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.

What do you call a dinosaur car accident?

A Tyrannosaurus wreck.

But seriously, what if dinosaurs hadn't been destroyed in a cosmic accident and invented cars and other technology.

The 19th century was an amazing time to be in any of the fields that spent their time digging up fields,

looking to expand our knowledge of Earth and its past, and one of the highlights of that was the growing discovery of fossils from ancient and giant lizards.

People came from all around to view the wired-together skeletons of these mighty and terrible extinct monsters,

helping fund many of our natural history museums and inspiring some of the most fascinating

tales of science fiction and fantasy.

From the outset people wondered what could have ended the story of these giant dinosaurs,

and the early theories ranged from their eggshells being too thin, or the ice ages wiping

them out, to them just being too big and aggressive for Noah to get on his arc.

About the time I was born, though, Nobel Prize winning physicist Louis Alvarez, his son

and geologist Walter Alvarez, and chemist Frank Asero and Helen von Michael discovered a sedimentary

layer with a very high concentration of

iridium which could be found all over the world. This is when the asteroid extinction hypothesis

gained traction, though it's been suggested at least as early as the 1950s. The evidence then

was hardly conclusive yet, but the reasoning goes as follows.

...