WARNING! This post may contain descriptions of hot tentacle sex!
Mimic is one of the Wikipedia articles I started a long time ago (you can see my original revision here) which has never quite been whipped into proper shape. Recently someone raised the issue of covering self-mimicry, that is, when a creature mimics another bodypart or object, like a leaf or an eye. In the course of reading around on the subject, I stumbled upon this rather beautifully-written essay about mimicry which rather neatly illustrates a rather important but unappreciated evolutionary principle.
The Viceroy and the Monarch, respectively.
The essay outlines Nabokov’s* critique of Batesian mimicry in the Viceroy butterfly. The conventional tale goes that the Viceroy mimics the Monarch butterfly, thus successfully avoiding predation because the Monarch is bitter and unpleasant to birds. As it turns out, though, the Viceroy itself is bitter and unpleasant to the taste (which Nabokov verified by direct taste-test), which removes the alleged impetus for selection. A more likely source of the apparent mimicry is the fact that butterfly bodyplans impose constraints on the evolution of wing patterns; there is not, in fact, infinite variation available, but a specific number of constrained patterns that can evolve. The odds that two highly simliar bodyplans will emerge merely by chance is therefore much higher.
This underscores the idea that evolution is constrained both by physics and by history. By now you may have already noted that no creature has evolved the ability to teleport or phase through rock†. But there is a kind of inevitability imposed by biological history as well. The mammalian body plan, for example, established some 200 million years ago, means that we’re committed to a certain evolutionary path, deviation from which is enormously difficult.
So, for example, cetaceans have diverged remarkably in their 50 million years of evolution away from the artiodactyls who are their evolutionary sisters, to the point where it’s faintly ridiculous to imagine that the mountain goat and the whale are not that distantly related. In that time, though the cetacean body plan has shown remarkable plasticity, it has also remained constrained in significant ways. Most notably, though cetaceans spend their entire lives in the ocean, they still cannot extract oxygen from the water, like other sea-dwelling creatures do. This, presumably, is because the mammalian commitment to a heart/lung system is fairly robust, and redeveloping gills simply requires too much reengineering of structure to be evolutionarily feasible. That is, the evolutionary landscape can be explored if adaptations do not have intolerable fitness penalties. Whales are warm-blooded and have large brains and bodies; their survival is contingent on maintaining these, and they have adapted to do so by developing thick blubber coats. Giving up on these for gills seems unlikely to happen.
On the other hand, observe the incredible plasticity in form amongst vertebrates, which can elongate their spines, lose limbs and digits, reshape skulls, etc., with relative facility. This, because the gene system that regulates the development of the body (e.g. Hox genes and other homeobox genes) can be tweaked with relative ease to produce phenotypically dramatic shifts in body size and shape. This means it would be comparatively easy, for example, for humans to evolve fins and webbed toes – lengthening the bones in the fingers and toes is a snap, as is disabling programmed cell death in the skin that normally grows between the digits in the developing embryo.
Most significantly, this teleology makes it next to impossible for any Earth-bound creatures to escape fundamental truths of life on this planet: the genetic code, the twenty basic amino-acids, ribosomes, and nucleic acids as information carriers.
It also means that it’s extremely unlikely any extraterrestrial life we encounter will operate along similar principles.
If life has multiple points of origin across the universe, it is likely that there will be, at the very least, subtle differences in the biological molecules employed by alien lifeforms. Even assuming they use similar polymer-based mechanisms for encoding genetic information, it’s unlikely they’ll use deoxyribose sugars or the specific purine/pyrimidine bases we use. More importantly, the complement of amino acids (if they are used at all) will vary. Even the chirality of amino acids (all Earthling amino acids exist as L-enantiomers, for reasons unknown. Maybe coincidence.) could have significant effects.
So when we encounter an alien, even if they’re carbon-based, proteinate, oxygen-breathing, sugar-metabolising multicellular lifeforms like us, it’s extremely likely they won’t resemble us at all biochemically. What this means is when Kal-El french kisses Lois Lane, he’s probably introducing a whole host of compounds into her mouth that she’s unable to process biochemically, and that might even be highly toxic to her. Akin to kissing a drum of oil. The wages of sin in this case could run the gamut of most kinds of poisoning you can think of. Even the dust produced by alien skin cells flaking off would be toxic, blessedly killing off allergenic dust-mites, but also instantly inducing lethal coughing fits as soon as you walk into your alien buddy’s studio apartment.
As for sex? You can forget it. Even assuming that anatomical incompatibilities don’t render you unable to perform the act or make it exceedingly unenjoyable, short of wearing a full-body condom (including the mouth) there’s no possibility of you groping and fondling a strapping Klingon warrior or cuddling with Oola the Twi’lek dancing-girl. Melding sensitive membranous flesh together might result in subsequent necrosis and organ failure in both parties, which seems like a high price to pay for a close encounter.
* Yes, that Nabokov. In addition to being one of the greatest writers of the 20th century, he was also, annoyingly, an equally distinguished lepidopterist. Fucker.
† Although apparently some biologists who consult for Marvel Comics think that these developments are due to arrive any day now.