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 ad-free, plus hours of bonus content,

check out go.nebula.tv slash Isaac Arthur and use my code, Isaac Arthur.

Once long ago, some human looked up at the starry skies and was the first to wonder if they

might be the realms of the gods or home to other worlds like ours, but some think that

maybe they instead looked up in surprise at those skies to see a visitor from those places

descending and preparing to change humanity forever.

In the last few centuries, the fields of archaeology and paleontology have opened our

eyes to the ancient wonders hidden from us by the sands of time.

Even before then we beheld places like the pyramids or stonehenge and wondered about

the people who built them and why they built them.

These megalithic structures are impressive even today, and even more so for being produced

by peoples who lacked modern technology.

It begs the question of why such extraordinary efforts were made, and also the question

of if we got help doing it from some older civilization, residents or

visitors from far away that have since vanished from our world. Our history is full of

stories of visitors coming to teach us or uplift us, making us, or making us more than simple

animals, or seeing that intelligence emerging and helping to nurture it along,

sometimes for benevolent reasons, sometimes for selfish ones, often for unknown or ambiguous

ones, but uplifting us from our primitive state and putting our feet on the path forward

...