Life and the Earth's
nature is a stranger yet;
The ones that cite her most
Have never passed her haunted house,
Nor simplified her ghost.
To pity those that know her not
Is helped by the regret
That those who know her
Know her less
The nearer they get."
Dickinson, cited by Margulis and Sagan in MICROCOSMOS, 1986.
can characterize living organisms by various chemical, physical, and biological
parameters, the harmony that puts it all together and forms the spark
we call life is far from understood. We see live organisms that may be
stages toward more complete organisms. Viruses, for example, are essentially
a piece of DNA or RNA coated with protein. It is by inserting this unit
into cells of other animals that viruses reproduce and continue their
lives. Infective viruses have been synthesized from elements. Several
scientists have synthesized DNA and RNA molecules that replicate themselves
in a test tube. But while we can concoct many of the molecules that must
have been in the primeval soup in which life evolved, we cannot yet compose
from elements a group of "cells that crawl out of a test-tube"
on their own, to paraphrase Margulis and Sagan.
We do know
a lot about the environmental conditions that sustain life. We have begun
to understand that not only does the environment sustain life, but life
in turn has made environmental conditions what they are today. We now
discuss some peculiar features and compounds on Earth that make our environment
particularly "fit" for our kind of life. Much of this discussion
is based on three sources:
Four Billion Years of Microbial Evolution, by Lynn Margulis and
Dorion Sagan, 1986.
Arrow and Evolution, by Harold F. Blum, 1951.
by James Lovelock, 1979
the "fitness of the environment," a concept originally proposed
by Lawrence J. Henderson in 1913. Certain aspects of the environment make
the Earth particularly advantageous for living organisms to live, develop,
and evolve. The Earth's size and its distance from the sun (a medium
yellow star), and the nature of the sun itself, determine the gravitational
force of the Earth and the amount and type of electromagnetic energy we
receive. These factors provide the conditions under which life evolved,
and that sustain life on Earth.
/ Discussion Questions:
List and draw a concept map of how Earth's gravity, its distance
from the sun, and the properties of sunlight affect factors critical
to a system of living organisms based on carbon, hydrogen, and oxygen.
List the characteristics of a "live" system. Which of
these are not attainable by artificially-created systems like a
What do you think we would have to look for on another planet to
determine if there are life forms on it similar to ours?
Think of some parts of the Earth with an extreme environment (extreme
temperatures, pressures, etc.). Do organisms live there?
this go here? Or should it be a notecard on symbiosis?]
adapt to extreme conditions through symbiosis. Photosynthesis is an original
adaptive mechanism developed in a species of bacteria called cyanobacteria
a billion years before plants evolved. These seem to have eventually partnered
with fungus-like organisms to evolve into the cells of plants. The cyanobacteria
have now become the part of the plant called chloroplasts which are the
photosynthetic organelles of plants! This gives evidence to a symbiotic
theory of evolution. A group of bacteria live in animals intestines. Another
species live in the seabed seven hundred meters below the surface of the
Gulf of Mexico. Bacteria are responsible for nitrogen fixing -- extracting
nitrogen from the air and making it into nitrates and NH3,
which can then react with water to provide nutrients.