Encyclopedia of Evolution.pdf - Online Reading Center

Are Humans Alone in the Universe?
When considering the evolution of intelligent life in the entire universe,
scientists are confronted by two stupefying vastnesses: the
improbability of the evolution of complex life, and the prodigious
expanses of the universe. In a universe with possibly a hundred
billion galaxies, each with billions of stars, even the least probable
event may be expected to occur occasionally, even frequently.
Humans will probably never know whether or not the universe
contains other life-forms with a human level of intelligence. Even if
there were thousands of other civilizations in the universe, could
they contact humans, or humans contact them? Light and other
forms of photonic transmission require millions of years to travel
among galaxies. Human observers would learn about these other
civilizations only if they had evolved to the extent that they could
have sent messages millions of years ago; and by now, they may no
longer exist. Another problem is the ability to recognize a generalized
transmission from another civilization. This is the idea behind
SETI—the Search for Extraterrestrial Intelligence—and the novel
Contact (see sagan, carl): one has to look everywhere for anything
that might be a signal, perhaps an irregularity in what had been
considered the lifeless throbbing of a pulsar.
Because of the enormous resources that deep space communication
and travel would require, the fact that humans have neither
been visited nor contacted from outer space means little. Science
fiction speculates about the possibility of travel through wormholes
or through other dimensions, but any travel through a wormhole or
a black hole would probably result in the destructive scrambling of
whatever goes through it.
The equation first formulated by astronomer Frank Drake
allows a rough estimate of how many planets with advanced civilizations
might exist in the Milky Way galaxy. The equation consists
of a series of probabilities: the fraction of stars that have planetary
systems, the number of planets in a system that have ecological
conditions suitable for life, the probability that life will evolve, the
probability that intelligence will evolve, and the probability that
advanced technology will develop. The problem with these calculations
lies with assigning values to the probabilities. Cosmologists
disagree greatly on even the order of magnitude of some of them.
There is little doubt that simple life could be widespread in
the universe. Bacteria and archaebacteria on Earth survive in a
tremendous variety of circumstances that humans would consider
destructive to life (see archaebacteria; bacteria, evolution
of). Any conceivable bacterial life might require a solid planet
with water. But there might be billions of such planets in the universe,
and many of them might have life that resembles bacteria.
Bacterial life-forms may have evolved even on Mars (see mars,
life on). However, the conditions that would allow the evolution
of complex life-forms, beyond the bacterial stage, might be vanishingly
rare. In order for complex life to evolve on a planet, the
planet must have relative stability for a long period of time. The
emergence of complex life on Earth was preceded by nearly three
billion years of microbial evolution. How likely is the existence of
other planets which, like the Earth, have had billions of years of
relatively stable conditions? It depends on the answers to questions
such as the following:
origin of life
How typical is the galaxy in which humans live? It is true that
there are a lot of galaxies. The first deep field photographs from
the Hubble telescope, in 1995, showed 1,500 galaxies from an area
of deep space just one-thirtieth the area of the full moon. However,
the Milky Way galaxy seems to be an unusually calm place to live:
• Gamma ray bursts from colliding neutron stars would destroy any
life for many hundreds of light-years around, perhaps life in an entire
galaxy. These bursts are common enough in the universe that
Earth should be hit by one every 200 million years, but Earth has
apparently not experienced any such radiation during the entire
three and a half billion years of life.
• The Milky Way galaxy is a disc galaxy with orderly revolutions of
stars; but many galaxies are elliptical galaxies, which have less
stable star orbits, and whose stars experience many collisions.
The Milky Way galaxy has had relatively few stellar collisions and
their attendant bursts of radiation. This has contributed to a long
period of stable conditions that have allowed complex and intelligent
life to evolve on Earth.
How typical is the solar system? The Sun and its planets
appear to be very unusual in the Milky Way galaxy, for reasons
such as these:
• The habitable zone. The solar system is in the habitable zone of
the Milky Way galaxy. If the Sun were nearer to the center of the
galaxy, all the stars would be very close together, and the Sun
would be dangerously close to many neutron stars and black
holes. If the Sun were near the edge of the galaxy, it may have
produced few or no atoms larger than helium.
• The size of the Sun. Many stars are giants and burn out as quickly
as 10 million years rather than the 10 billion that the Sun will persist.
Ten million years would not be enough time for complex life
to evolve. Many stars are hotter than the Sun and would produce
much ultraviolet radiation, which would destroy life on its planets.
Many stars are small enough that they just convert hydrogen
to helium, perhaps up to bismuth. This is mostly what is happening
in the Sun now. However, the supernova that produced the
solar system generated many heavier elements. Because of this,
according to geologists Peter Ward and Donald Brownlee, the
Sun has a 25 percent greater amount of heavy elements than a
typical star its size.
• The stability of the Sun. Many stars fluctuate in energy output;
in contrast, the Sun has been mild and stable, not producing the
bursts of energy that could easily wipe out advanced life-forms.
Binary star systems are very common (two-thirds of sun-sized
stars are in binary or multiple groups), but complex life could not
evolve on planets associated with binary systems because climatic
conditions would be very unstable.
• The rarity of asteroids. Early in the history of the solar system,
the Earth, Moon, and Mars were bombarded by asteroids. This
bombardment stopped about four billion years ago (see asteroids
and comets). The first evidence of sediments on Earth
comes from this time, when the Earth cooled and the oceans
formed. Part of the reason this bombardment stopped is that
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