So now you have some idea of what these creatures are. Here’s where they came from: judging by nuclear introns and mitochondrial satellites, we think that vampires split off the human lineage something less than a half-million years ago, and persisted (albeit in small numbers) into the beginning of historical times. We trace their genesis to a paracentric inversion mutation on the Xq21.3 block on the X-chromosome, resulting in functional changes to genes that code for protocadherins. PCDH-Y is a protocadherin, and as I’ve mentioned they play a critical role in the development of the central nervous system. They occur in the headwaters of CNS development, as it were, and a relatively small change far upstream can lead to a whole variety of interrelated cascade effects. These include many of the features you’ve heard about today.
Now I’m not saying that a single mutation made all these improvements in a single lucky step; evolution doesn’t work that way. What I am saying is that a headwater mutation had such a huge impact on so many aspects of CNS development that basically the whole deck of cards got shuffled; suddenly there was far more variation for natural selection to work on, and so vampires could arise relativel quickly from that background chaos.
However, natural selection doesn’t optimise anything. "Survival of the fittest" is a profound misnomer: it would be more accurate to say "survival of the least inadequate". It doesn’t matter whether a given adaptation is the best possible solution; all that matters is whether it works better than the competition. Overall, vampires did work better than the competition, but that doesn’t mean they didn’t have a few design flaws. You’ve encountered the two biggies: the broken pathway which forced them to eat other hominids, and the defect that killed Donnie, the so-called crucifix glitch. It is this glitch that doomed them from the moment we developed Euclidean architecture. Vampires would have been barred from approaching any human dwellings that featured quartered windows, supporting crossbeams, and so on. (You can imagine how a resurrected vampire would react to a modern-day office building, with its facades of repeating windowframes. Take my word, it’s not a pretty sight.) And you can be damn sure that our ancestors figured that out pretty early in the game. The cross is not an exclusively Christian icon: it’s been used as a religious symbol back into prehistoric times. Now we know why.
You might wonder how such a lethal trait could get so fixed in the population to begin with. Shouldn’t natural selection have weeded it out sooner? The answer is surprisingly simple: the trait wasn’t lethal, not at first. An aversion to crosses is no disadvantage in a world where crosses don’t exist, and you don’t find many right- angles in nature. Any biology undergrad will tell you, neutrally selective traits can become fixed in small populations through a simple process called genetic drift. In this case the trait wasn’t even neutraclass="underline" the same crosswiring responsible for the crucifix glitch was also involved in vampiric pattern-matching skills, and that was a trait that natural selection would have actively promoted — right up until the point that their prey discovered geometry.
It’s tempting to speculate that this was also the source of the myth that vampires can’t enter someone’s house uninvited. It would be more accurate to say that vampires can’t come into your house unless they keep their eyes closed; and since that would make them extremely vulnerable to attack, they would only be advised to do that when the house’s inhabitants didn’t wish them ill.
We can also draw tentative conclusions about some of the other vampire myths that have sprung up over time. The whole bloodsucking aspect remains an open question: technically vampires are closer to what you might call obligate cannibals, eating human flesh rather than simply drinking the blood. However, given that the only thing they really needed from us was a certain type of protein, it’s theoreticaly possible that a blood diet could meet that need, although they’d have to drink a lot of the stuff. Perhaps this was a deliberate conservation strategy; drinking the blood leaves you with an anemic victim that can recover over time and serve as a future food source, while eating the flesh basically relegates your victim to single-serving status; and as we’ve seen, vampires could feed on other species to meet most of their dietary needs. They were much smarter than us, smart enough to figure out the virtues of resource conservation (a concept that baseline humans seem to have a hard time grasping even now).
Photosensitivity. None of our subjects developed xenoderma pigmentosum (a rare photosensitive skin condition which some have linked to vampirism in the past). Vampires do have very sensitive night vision, however, and their pupils don’t react as quickly to ours to changes in light intensity; they can be easilly snowblinded, as you would be if someone shone a light in your face while you were wearing night-vision goggles. It wouldn’t cause them to burst into flame when struck by the sun’s rays, but it might explain a general aversion to bright light. A crowd of peasants with torches might present a real problem to these creatures.
None of our subjects developed any kind of aversion to garlic, or to any of the Amaryllidaceaen species. It’s possible that vampires themselves spread this rumour, to engender a false sense of security among their prey; why bother building crucifixes if you think some garden weed is going to protect you? It’s also possible that the whole story is pure fiction.
A lot of other myths—that vampires can fly, or shapeshift, or that they don’t reflect in mirrors—are likely to be mostly fiction as well. They do reflect in mirrors: that was one of the first things we tested. But it’s worth remembering that these creatures are both faster and more intelligent than we are, and their superlative pattern-matching skills would give them a real advantage in "blending in" via crypsis; it’s quite likely that one might seem to disappear simply by fading into shadow, or adopting a posture that broke up its outline against the background. Combine such a vanishing act with, say, the flushing of some startled animal caught in its path, and a primitive human might think that some kind of shape-shift had occurred.
Reproduction: as the classic mythology would have it, vampires reproduce by turning their victims into other vampires. Revisionists and horror writers have played around with the idea of vampirism as a kind of viral infection, an STD transmitted from saliva to blood. Biologically, of course, there are some problems with this idea: if you create another vampire every time you feed, it won’t be long before all your prey have been turned into vampires, all of which will get very hungry very fast. However, the idea isn’t as absurd as it may seem on the surface. Lateral gene transmission is not unheard of in nature; certain microbes are known to act as carriers for the DNA of other species, transmitting them from one host to another; and in any event, it appears that predator and prey share many of the same genes anyway; perhaps the only thing that needs to be transmitted is some kind of catalyst to activate them. More conventionally, vampires and humans never achieved complete reproductive isolation in any event; there’s no reason why interbreeding couldn’t produce vampire offspring, especially if the critical vampire genes were heterozygously dominant. This is one of the strongest arguments of the syndrome-not-subspecies contingent, who argue that if it walks like a duck, quacks like a duck, and has sex with ducks, it must be a duck even if it looks like a turkey vulture.