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.

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About 300 million years ago, there existed a small, shrew-like species who really wish the

Earth wasn't so full of big, pesky dinosaurs, and while that creature

is long extinct, its descendants now dominate the planet, as it was with them, so it likely

will be with us in the future, and maybe Kordantans, a creature resembling a mix

of rat, a possum, and true, from which every future mammal descended.

Back then this creature and its siblings were a species, and as far as we know, the

only mammals, so mammals would have been a species and not a class yet.

Millions of years later that species would have become a genus composed of mini-species,

millions more and it would have become a family composed of mini genus each of many species,

then eventually an order composed of many families

and finally a class. That rat-like ancestor of ours existed just a few hundred million

years ago when the universe was around 2% younger than it is now, and it would seem like

it will not take nearly that long for humans to split out into a genus, then a family, then

an order, then

a class all on its own.

...