(Note to everyone who is still sitting on the edge of their seat waiting for the promised follow-up to my last post: be patient. It's Halloween today so I'm doing a Halloween post.)
This past weekend I took my son to see the original Invasion of the Body Snatchers (1956) at Amherst Cinema. It was extremely entertaining for both of us. Some old "classic" films don't actually age well, but Body Snatchers held up very well indeed. I suspect it's because the movie has some really good actors in it, and the only fantastic elements shown on screen are some big paper-mache seed pods and a couple of wax dummies which are supposed to look like wax dummies. There's no monster suit to see the zipper on.
However, it did make me start wondering about the actual biology of the alien "Body Snatcher" organisms. From the film we know that they arrived on Earth in the form of seeds, drifting through space (more on this anon). The seeds grew into the big pods which (somehow) duplicate the forms and steal the minds of nearby living things. (Exactly what happens to the originals afterward is never mentioned; we don't ever see the Pod People disposing of bodies.)
While nobody in the film actually gets around to dissecting one of the duplicates, the movie strongly suggests that their impersonation of humans is complete, right down to the internal organs. The only difference between the Pod People and real humans is their lack of genuine emotion. (Though one wonders what a DNA test would reveal.)
But I find myself wondering: what are the Pod People for? What evolutionary purpose do they serve? What part in the Body Snatcher life cycle do they play?
On Earth we see the duplicates actively cultivating the pods and distributing them, but that requires a species capable of agriculture for them to mimic. What happens if the pods grow on a planet — or even in a remote location — with no intelligent life? What can the Body Snatchers accomplish by duplicating, say, mountain lions, or pronghorn antelopes?
Much depends on how innately intelligent the Body Snatchers are. If they are an intelligent species, then duplicates of animal species could still act as "farmers" to cultivate and spread the pods. The trouble is, I don't believe that. How can you have intelligence when your brain is a perfect duplicate of the brain of a species which isn't very intelligent? Unless you assume that the intelligence of the Body Snatchers comes from having an immaterial soul — but the single distinguishing feature of the duplicates is their utter soullessness.
One might posit that the Body Snatchers only take over intelligent life; their seeds remain dormant or are harmless weeds until a proper host evolves on the target world. The difficulty there is how such a system could evolve. The sheer time scale of interplanetary seed dispersal means you simply don't get many generations of Body Snatchers to evolve into perfect duplicators of alien life. If you limit their impersonations to intelligent life, the number of suitable target worlds is so tiny that it's a wonder any Bodies ever get Snatched.
So the Body Snatchers must have a simpler role: mere instinctive protection of the growing pods and maybe a little seed dispersal. That's still an enormous advantage. A plant which has animals selflessly defending it from predators and pests gains an immense boost in reproductive success. Consider how much of the Earth's surface is covered by corn, which worked out that trick in collaboration with the ancient human inhabitants of Mexico.
In that scenario, the intelligence of the human-duplicate Pod People in the film may be the first time the species has ever been truly intelligent. The invasion of Earth isn't just bad news for humanity, it's a menace to the entire Galaxy! A plant species which can enslave an entire technological civilization to disperse its seeds across space can literally conquer all lifebearing worlds. Only a technologically superior civilization might — might — be able to fend them off. Maybe that's what the terrifying robot Gort in The Day the Earth Stood Still is really for: not preventing interplanetary war, but making sure Earth hasn't been colonized by Body Snatcher pods. He's a flying-saucer piloting Department of Agriculture.
Other Times, Other Places
Maybe I'm overstating the Body Snatcher menace. In order to get their seeds onto other worlds, the Snatcher plants must send out an absolutely enormous number of spaceborne seeds. This, in turn, suggests that it's not uncommon for a planet to be hit by more than one seed in its history. Is it really likely that the Santa Mira infestation of 1956 is the first time a Body Snatcher sprouted on Earth?
Perhaps the native life forms can fight back. After all, when the Body Snatchers enslave native organisms to defend the growing pods, that's obviously going to be bad for those organisms. They won't reproduce, and may not live as long (the Pod People don't love, and seem to be rather careless drivers). Maybe an infestation gradually burns itself out, occupying a region but gradually getting out-competed by un-duplicated native organisms. And of course there would be strong evolutionary pressure to develop resistance to duplication.
And even during historical times it's possible that humans beat back other infestations, without even really understanding what they were accomplishing. Consider the various "witch panics" in Europe, Africa, and elsewhere during the Early Modern period. Were those "witches" who aroused such unreasoning hatred really Pod People? Ironically, a modern, mobile, high-technology, tolerant society might be more vulnerable to the interstellar plant menace than an isolated medieval culture.
Still: make sure to check under the bed before you sleep tonight. Might be a pod under there.