Alien life 'may exist among us'

He has called on scientists to launch a "mission to Earth" by
trawling hostile environments for signs of bio-activity.

Weird life could even be living among us, in forms which we don't yet
recognise, he told the American Association for the Advancement
of Science (AAAS) meeting in Chicago.

"We don't have to go to other planets to find weird life.

"It could be right in front of our noses - or even in our noses,"
said the physicist.

"It is entirely reasonable to expect we will find a shadow
biosphere here on Earth.

"But nobody has actually taken the trouble to look.

"The question is why? The cost is not expensive - it would be a
fraction of the money we spend searching for extraterrestrial
life."

'Second genesis'

Professor Davies was one of the speakers at a symposium exploring the
possibility that life has evolved on Earth more than once.

How do we know we are dealing with separate Earth genesis and not a Mars genesis? Professor Paul Davies, Arizona State University

The descendants of this "second genesis" may have survived until
today in a "shadow biosphere" which is beyond our radar because its
inhabitants have biochemistry so different from our own.

"All our microscopes are customised for life as we know it
- so it's no surprise that we haven't found microbes with different
biochemistry," said Professor Davies.

"We don't quite know how weird life would look. It's as wide as the
imagination and that's why it's really hard to look for."

If it exists, weird life could be based on DNA and RNA - but with a
slightly different genetic code or different amino acids.

At the other end of the spectrum, we could find creatures which have
more drastic differences.

"Maybe one of the elements life uses - carbon, hydrogen, oxygen,
nitrogen, phosphorus - could be replaced by something else," said
Professor Davies.

"When I say that, everyone immediately thinks of silicon life -
because of Star Trek. But I'm not talking about anything that drastic.

"For example, most of the jobs that can be done by phosphorus
can be done by arsenic."

Arsenic may be poisonous to humans, but it has chemical properties
which might make it ideal in a microbe's machinery, he said.

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'Mission to Earth'

So how do we go about hunting for something we have never seen
before?

"There are two possibilities," said Prof Davies, Director of the
BEYOND Center for Fundamental Concepts in Science.

Mono Lake in the US is home to arsenic-fuelled microbes

"One is that weird life is ecologically isolated, in niches
beyond the reach of mankind."

In this case, we must begin trawling the world's most inhospitable
environments - deserts, salt lakes, and areas of high pressure,
temperature or UV radiation.

"We could have a 'mission to Earth'. There's a big long list of
places we could be looking," observed Professor Davies.

"For example, if we are looking for arsenic life, we could head for
environments which are both arsenic rich and phosphorus poor -
such as deep ocean vents.

"There is also a heavily contaminated lake in California
which is arsenic rich - Mono Lake - and we do find microbes in there
which get their energy from arsenic.

"But they don't actually incorporate the arsenic into themselves.
They spit it back out again. They smoke but they don't inhale."

On the other hand, it could be that "weird life" is actually all
around us - intermingled with carbon based life.

"In that case it's going to be really hard to detect - you have to
find some way of filtering everything else out."

This laborious process has been used to search for unknown organisms
in seawater - by painstakingly filtering everything else away.

If we did discover something unprecedented, "we'd all start
arguing" said Professor Davies, a theoretical physicist.

"The question would be whether this life was truly different, or
whether there was a common precursor a deep branch on the main tree of
life.

"Also, how do we know we are dealing with separate Earth genesis and
not a Mars genesis?

"We know rocks do get traded between the two planets, and life could
hitch a ride.

"Personally, I'm only interested in establishing
whether life happened more than once. If we find it has happened twice
from scratch then its going to have happened all around the universe.

"It's going to be teeming with life and there's a very good chance we
are not alone."

Life in the lab

Another way to determine what alternative life might look like
is to try to invent it ourselves.

If we can create new molecules which can behave in life-like way, we
may then go out and look for these in the environment, says
Professor Steven Benner, of the University of Florida.

His team have created perhaps the closest yet to a man-made
alternative form of life.

"We are announcing the first example of an artificial
synthetic chemical system capable of Darwinian evolution," he told
the conference.

"Is it alive? Well, I can tell you that it is not self-sustaining.

"You have to have a graduate student stand there and feed it from
time to time, but it is evolving."

The molecule is essentially a modified version of our own DNA
double helix - but with six "letters" in its genetic alphabet, instead
of four.

These nucleotides pair up in strands, which can replicate,
though only with the help of polymerase enzymes and heat.

"Sometimes mistakes are made in pairing and these mistakes are
maintained in the next generation - it is evolving," said
Prof Brenner.

"The next step is to apply natural selection to it, to see if it can
evolve under selective pressure.

"The accepted definition of life is a molecule capable of
Darwinian evolution, so we are trying to put together molecules
that are capable of doing it."

But he questioned whether our definition of "living" is
perhaps too "Earth-centric".

"Remember - just because you are a chemical system which is
self-sustaining and capable of Darwinian evolution, that
doesn't mean that is the universal definition of life," he said.