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 exists 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 in hundreds of years, not hundreds of millions.

There once was a single mammal, possibly the Morganyu Korditans, 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 single 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.

...