Life in the Universe

The great Fraser Cain, creator of Universe Today and animal advocate, believes we are alone in the universe. His reasoning:

If life had evolved elsewhere, it would inevitably evolve to intelligence. Any intelligent species would eventually send out von Neuman probes throughout the galaxy. It would only take ~10 million years (a very short time compared to the ~14 billion year history of the universe, or even the ~4.5 billion year history of Earth) for an intelligent species' probes to reach every corner of the galaxy. 

To quote Enrico Fermi: Where is everybody?   [OTOH]

Hence, Fraser concludes, since we see no evidence, there must not be any other life out there.

Fraser could very well be right. But if I had to bet my life, I'd bet he's wrong.

The Earth formed 6,000 ~4.5 billion years ago. That's 4,500,000,000 years ago. For the first millions of years, the Earth was uninhabitable - no liquid water, many asteroid strikes (including the biggie), non-stop volcanism, etc. 

Almost immediately after Earth was cool enough for liquid water, life came into existence -- even before the Late Heavy Bombardment! 

That makes the question all the more compelling: Where is everyone?

I think the answer lies between "life evolves" and "technology spreads throughout galaxy."

There are many filters between the evolution of life and a technological civilization spanning the galaxy. Just two examples:

1. For ~75% of the history of life on earth, there were only unicellular organisms. Life itself evolved almost immediately, but then took literally billions of years to evolve into the simplest multicellular organism. It took relatively forever to go beyond single cells, even on a planet that is basically perfect for life. 

If life existed on Mars (which seems more likely than not), that life was always simple. If life exists on Enceladus or another watery moon, it is and always will be simple. It seems entirely probable that multicellular life would only evolve under the rarest circumstances.

2. Even given multicellularity and many other assumptions, technological intelligence certainly doesn't seem inevitable or even likely. Dinosaurs dominated Earth for about 180,000,000 years; humans have existed for only about 300,000 years, ~0.004% of Earth's existence (and humans have been "technological" for a vanishingly small fraction of that time; we're still not close to sending out von Neuman probes). Give just the tiniest nudge to the Chicxulub meteor* and dinosaurs would still rule the Earth. 

There are many other reasons why technological life might be vanishingly rare, even with life "eager" to come into existence. (Complex cells, land+water, oxygen, moon, complex animals, not going extinct, relatively stable climate, fire, not going extinct again, energy capture, metallurgy, institutions.) For more, see the book Rare Earth.  

What data we actually have shows: 

1. Life existed for 100% of the billions of years Earth has been habitable. 
2. We've been able to search for life elsewhere ~0% of that time.
3. We have been von-Neuman-capable for exactly 0% of that time.

This is not to say technological life is impossible outside of Earth.

But it would necessarily be exceedingly rare compared to life itself. Other technological civilizations could be out there, and we might still be millions of years from their probes reaching our solar system. 

Or we could be the first. Someone has to be the first. Even in a universe where life is common, it would appear to the first technological species that the universe is devoid of any other life. 

PS: Just to be clear, I don't want there to be other sentient life in the universe, as suffering seems to be the natural state of life. Here is a good insight into how we take our biases and make them "universal" truths.

*The animation at that link is funny.