An advanced civilization passes through eight stages:
- A congenial star system
- Reproductive molecules
- Simple single-cell life
- Complex single-cell life
- Sexual reproduction
- Multicellular life
- Tool-using animals with big brains
- Colonization explosion
Now, we haven’t observed any intelligent extraterrestrials. That implies that at least one of these steps is very improbable, a “filter” that prevents life from colonizing space.
We’re on step 7. If the filter is among steps 1-6, then we’re not likely to meet any neighbors — something prevents most life forms from getting as far as we have. If the filter is in step 8, then it appears some catastrophe must strike us soon. Our future, it seems, must be either lonely or ruinous.
“The larger the remaining filter we face, the more carefully humanity should try to avoid negative scenarios,” writes George Mason University economist Robin Hanson. “Our main data point, the Great Silence, would be telling us that at least one of these scenarios [e.g., nuclear war, ecological collapse] is much more probable than it otherwise looks.”