PASADENA, Calif. (AP) _ The evolution of life on Earth suggests intelligent life must abound in the universe, some scientists said Friday, but others said that if aliens existed they would have arrived here by now.

''In a universe of 100 billion galaxies, each of which have a few hundred billion stars, the idea that our sun is the only star with an inhabited planet is laughable,'' Cornell University astronomer Carl Sagan said.

''Where do we come off to imagine we're the only kind of life in the universe?''

Sagan and other scientists debated the question of intelligent life in outer space as about 1,200 people met at the annual conference of the Committee for the Scientific Investigation of Claims of the Paranormal, a group that debunks claims about psychic powers and other supernatural phenomenon.

While all the speakers said there had yet to be a single confirmed report of an unidentified flying object, or UFO, that was from another planet, most said they believed intelligent life must exist beyond Earth.

''I'd be willing to bet one hundred bucks that between now and the time I die we'll find evidence of intelligent life,'' said Al Hibbs, a retired space scientist at the National Aeronautics and Space Administration's Jet Propulsion Laboratory.

Because there is evidence that planets may exist around a dozen nearby stars and because the chemical evolution that produced life on Earth exists throughout the known universe, the probability for intelligent life elsewhere is 100 percent, said Frank Drake, dean of natural sciences at the University of California, Santa Cruz.

''We are, as best we can tell, the result of completely normal processes, therefore life should be abundant in the universe,'' said Drake, who also is president of the Search for Extraterrestrial Intelligence Institute, or SETI, a NASA-funded effort to search for radio signals from alien civilizations.

But Robert Rood, a University of Virginia astronomer, argued that if advanced, intelligent civilizations existed elsewhere in our Milky Way galaxy, at least one would have colonized the galaxy and reached Earth by now.

''Even at five centuries (of travel time) to reach the nearest star, taking a few thousand years there until it got crowded, and then going to the next star, it takes only 20 million to 30 million years to get across the galaxy,'' a mere instant in the galaxy's multibillion-year history, Rood said.

He also said it was possible that the formation of Earth's moon in a way that created ocean tides on Earth may be unique, thus making the evolution of the first life forms in tidal pools a fluke that occurred only here.

Jill Tarter, a SETI Institute astronomer, said the organization was seeking $65 million from NASA to finance a new, 10-year, systematic search of the heavens for radio signals that could be signs of an alien civilization. She said the search should begin in 1992.

Radio and optical astronomers in seven nations have conducted 48 separate searches for signs of extraterrestrial life in the last 27 years, ''yet no one has found anything,'' Tarter said.

However, she said, such searches were not systematic.