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Alien Intelligence Depends on Time Needed to Grow Brains By Leslie Mullen Astrobiology Magazine posted: 09:15 am ET 02 December 2002
Spock: To hunt a
species to extinction is not logical.
Gillian: Whoever
said the human race was logical?
Star Trek, The Voyage Home
We expect aliens to be a whole lot smarter than
us. Not only will they possess the wisdom of the ages, but they will travel at
warp speed, have the ability to transform (or destroy) entire planets, and their
civilizations will span across galaxies.
Until we find alien life, however, we can only
guess at how many intelligent civilizations may be out there. Frank Drake made a
stab at guessing the number in 1961, when he formulated the "Drake
Equation".
According to this equation, there could be a
million intelligent civilizations in the Milky Way galaxy, and probably billions
of such civilizations throughout the universe.
The Drake Equation is based, in part, on an
estimate of the number of planets in the galaxy that might harbor life. Such
planets would have to exist in "habitable zones" -- those regions around stars
that would best support life as we know it. These planets would be the most
likely places where life capable of achieving intelligence is fostered and
sustained.
To understand how intelligence develops, we have
only one example to study: the development of human intelligence on Earth. The
first life on our planet probably arose about 3.8 billion years ago, less than a
billion years after the Earth itself formed.
But multi-cellular life didn't appear until
nearly 3 billion years after that, and the first animal life didn't form until
the Cambrian Explosion 600 million years ago. Intelligent life -- which we
broadly define as human civilization -- didn't develop until a few tens of
thousands of years ago. Christopher McKay, a planetary scientist with the NASA
Ames Research Center, has defined intelligence as the ability to build a radio
telescope. If we go by McKay's definition, then truly intelligent life on Earth
didn't show up until the twentieth century.
Since intelligent life took a long time to
develop on Earth, some believe it will take just as long on other worlds.
The paleontologist Peter Ward and the astronomer
Donald Brownlee expressed this belief in their book, "Rare Earth: Why Complex
Life is Uncommon in the Universe." Intelligent life on Earth, they say, is due
to a long chain of events that greatly relied on happenstance. The odds of such
a chain of events occurring on other worlds seem to be impossible. Thus, as the
title of their book indicates, they believe that simple, microbial life may be
common in the universe, but complex life will be rare. They certainly don't
expect to find very many advanced alien civilizations out there.
Other scientists disagree with this conclusion.
They suggest that animal life -- or something resembling it -- may
have developed more rapidly on other worlds. One proponent of this theory is
McKay, who wrote the essay, "Time for Intelligence on Other Planets," in order
to determine the shortest possible time it would take for intelligence to
develop after the origin of life.
Crunching the numbers
Although the traditional view of evolution is as
a constant push toward greater complexity, the fossil record on Earth shows
instead that there were periods of rapid changes followed by long periods where
nothing much happened at all. McKay says such a drawn-out style of evolution
need not be universal. By removing what he calls evolution's "spurious" time
periods, he says that intelligent life could take as little as 100 million years
to develop.
"Nothing in our understanding of evolution
suggests that these periods of stasis are required," says McKay. "We believe
they represent mere historical happenstance."
Another limiting factor for evolution on Earth
was a lack of oxygen. The early Earth had very little free oxygen until
cyanobacteria and other photosynthetic life forms began producing it about 2
billion years ago. Oxygen may be the key to tissue multi-cellularity, and thus
the formation of large, multi-celled organisms capable of developing a brain.
The build-up of oxygen also led to the development of an ozone layer, shielding
life on Earth from the Sun's harmful UV rays.
But this need not be the case on other worlds.
Perhaps some planets begin with substantial amounts of atmospheric oxygen.
Slower tectonic activity would make more oxygen available, as would a less
iron-rich geography. A planet with early access to oxygen might see life, and
intelligence, evolve much faster than on Earth.
