Encyclopedia of Evolution.pdf - Online Reading Center

• Chemical composition of the Earth. Most known planets from other
solar systems are gaseous. But if the solid planets were like many
asteroids, which have a great deal of carbon and water, then the
Earth would have either a massive greenhouse effect from carbon
dioxide, or would have been completely covered with deep
oceans, not having the shallow waters that appear necessary for
the evolution of life.
• Size of the Earth. If the Earth had been a little smaller, it would
by now have completely cooled and lost its magnetic field. This
would have allowed cosmic radiation to rip away the atmosphere
and water and life. This appears to be what happened
on Mars. On a larger planet, gravitation would be so strong that
complex life might not be possible. On a larger planet, all geological
forms might collapse underneath an ocean; not only would
there be no terrestrial life but also no erosion of nutrients into
the ocean. As a result, the entire ocean would be nutrient-poor,
just as the middle of the oceans on Earth is today.
• Habitable zone of the solar system. Within the solar system, Earth
is in just the right place, the only planet in the habitable zone. If
the Earth were one percent further away from the sun, it would
experience a runaway ice age; if it were five percent closer, it
would experience a runaway greenhouse effect, sometime during
its history. If the Earth were closer to a smaller star, the star’s
gravity would hold the Earth in an orbit in which one side would
always face the sun—just as the moon always faces the Earth.
Under such conditions, one side of the Earth would burn up, the
other would freeze, and extremely strong winds would result.
As some observers have said, humans should “thank their lucky
star,” but also their extremely lucky solar system and Moon. Without
the concatenation of all of these unlikely events, the Earth
would have had an extremely unstable history, losing its oceans,
or fluctuating in temperature so greatly that only bacterial life
gist Simon Conway Morris to comment that the universe
“smells faintly of mothballs.” There are at least 27 kinds
of organic molecules, some quite complex, in the tails of
Halley’s and Hale-Bopp comets.
• Organic molecules are common in the carbonaceous chondrite
asteroids left over from the initial formation of our
solar system. Even in the 1830s it was known that carbonaceous
meteorites contained organic molecules. The
Murchison meteorite, a carbonaceous meteorite that fell in
Australia in 1969, contained at least 74 kinds of molecules,
of which eight are amino acids found today in living cells,
as well as fatty acids, glycerol, and purine and pyrimidine
bases (found in nucleic acids such as DNA). This was also
true of the Tagish Lake meteorite, which fell in Canada in
2000. Careful analysis discounted the possibility that these
molecules were terrestrial contaminants.
The nebular and comet-tail chemicals are very sparse—
only a few molecules per cubic meter. How could they be
concentrated and delivered to the Earth? An immense
amount of comet dust rains on Earth: about 40,000 tons
per year. However, it is unlikely that a significant amount
of organic material would have survived on meteorites. The
origin of life
could have survived. On a cosmic scale, even the nearly complete
freezing of the Earth that occurred most recently about 700 million
years ago is a mild occurrence (see snowball earth).
According to Peter Ward and Donald Brownlee, when
you consider all of these factors, it is possible that the Earth is
the only planet that has been stable enough for complex life to
evolve—even in the entire universe. At the very least, they claim,
the universe is not like Star Trek, full of humanoids with whom
humans can make contact. Some people have used these very
same data to claim that the Earth has been prepared for our
arrival by a higher intelligence (see anthropic principle). However,
such a principle is unnecessary. While it is unlikely for all
of these lucky things to have happened right here in this part
of this galaxy, it could very well be that humans exist and think
about such things in this place and not somewhere else simply
because this place is where the luck happened to occur. At the
same time, it might also mean that the rest of the universe, even
if chock-full of bacteria, is a very lonely place for creatures with
higher intelligence.
Further Reading
Basalla, George. Civilized Life in the Universe: Scientists on Intelligent
Extraterrestrials. Oxford, U.K.: Oxford University Press, 2005.
Conway Morris, Simon. Life’s Solution: Inevitable Humans in a
Lonely Universe. Cambridge University Press, 2003.
Jackson, Randal. “PlanetQuest: The search for another Earth.” Jet
Propulsion Laboratory, California Institute of Technology. Available
online. URL: http://planetquest.jpl.nasa.gov/index.cfm.
Accessed April 24, 2006.
Sagan, Carl. Cosmos. New York: Random House, 1980.
Ward, Peter D., and Donald Brownlee. Rare Earth: Why Complex
Life is Uncommon in the Universe. New York: Copernicus, 2000.
presence of organic molecules in outer space does not explain
where terrestrial organic molecules came from. It demonstrates
that the universe has produced immense amounts
of the very kinds of organic molecules from which life is
made—and this could have happened on the early Earth as
easily as anyplace else in the universe. Because organic molecules
are so common in the universe, the presence of PAH
(polycyclic aromatic hydrocarbons) in the famous Mars
meteorite is not itself evidence of life.
The study of life outside of the Earth is called astrobiology
(“star-life”), formerly called exobiology (“outside life”).
Astronomer Jonathan Lunine notes that since the discovery of
ALH84001, astronomers and the National Aeronautics and
Space Administration (NASA) have had a renewed interest in
astrobiology and even in the possibility of panspermia.
How
Assuming that life evolved from organic molecules that
formed on the Earth, how could this have occurred? The
question is not new. Charles Darwin wrote a letter to Joseph
Hooker, dated February 1, 1871, in which he made his
famous reference to life originating in a “warm little pond”: