, appears to have originated early in the
history of the Earth, possibly before the end of the late bom-
bardment 3.8 Gyr ago and almost certainly not later than
3.4 Gyr ago. This suggests that the time required for the on-
set of life was brief. If the Greenland sediments are taken
as evidence for life, it suggests that, within the resolution of
the geological record, life arose on Earth as soon as a suit-
able habitat was provided. The microbial mats at 3.4 Gyr
ago put an upper limit of 400 million years on the length of
time it took for life to arise after clement conditions were
present.
In principle, tt is possible to determine which organ-
ism on the Earth is the most similar to the last universal
common ancestor. To do so, we must determine which or-
ganism has changed the least compared to all other organ-
isms. For example, if sometaxonof organism contains a
certain mutation, but many do not, we can trace the muta-
tion to an ancestor common to all organisms in that taxon.
Within this related group of organisms, the most primitive
FIGURE 5 A stromatolite formed by cyanobacteria over
1 billion years ago from the Crystal Springs formation, Inyo
County, California. Stromatolites are an important form of fossil
evidence of life because they form macroscopic structures that
could be found on Mars. It is therefore possible that a search for
stromatolites near the shores of an ancient Martian lake or bay
could be conducted in the near future. Expecting microbial
communities to have formed stromatolites on Mars is not
entirely misplaced geocentricism. The properties of a microbial
mat community that results in stromatolite formation need only
be those associated with photosynthetic uptake of CO 2. There
are broad ecological properties that we expect to hold on Mars
even if the details of the biochemistry and community structure
of Martian microbial mats were quite alien compared to their
terrestrial counterparts. Within stromatolites, trace microfossils
can sometimes be found.traits can be established based on how widespread they are.
Traits that are found in all or most of the major groupings
should be primitive, particularly if these traits are found in
groups that diverged early. Traits found in only a few re-
cently related groups are probably younger traits. This line
of reasoning applied to the entirephylogenetictree would
indicate which organism extant today has the most primi-
tive set of traits. This organism would therefore be most
similar to the common ancestor. Studies of this type have
indicated that the organisms alive today that are most sim-
ilar, genetically and hence presumably ecologically, to the