Other factors affecting Earth's evolution were
cataclysmic events such as asteroid impacts. Such events would kill off complex
life, but these events could also clear the way for the development of more
advanced forms of intelligence. The creatures with superior brains may have been
better able to save themselves from the sudden changes in their environment
caused by these events.
Many of the factors that went into the
development of life on Earth remain a puzzle to us, so there may be many other
characteristics of a planet, or even a solar system, that affect the development
of intelligence. For instance, some scientists have noted that intelligence did
not arise on Earth until the Sun hit middle age. Perhaps, they suggest,
intelligence cannot evolve until the planet's star reaches a certain stage in
its own evolution.
Chris McKay, however, says he has not heard a compelling argument as to why human level intelligence needed the Sun to be middle aged.
"I would say that the build-up of oxygen is the
only good environmental requirement," says McKay.
Why only one?
The Earth's fossil record indicates that, despite
periods of stasis or of setbacks like asteroid impacts, most organisms evolve
toward greater complexity. Some of Earth's life forms have gone extinct, while
others became cornered in evolutionary dead ends. But as a whole, evolution has
moved toward increasing the complexity of the central nervous system,
culminating in the development of the brain. (The "brain" as an organ within the
skull did not develop until the emergence of the first vertebrate
animal.)
Since evolution seems aimed towards the
development of intelligence, a planet should be able to evolve not just one, but
many intelligent species over time. Yet on Earth, humans were the only species
who developed "radio telescope-building" intelligence.
"It might be argued that among mammals, humans
developed intelligence first and are thereby effectively precluding the
development of intelligence in any other species," says McKay. "It follows from
this argument that intelligence evolves once and only once on a planet, because
once evolved it changes the rules of the interaction between species and
effectively dominates the planet from then on."
Human intelligence may never have developed if
the dinosaurs had not gone extinct. During the age of the dinosaurs, our
ancestors were small, rodent-like creatures scavenging for food in the low
grass. Perhaps we had to wait for the dinosaurs to disappear before we could
evolve beyond a certain point. However, says McKay, this theory still does not
explain why the dinosaurs didn't become the Earth's first telescope-builders.
They dominated the planet for over 150 million years, occupying all the niches
mammals currently occupy.
"That is more than twice the time between the end
of the Cretaceous and the construction of the first radio telescope," says
McKay. "One might speculate that perhaps Stenonychosaurus (also known as
Troodon) or her progeny did build radio telescopes but their civilization was
destroyed by some internal or external catastrophe. Perhaps the lifetime of
their civilization was so short, compared to the resolution of the geologic
record, that it is simply lost without trace in the depths of time. It is
difficult to say what evidence would survive of human civilization - if it was
terminated now - after 65 million years of tectonic activity, erosion, and sea
level change."
Since it seems that intelligence only evolved
once on Earth, despite other opportunities to do so, perhaps not many forms of
intelligence could evolve on other planets. McKay says that, considering the
Earth's evolutionary history, the odds for developing intelligence elsewhere may
be less than one in three (65/215). Still, given the potential number of
habitable planets in our Galaxy alone, that could mean there are many millions
of intelligent species out there.
"The odds I computed are just a rough upper limit
based on the history of Earth as we now know it," says McKay. "For us to be the
ONLY intelligent radio builders in the galaxy, the odds would have much lower --
about 1 in million."
Despite all the rationale behind one viewpoint or
another, the question of how many intelligent civilizations are out there can
only be answered if we discover alien life. NASA is planning to launch the
Terrestrial Planet Finder in 2012. This satellite will operate for 6 years,
searching for Earth-sized planets around distant stars.
In the meantime, scientists with the Search for
Extra Terrestrial Intelligence (SETI) continue to explore the electromagnetic
spectrum for alien transmissions. The SETI Institute recently published SETI
2020, a book detailing the focus of SETI strategies between now and the year
2020.
More Astrobiology
News | Astronotes
This story is presented in cooperation with Astrobiology Magazine, a web-based publication sponsored by the
NASA astrobiology program.