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.tv

slash Isaac Arthur and use my code, Isaac Arthur. This episode is sponsored by Raycon.

We often wonder if in all the billions of plants in this galaxy, another intelligent

species might have arisen on another world, but what about the possibility of two arising

on the same one. A few months back in our Valentine's Day episode on multi-species habitats, we examined the classic

science fiction trope of marrying aliens

and having hybrid children with them. It's also fairly common in fantasy genres too,

usually half human, half-elven children and such. However, a less common trope in science

fiction is two separate species coming about on the same planet of origin and not in a

breeding at all.

This is much more common in fantasy though, and certainly in human mythologies, for presumably

two reasons.

First, the action on fantasy tends to be limited to a single world, so your nominal aliens

have to be from there too, and second and probably more importantly, these are usually predicated on

one or more deities having directly created the intelligent species in question, so we need

not be concerned about the seeming improbability of two separate species, both getting intelligence

at roughly the same time. This is not like us and the Neanderthals, who are simply another

branch of intelligent hominids

we share a common ancestry with, and indeed with whom we have increasing evidence of interbreeding,

but rather something very far apart.

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