The annals of science fiction are filled with advanced extraterrestrial
creatures like Klingons and Wookies, Vogons and Romulans, all carrying
on in a human sort of way. And while screenwriters and novelists weave
stories around these characters, some people scour the heavens for signs
that such highly evolved beings really are out there.

But a new book by two University of Washington scientists contends that,
contrary to popular thought, we just might be alone and Earth might be
unique, if not in the universe at least in this celestial neighborhood.

In “Rare Earth,” published this month by Copernicus Books/Springer,
paleontologist Peter Ward and astronomer Donald Brownlee examine the
remarkable confluence of conditions and events that deposited life-forming
chemicals on Earth, allowed simple life to gain a foothold and then
protected the planet sufficiently and created just the right environmental
factors for advanced life to slowly evolve.

“It seems like something a lot of people don’t want to hear, yet nearly
everyone who works in these areas has remarked at one time or another how
unusual the Earth is,” said Brownlee, an expert on comets, the space bodies
that might have delivered the first organic chemicals and life-sustaining
water to Earth.

In fact, he and Ward, whose extensive research on the fossil record has
provided key insights into prehistoric mass extinctions, frequently
discuss the Earth’s unusual character with students in their astronomy
and geological sciences classes.

The scientists don’t argue that life is rare. In fact, recent evidence
showing simple microbial life can survive extreme conditions on Earth
is an indicator that such life also might be widespread in the galaxy and
the universe.

“But you need to have a vast amount of time to let evolution ramp up to
animals, and we think there are only a small number of planets where that
could happen,” said Ward.

The key, he said, is having near equilibrium in such things as temperature
and water content over enormous time spans.

Microbes have shown they can live in some of the harshest Earth environments
imaginable, while advanced plant and animal life requires a delicate balance
of conditions.

“For 90 percent of the age of this planet, life was slime at the bottom of
the ocean,” Brownlee said. But that life was given a one-in-a-million
opportunity to gradually evolve to the complexity it enjoys today.

“The underlying theme of the book is that the Earth is a very charmed
planet,” he said. “We know of no other body that is even remotely like
Earth.”

Factors that made advanced life possible include the Earth’s having:

  • The proper distance from the sun to allow development of habitat for
    complex life and ensure that water remains liquid, not vapor or ice.

  • The proper mass to retain atmosphere and ocean.

  • Plate tectonics, which act as a sort of atmospheric thermostat, build
    land masses and enhance biotic diversity.

  • A neighbor the size of Jupiter, not too close and not too far away, that
    can use its gravity to protect the planet from too many life-extinguishing
    collisions with comets and asteroids.

  • A stable orbit unperturbed by giant planets.

  • A large moon at the right distance to stabilize tilt, thus ensuring
    seasonal climate fluctuations that are not too severe.

  • Enough carbon to support development of life but not so much to allow
    for runaway greenhouse conditions.

In addition, Brownlee and Ward contend, the solar system’s position in the
Milky Way galaxy also is key to life development on Earth. At the edge of
the galaxy, many stars are too metal-poor for planet formation, while
the center of the galaxy has extreme energy processes that would hinder
complex life.

The “charmed” conditions on Earth won’t always be present. Someday, some
way, evolution on Earth will end. That could be when the sun gets so hot
that life can no longer survive, when ultimately the ocean boils and surface
rocks melt.

“There will be a time when there will be no record of life ever having
existed on Earth,” Brownlee said.

He and Ward acknowledge that their assumptions about how uncommon
advanced life might be in the universe are based on observed conditions
that allowed evolution on Earth. But this is the only place in which
advanced life is known to have occurred, and it is one of only a handful
of places in the solar system where even microbial life is suspected,
making this planet the ultimate laboratory on advanced life.

A key condition for life on Earth is the presence of carbon, because of its
unique properties.

“Probably all life is based on carbon,” Brownlee said. While he concedes the
possibility that life has evolved elsewhere based on an element such as
silicon, he remains skeptical of that theory.

“Many things are possible. You can never imagine everything the universe
can do. But we know it didn’t happen here,” Brownlee said. “If things have
to obey physical and chemical laws, then there really aren’t a lot of options
in nature.”

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For more information, contact:

Ward at (206) 543-2962 or argo@u.washington.edu

Brownlee at (206) 543-8575 or brownlee@bluemoon.astro.washington.edu