Peter Ward is a paleontologist and professor in the department of Earth and space sciences at the University of Washington. He is the co-author, with Donald Brownlee, of two books, Rare Earth and The Life and Death of Planet Earth.

In this installment of the series, Dr. Ward talks about why he thinks humanity is unlikely ever to make contact with or detect communications from other intelligent species in our galaxy.

In part I of this series, Frank Drake discussed the history and content of the Drake equation. Subsequent parts will include the comments made by David Grinspoon and the question-and-answer period that followed the opening remarks.

Peter Ward: Well the first thing that you should find striking about us sitting at this stage is not that David is here, because he's a planetary scientist, but that the person in the middle, me, is a paleontologist. And I think it says an awful lot about the evolution of astrobiology as a field that someone who studies the fossil record should be sharing the stage at all with Frank Drake. If someone told me even six or seven years ago that this would be happening, I'd have laughed. And maybe I still should laugh.

I wrote the book Rare Earth because I began to think that maybe paleontology did have something to say. Luckily for me, I dragooned my friend Don Brownlee, who knows an awful lot about astronomy, to help me not make the more egregious mistakes, although many did creep in.

The first conference I went to was a science-fiction convention, and the book had been out about a month. And I got on the stage, and my moderator - I was in a room full of people in wookie costumes and alien costumes, and they're all there - and I was introduced and the round of booing was unbelievable. Because I was the person that was there to take their aliens away from them. And I was lucky to escape with my life.

Well, my sense is that we really should rename SETI, "SETL," the search for extraterrestrial life. And the reason I say that is that I think that even if we never do find intelligence out there, we would all settle for finding an alien, even if it were a bacteria. Because in our lifetime this would be simply stated, the greatest discovery of all time.

Simply put, we'd like to know, is other life Earth-like? Does it have DNA or a DNA equivalent? Or is something radically different? And one of the greatest questions we have to face is what types of life are permitted by chemistry. If we assume that chemistry and physics are the same throughout the known Universe, chemistry and physics have rigorous rules that life will have to follow.

Will life that is not Earth-like be more fragile or more durable? Because Earth-like life is very fragile, and seemingly, the more complex it gets, the more fragile it becomes. A bacterium can withstand temperatures from below freezing to well above boiling. But I say that any of you put into below freezing to above boiling for a very long time at all, will be turned into very different states of matter, none of which are thinking for very long.

We complex beings can inhabit a very narrow range. Secondly, it takes us a long time to evolve. So we need, then, conditions that are narrow for long periods of time. And therein lies the problem.

The second aspect that we tried to bring into Rare Earth is that all parts of the galaxy are not the same. Just as you live in neighborhoods - some are good and some are bad - we believe that the entire galaxy as a whole can be looked at as neighborhoods. People always say: The galaxy has 400 billion stars. The numbers are so huge; surely we can find all types of life out there.

But are all 400 billion stars in real estate portions of the galaxy that will allow life to exist for long periods of time? And our thoughts are that perhaps many of the stars in the centers of galaxies live in very hostile neighborhoods. Hostile because life, being fragile, can find itself succumbing to catastrophe.

And this is where paleontology enters. Our Earth has seen major catastrophes over the last 500 million years. Five of these have wiped out over half the species. One of these over 90 percent of the species. Now this comes close to annihilation of even the least-complex life forms, when you kill 90 percent. If you simply up the ante of whatever the conditions were, you can conceive of killing every animal and plant on the planet.

Well, surely that catastrophe has befallen other planets. And where in galaxies do those neighborhoods exist where you can die out? What are the times between catastrophes? How long can you have a planet that allows it to be the cr�che that allows the evolution of life, first of all, and secondly the evolution of complex life? Our ignorance is so daunting. And perhaps the most ignorant part of it is in the biology.

We're doing much better in astronomy, because we're discovering planets. But I ask you this question: If we replay the tape of Earth - let's start out with the cosmic gas, the nebula that eventually produced our solar system; let's redo it 100 times - do we get life on this planet all 100 times? Do we get it once? Or do we get it some number in between? And there is no-one in this room, no-one on this planet that can give us even a close estimate.

I hear over and over, and I've stated it myself: Life must be easy to make, because we got it on Earth almost as soon as it could have evolved. Where's the science behind that? Those who are attempting to produce life in a test tube are getting nowhere. We are much more realistic in understanding the physical environments than the biological environments.

And finally, intelligence. How often does intelligence arise? As a paleontologist, I can tell you there've been a lot of species on this planet in the past. There are easily 10 million species on the planet today. The average species of mammal lasts 5 million years. Clams last 10 million.

But 10 million years is a good long time for a species. If there's 10 million species on the planet now, and there's been 500 million years of evolution to play with, we can run up to hundreds of millions, perhaps a billion species on planet Earth. And out of perhaps 500 million species, one has been able to build a radio telescope, the definition of intelligence that radio astronomers give us.

Now are we saying that Earth life is particularly bad and stupid, that it doesn't produce more intelligent species than one in 500 million, and in over 500 million years of animals we get but one set of intelligence? We have but one planet to play with and one set of numbers.

My own sense is there are indeed other intelligent species in our galaxy. How many there are is what we're all here to debate. The numbers are so huge. But it wouldn't surprise me in the least if, a thousand years in the future, after much looking, we've never heard from anybody. Because the number may be so small, the distances so vast, that we just never get to run into them. I would much greater put my bets on a bacterium than I would on an ewok.

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