WORLD OF MICROBIOLOGY AND IMMUNOLOGY Life, origin of349•
leaf-like in appearance. Fruticose lichens rise above their sub-
strate, and are much branched and bushy in appearance.
Most lichens regenerate asexually as lichen symbioses,
and not by separate reproduction of their mycobiont and phy-
cobiont. Reproduction is most commonly accomplished by
small, specialized fragments of thallus known as soredia, con-
sisting of fungal tissue enclosing a small number of algal cells.
The soredia generally originate within the parent thallus, then
grow out through the surface of the thallus, and detach as
small bits of tissue that are dispersed by the wind or rain. If the
dispersing soredium is fortunate enough to lodge in a favor-
able microenvironment, it develops into a new thallus, geneti-
cally identical to the parent.
Because they are capable of colonizing bare rocks and
other mineral substrates, lichens are important in soil forma-
tion during some ecological successions. For example, lichens
are among the first organisms to colonize sites as they are
released from glacial ice. In such situations, lichens can be
important in the initial stages of nitrogen accumulation and
soil development during post-glacial primary succession.
Lichens are important forage for some species of ani-
mals. The best known example of this relationship involves
the northern species of deer known as caribou or reindeer
(Rangifer tarandus) and the so-called reindeer lichens
(Cladina spp.) that are one of their most important foods,
especially during winter.
Some species of lichens are very sensitive to air pollu-
tants. Consequently, urban environments are often highly
impoverished in lichen species. Some ecologists have devel-
oped schemes by which the intensity of air pollution can be
reliably assayed or monitored using the biological responses
of lichens in their communities. Monitoring of air quality
using lichens can be based on the health and productivity of
these organisms in places variously stressed by toxic pollu-
tion. Alternatively, the chemical composition of lichens may
be assayed, because their tissues can effectively take up and
retain sulfur and metals from the atmosphere.
Some lichens are useful as a source of natural dyes.
Pigments of some of the more colorful lichens, especially the
orange, red, and brown ones, can be extracted by boiling and
used to dye wool and other fibers. Other chemicals extracted
from lichens include litmus, which was a commonly used
acid-base indicator prior to the invention of the pHmeter.
Some of the reindeer lichens, especially Cladina
alpestris, are shaped like miniature shrubs and trees.
Consequently, these plants are sometimes collected, dried, and
dyed, and are used in “landscaping” the layouts for miniature
railroads and architectural models.
In addition, lichens add significantly to the aesthetics of
the ecosystems in which they occur. The lovely orange and
yellow colors of Caloplacaand Xanthorialichens add much
to the ambience of rocky seashores and tundras. The intricate
webs of filamentous Usnealichens hanging in profusion from
tree branches give a mysterious aspect to humid forests. These
and other, less charismatic lichens are integral components of
their natural ecosystems. These lichens are intrinsically impor-
tant for this reason, as well as for the relatively minor benefits
that they provide to humans.LLife, origin ofIFE, ORIGIN OF
The origin of life has been a subject of speculation in all
known cultures and indeed, all have some sort of creation
idea that rationalizes how life arose. In the modern era, this
question has been considered in terms of a scientific frame-
work, meaning that it is approached in a manner subject to
experimental verification as far as that is possible.
Radioactive dating suggests that Earth formed at least 4.6 bil-
lion years ago. Yet, the earliest known fossils of microorgan-
isms, similar to modern bacteria, are present in rocks that are
3.5–3.8 billion years old. The earlier prebiotic era (i.e., before
life began) left no direct record, and so it cannot be deter-
mined exactly how life arose. It is possible, however, to at
least demonstrate the kinds of abiotic reactions that may have
led to the formation of living systems through laboratory
experimentation. It is generally accepted that the develop-
ment of life occupied three stages: First, chemical evolution,
in which simple geologically occurring molecules reacted to
form complex organic polymers. Second, collections of these
polymers self organized to form replicating entities. At someA lichen growing on a rock.womi_L 5/6/03 3:32 PM Page 349