Encyclopedia of Evolution.pdf - Online Reading Center

origin of life
Are Humans Alone in the Universe?
(continued)
Jupiter has so much gravity that it has stabilized the asteroid
belt and licked up many comets and asteroids from the solar system.
Jupiter is just near enough to clear away asteroids from the
Earth’s path, but far enough away not to bother the Earth. Jupiter
disturbs just a few asteroids into the path of the Earth, punctuating
the history of life (see mass extinctions) but not often enough
to destroy life.
• The stability of planetary orbits. As of May 2007, 212 planets had
been detected that revolve around other stars. The same Doppler
effect that causes the red shift of expanding universe (see universe,
origin of) also shows that these stars are being tugged by
the gravity of planets going around them. The sizes and distances
of these planets from their suns can be calculated; if the residual
variation shows a pattern, it can be assumed that the star has two
or more planets. In 2005 a photograph of a large gaseous planet
around a distant star was published. It may be impossible to detect
planets as small as the Earth around other stars, either by the Doppler
effect or photographically. The planets so far detected are frequently
“hot Jupiters” (as big and gaseous as Jupiter, close to the
star, perhaps representing a failed binary system). Such huge planets
would destabilize the orbits of smaller planets, causing them to
crash into the large planet, into the star, or be thrown into interstellar
space. A solar system of hot Jupiters would be very unstable,
and any earthlike planets would certainly experience wild swings
of climate which would prevent the evolution of complex life. The
eccentricity (departure from a circular orbit) of the Earth is only
0.0167, which is just enough to produce climatic effects. Some of
the planets detected around other stars have an eccentricity as
Where
There are three possibilities for where life originated:
A. Life evolved on Earth only. According to the Rare Earth
hypothesis (see essay, “Are Humans Alone in the Universe?”),
the conditions necessary for complex life are so
uncommon that the Earth may in fact be the only planet
on which complex life has evolved. The authors of this
hypothesis assert that, although complex and intelligent
life might be unique to the Earth, bacterial life might be
common in the universe.
B. Life evolved someplace else, and was then transported to
the Earth. This hypothesis is called panspermia (“seeds
everywhere”). Swedish chemist Svante Arrhenius brought
up this idea early in the 20th century. Biochemists Francis
Crick and Leslie Orgel have written about the extraterrestrial
origin of the organic molecules that produced life on
Earth, even though life itself evolved here. Astronomers
Fred Hoyle and Chandra Wickramasinghe have quite seriously
suggested that the first cells came to the Earth from
outer space, from a “life cloud”. The Martian meteorite
high as 0.93. Such planets, even if otherwise suitable, would have
wild swings of climate that would prevent the evolution of complex
life. However, in 2007, an Earth-like planet was detected orbiting a
star about 20 light years away.
• The availability of comets. Comets may have played an important
role in bringing water to the primordial Earth (see origin of life).
How can scientists know whether other solar systems even have
comets? The Oort cloud extends halfway to the next star (two
light-years into outer space). Some of those comets may be as
likely to go to Proxima Centauri as to the Sun. Based on chemical
analysis, all the comets known in our solar system seem to
be from the Oort cloud and Kuiper Belt. Apparently there are no
comets from Proxima Centauri coming into the solar system, suggesting
that Proxima Centauri has nothing that corresponds to the
Oort cloud. The star Beta Pictoris appears to have comets falling
into it, based on bursts of different colors of light. But how typical
is this of solar systems?
• The Moon. The Moon stabilizes the 23-degree tilt of Earth. This
stable tilt is what causes the regular alternation of seasons.
Billions of years ago, the Moon was closer and the Earth spun
faster, resulting in extreme winds. The Moon has been moving
away from the Earth, and the Earth’s rotation has been slowing
down, so that winds are not now deadly. The Moon has allowed
the Earth to have stable conditions for long enough that complex
life has evolved. A bigger or smaller moon, or multiple moons,
would be unlikely to produce this effect.
Therefore solar systems like the one in which humans live, suitable
for the evolution of complex and intelligent life, might be very rare
in this or any other galaxy.
How typical is Earth? The Earth may be a very unusual planet
in the Milky Way or any other galaxy.
ALH84001 contains organic molecules, and structures
that may be bacterial fossils. Despite these suggestions,
it is unlikely that Martian bacteria would have survived
being ejected from Mars, the journey through outer space,
and falling through the atmosphere. It is therefore unlikely
that meteorites brought the molecules of life to the Earth.
C. Life evolved on Earth, but also in other places. This idea
is popular among scientists because the universe is, in fact,
full of organic molecules. On the Earth today, all organic
molecules have a biological origin: Even petrochemicals
are the products of plants that died millions of years ago.
But organic molecules can be produced during the same
processes that form stars and solar systems. The most
common elements in universe are hydrogen, helium, oxygen,
carbon, nitrogen, and neon; all but the last of these
predominate in organic molecules:
• Spectral analyses of starlight through nebulae reveal the
existence of at least 62 kinds of organic molecules in those
nebulae. Organic chemicals (for example, naphthalene)
are common in nebulae, which has led evolutionary biolo-