Where are the extraterrestrials?

Dennis Overbye writes in the NY Times that not everyone is excited by the possibility of primitive life on Mars or elsewhere:

In an article published in Technology Review in 2008, Professor Bostrom declared that it would be a really bad sign for the future of humanity if we found even a microbe clinging to a rock on Mars. “Dead rocks and lifeless sands would lift my spirit,” he wrote.

Why?

It goes back to a lunch in 1950 in Los Alamos, N.M., the birthplace of the atomic bomb. The subject was flying saucers and interstellar travel. The physicist Enrico Fermi blurted out a question that has become famous among astronomers: “Where is everybody?”

The fact that there was no evidence outside supermarket tabloids that aliens had ever visited Earth convinced Fermi that interstellar travel was impossible. It would simply take too long to get anywhere.

The argument was expanded by scientists like Michael Hart and Frank Tipler, who concluded that extraterrestrial technological civilizations simply didn’t exist.

The logic is simple. Imagine that one million years from now Earthlings launch a robot to Alpha Centauri, the closest star system to our own. It gets there in a few years, and a million years later sends off probes to two other star systems. A million years after that, each of those sends off two more probes. Even allowing for generous travel times, in 100 million years roughly a nonillion stars (10³⁰) could be visited. The galaxy contains maybe 200 billion stars, so each could be visited more than a trillion times in this robot crisscrossing.

I think that this is correct. If Earth-like planets are common, then it would only take 100M years for an advanced civilization to colonize the galaxy.

It seems reasonable to assume that primitive life has evolved on 100s of other planets in our galaxy. But it is doubtful that any of them evolved into an advanced civilization.

Earth has many strange features that have been essential to human life, and are unlikely elsewhere. We have a single sun, a single large moon to cause tides and stabilize the orbit, a Jupiter to clear out other junk, water covering 2/3 the Earth so sea and land life is possible, etc. We do not know where the water comes from.