The sand worms of Dune. The huge worm that Han Solo flies into in TheEmpire Strikes Back. The gigantic aquatic beasts of The PhantomMenace. The Sarlac in Return of the Jedi.
One of the classics of ecology is Paul A. Colinvaux' Why big fierce animals are rare: an ecologist's perspective(Princeton, 1978). Every step in the food chain requires about ten times as much biomassin the step below. So carnivores as big as we find in science fiction wouldrequire perhaps millions of times their own mass to sustain them. Then theywould be rare, not so common that hapless humans blunder into them allthe time.
Not only are many carnivores in science fiction huge, but often they live inenvironments totally lacking in food. The huge worm that Han Solo flies into in TheEmpire Strikes Back lives on an airless asteroid. How did it get there?Where does it find food? How does it reproduce?
The sand worms of Dune and the Sarlac in Return of the Jedi livein deserts. So where do they get the biomass to sustain themselves? Dunemakes an attempt to describe a food chain, but the sand worms are the size ofblue whales and so common that even minor disturbances anywhere on the surface attractthem. There's just not enough biomass to sustain them.
The beasts of the Alien series. Star Trek: The Next Generationhad an episode involving a life form that reproduced by taking over the bodiesof visiting aliens.
This is a pretty iffy source of food and reproduction.
Obviously intelligent beings have their own motivations and methods. But thestarseeds in Larry Niven's novels and the arthropods in the film StarshipTroopers have purely biological methods of getting into space.
How did these things evolve? Half a wing will enhance gliding ability. Halfan eye will detect light. What evolutionary benefit is there to launchingoffspring at half of escape velocity? (One astute reader answered this one: it's a great way of dispersing seeds planet-wide.) In Niven's novels, there is a possibilitythat the starseeds were bioengineered by an ancient extinct race. The arthropodsof Starship Troopers are also intelligent and might have bioengineeredspace travel as well. On the other hand, there were plenty of Star Trekepisodes where spacefaring critters turned up.
If these organisms evolved on planets, how did they get into space? If theyevolved in space, how? Even in very dense stellar clouds, the density is muchless than earth's atmosphere. There just isn't enough density to create complexmolecules. (The most complex known interstellar molecules have a dozen or soatoms.) Where do they get food? What selection pressures lead to the evolutionof complex life forms? Why doesn't radiation kill them or scramble their genes?
The beasts of the Alien series. Spock in Star Trek: The Search forSpock.
Bamboo can grow up to a foot a day, but bamboo is little more than a bundleof cellulose tubes. The idea that complex organs can differentiate and developwithin hours, and that skeletons and muscles can grow at that sort of pace isfar fetched in the extreme.
Not only did the beasts of the Alien series and Spock in Star Trek:The Search for Spock grow to full size within hours, but they grew withouteating. The Alien popped out of a crewman's chest, slithered into theductwork as a foot long larva, and within a short time was as big as afull-grown human. When the first crewman met his demise, the Mad magazinesatire quipped: "Considering that 'in space, no one can hear you scream,'he sure made a hell of a racket!"
So where did that extra mass come from? Did it grow by licking mildewoff the ducts?
The Sarlac in Return of the Jedi lives like an ant lion on unfortunatecreatures that blunder into its mouth. But then it digests them alive, slowly,over a thousand years. That means that not only does this thing survive in afood-poor environment, but it has enough surplus energy to keep its prey alivefor a thousand years.
Humans in desperate situations have been known to eat other humans. And thesix billion of us now crowding the planet would make an awful lot of bologna forinvading aliens. If the aliens are bent on exterminating us, that might makesense.
Necessity and opportunity are one thing. A preference for human beings asfood, like the Morlocks in The Time Machine or the aliens in the TVseries V, is another matter altogether. Humans are high up the food chainand therefore an inefficient source of food. Even if you feed them a vegan diet,they can't eat a lot of common plants like grass, have a tendency to escapefrom cages and attack their keepers, and take a really long time to grow. Cows,on the other hand, can eat grass, are dumb and docile, and grow very fastcompared to humans.
A variation on this theme is the Matrix trilogy, which postulates that our reality is really a computer-generated world, created by a supercomputer that uses humans for electrical power, tapping the energy of their nervous systems. Wouldn't electric eels make a lot more sense? They pack a lot more electricity into a much smaller space than people.
The snake in Anaconda, the velociraptors in Jurassic Park, themonsters in Alien and the shark in Jaws scarfed down people likethey were popcorn. Cold-blooded creatures like snakes and sharks have slowmetabolisms and could probably go for a month after eating a human. Activecreatures like velociraptors and Aliens need more food, but they're onlyabout the size of a human, and we're pretty active. Even allowing for themhaving faster metabolisms, they might need three or four times as much food as ahuman, not their own weight in food every day.
Prosthetics are getting better and better, and I have no doubt they'll eventually be fully functional, even superior to natural body parts.
The problem is that when you mix bionic and natural tissue, the result will only be as strong as the natural tissue. So Jaws in the James Bond flicks, despite his steel dentures, will only be able to bite as hard as his jaws and jaw muscles will allow. He might rival a good bolt cutter, but he won't be able to bite through thick cables. Spider Man may have incredibly strong spider adhesion, but when he hangs from a bridge and holds a cable car in his grip, the part in between is only as strong as Peter Parker's muscles and bones.
It doesn't matter how strong the bionic parts are. If they're connected to normal bone or muscle, the result is only as strong as the weakest component. So when the bionically altered villain in Spider Man 2 stands on his human legs and uses his bionic arms to rip a bank vault door off its hinges, I wonder what steroids he used to beef up his leg muscles, and what's anchoring the bionic parts to his body.
First of all, why would an alien consider a human sexy, or vice versa? Would youregard a manatee or an elk as sexy? That's a rhetorical question and mostdefinitely does not require an answer. If you answer "yes"there are Web sites for that sort of thing. Please do not send me the URL.
Assuming we can get past cultural and aesthetic issues, why do we assume aliens haveanything even remotely like the apparatus humans have? And once we get past thathurdle, the chances of actually creating offspring are just about nil.
Even closely related species on earth cannot interbreed, and these arecreatures with nearly identical DNA. It might seem like inter-species breedingwould be favored by evolution, since it would generate a much wider variety ofgenetic lines for natural selection. However, it would also result in dilutionof successful genetic lines with genes of unknown quality, and the negativeresults would outweigh the positive ones.
How do we know the aliens have DNA? Could there be variations on DNA thathave more nucleotides than the four used on earth? Or fewer? Do they have thesame number of chromosomes as humans? Are traits coded at the same places and inthe same way, or would a human gene for red hair produce extra fingers on analien? As the late Carl Sagan pointed out, a mating between a human and apetunia would have a better chance of succeeding than a mating with anextraterrestrial.
Even among terrestrial organisms there are wild variations. Among mammals,offspring with unlike sex chromosomes are males. Among birds, they are females.So what rule do aliens follow? Spock is half Vulcan, half human. BelanaTorres was half Klingon, half human. (I don't even want to imagine how desperateyou'd have to get to make love to a Klingon.) So just what sex are these people,anyway? And what kind of plumbing do they have? That question inspires a remakeof an old limerick:
A Thoat from the plains of Barsoom
Took a Romulan up to his room.
They got into a fight
And argued all night
Who'd do what, and with what, and to whom.
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