Homo sapiens vampiris was a short-lived Human subspecies which diverged from the ancestral line between 800,000 and 500,000 year BP. More gracile than either neandertal or sapiens, gross physical divergence from sapiens included slight elongation of canines, mandibles, and long bones in service of an increasingly predatory lifestyle. Due to the relatively brief lifespan of this lineage, these changes were not extensive and overlapped considerably with conspecific allometries; differences become diagnostically significant only at large sample sizes (N>130).
However, while virtually identical to modern humans in terms of gross physical morphology, vampiris was radically divergent from sapiens on the biochemical, neurological, and soft-tissue levels. The GI tract was foreshortened and secreted a distinct range of enzymes more suited to a carnivorous diet. Since cannibalism carries with it a high risk of prionic infection, the vampire immune system displayed great resistance to prion diseases, as well as to a variety of helminth and anasakid parasites. Vampiris hearing and vision were superior to that of sapiens; vampire retinas were quadrochromatic (containing four types of cones, compared to only three among baseline humans); the fourth cone type, common to nocturnal predators ranging from cats to snakes, was tuned to near-infrared. Vampire grey matter was «underconnected» compared to Human norms due to a relative lack of interstitial white matter; this forced isolated cortical modules to become self-contained and hypereffective, leading to omnisavantic pattern-matching and analytical skills.
Virtually all of these adaptations are cascade effects that— while resulting from a variety of proximate causes— can ultimately be traced back to a paracentric inversion mutation on the Xq21.3 block of the X-chromosome. This resulted in functional changes to genes coding for protocadherins (proteins that play a critical role in brain and central nervous system development). While this provoked radical neurological and behavioral changes, significant physical changes were limited to soft tissue and microstructures that do not fossilise. This, coupled with extremely low numbers of vampire even at peak population levels (existing as they did at the tip of the trophic pyramid) explains their virtual absence from the fossil record.
Significant deleterious effects also resulted from this cascade. For example, vampires lost the ability to code for e-Protocadherin Y, whose genes are found exclusively on the hominid Y chromosome. Unable to synthesise this vital protein themselves, vampires had to obtain it from their food. Human prey thus comprised an essential component of their diet, but a relatively slow-breeding one (a unique situation, since prey usually outproduce their predators by at least an order of magnitude). Normally this dynamic would be utterly unsustainable: vampires would predate humans to extinction, and then die off themselves for lack of essential nutrients.
Extended periods of lungfish-like dormancy (the so-called «undead» state)—and the consequent drastic reduction in vampire energetic needs— developed as a means of redressing this imbalance. To this end vampires produced elevated levels of endogenous Ala-(D) Leuenkephalin (a mammalian hibernation-inducing peptide) and dobutamine, which strengthens the heart muscle during periods on inactivity.
Another deleterious cascade effect was the so-called "Crucifix Glitch" — a cross-wiring of normally-distinct receptor arrays in the visual cortex, resulting in grand mal-like feedback siezures whenever the arrays processing vertical and horizontal stimuli fired simultaneously across a sufficiently large arc of the visual field. Since intersecting right angles are virtually nonexistent in nature, natural selection did not weed out the Glitch until H. sapiens sapiens developed Euclidean architecture; by then, the trait had become fixed across H. sapiens vampiris via genetic drift, and—suddenly denied access to its prey—the entire subspecies went extinct shortly after the dawn of recorded history.
