Hello, SFIA audio listeners. In this month's Nebula exclusive, Big Alien Theory,

we're asking the reason alien civilizations might be rare is because most aliens are huge.

To hear it and every episode early and add free, plus hours of bonus content,

check out go.nebola.t.v.t.v. And use my code, Isaac Arthur.

This episode is sponsored by Audubbler.

It is often wondered if when we should ever meet aliens they might look or act like us

in whole or part as an example of convergent evolution.

But how likely is this?

So today we will be looking at the notion of convergent evolution, the idea that

life forms will tend to evolve the same traits, given long enough.

This is probably best known from science fiction examples of nearly every alien and robot

looking roughly humanoid.

Of course in reality, this is because we need to put makeup on a human or stuff them into

a costume for the role, but the common science fiction reason given is that the humanoid

form is very efficient and likely evolves most frequently.

Now Convergent evolution is a real thing in science, as is divergent evolution, where

species branches in the vast numbers of very different critters.

It's a bit different than in sci-fi though, as it is usually convergence on a specific

useful trait or organ, like the eyeball, which is thought to have evolved

independently on as many as 40 separate occasions. Light interacts with so many different

organic molecules, and knowing when you're in it, out of it, or crossing the boundary,

can be such important survival information, it's hard for the creature to evolve very

...