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Our
planet project is guided by
the laws of science as we
know them. However, we do
not know how life began.
Is
life a disease of matter,
something that will appear
anywhere in the universe if
favorable conditions exist?
Is its appearance a miracle,
a conscious act of Divine
creation? Is it an accident?
For the purposes of this
course, the appearance of
life on your planet does not
have to be explained. Your
life forms must be adapted
to their environmental
conditions, however, and
must interact in simple
ecological communities. This
will be challenging enough!
Scientists
have several ideas about
how life may have developed
here.
They include
The
Warm Little Pond Theory
After
Charles Darwin published his
book On the Origin of
Species a natural
question was, What have we
evolved from? This led
to questions about how life
began. One idea was
that a warm, quiet lake or
pond that contained a rich
mixture of organic
(carbon containing)
molecules could have been
the place where life began.
In 1953, Chemists Stanley Miller and
Harold Urey at the University of Chicago
performed an interesting experiment.
They put some water and simple inorganic
molecules (such as ammonia, water vapor
and methane) in a sealed flask
and ran electrical sparks through the
mixture. After a few days they
tested the contents of the flask and
found that more complex organic
molecules had formed, including some
amino acids, which are the building
blocks of proteins. Inspired by
these results, other scientists did
variants of this experiment, and were
able to show that under similar
conditions a number of more complex
organic molecules would form. However,
it is a long way from complex molecules
to living cells, and it is now believed
that the early atmosphere differed from
the one provided by Miller and Urey.
This interesting idea is still being
tested and discussed.
The
Extremeophiles
Now,
world builders, as you are resting
up from visiting your hot, violent
young planet, surely you
remember the lava fountains,
the smash of incoming
asteroids, the hot, unbreatheable
air, and the general
inhospitality of the place.
As scientists have
discovered life forms living under
difficult conditions, they
have come up with some new
theories. These
theories have been supported
by research into DNA, the
molecules that are the
genetic code for life forms.
By studying and comparing
DNA from different life
forms, the scientists are
mapping out the
relationships of organisms
to each other.
Some
scientists wonder if life
could have originated in or
around deep sea volcanic
vents, where hotter than
boiling water wells up,
carrying metals, minerals,
and sulfur. Rcently discovered life
forms cluster around vents
in our present day oceans,
and their discovery excited
and amazed scientists.
These are life communities
that still have autotrophs
and heterotrophs,
but do not use sunlight as
their basic form of energy.
On a violent, hot, and
unstable earth, these
organisms might have been
the first life forms.
They would have been
protected from much of the
falling space debris, and
also the ultra violet
radiation, if they were deep
under water.
Panspermia
Other scientists
wonder if perhaps life came to earth
from other planets, perhaps from an
earlier and well watered Mars, or
planets from another solar system.
Our own space journeys have shown that
microbes on the outside of, for
instance, a space camera, are still
alive when they return to earth despite
spending time in the low temperatures,
dehydrating conditions, and airless
environment of space. We do not
know how long microbes could survive in
space. However, a planet is a very
small target out in the vastness of
space, and many spores would be needed
if even a few were to land on our planet
by chance
A related
theory asks if the organic molecules
might have been assembled in the cold
ices of comets. Amino acids have
been found in this ice, and also in
meteorites, and some of these molecules
have survived their fiery descent
through our atmosphere.
Creating
Structure on Clay
Life on
our planet is cellular. This means
that the organism contains itself in a
little packet, a cell wall or a cell
membrane, that encloses the molecules
necessary for its life and
reproduction. The cell has to be
able to let the useful chemicals in
and keep the other materials outside
itself. However, this poses an
unsolved problem: How and why
would a cell membrane evolve if there
was nothing inside it? And how
could the complex reproductive molecule
evolve unprotected in a boundless water
environment? These are important
unanswered questions, but some
scientists think that the molecules could
have evolved structurally if they built
themselves in clay, which has very
small, thin, flakes that might have
provided a structured, sheltered
environment in which the complex
molecules could have become assembled.
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