The difference in external diagnostic features between humans and vampires were actually pretty subtle, both because vampires never lasted long enough to diverge greatly from the human baseline, and also because natural selection is going to promote superficial similarity. However, while virtually identical to modern humans in terms of gross physical morphology, vampires were radically divergent from humans on the biochemical, neurological, and soft tissue levels. This limited significant physical changes to soft tissue and microstructures that do not fossilize. This is one of the reasons why it's so difficult to identify these creatures in the fossil record—the other reason being that they sat at the very apex of the food pyramid, which means that they were quite rare even at peak numbers.
More gracile than Homo sapiens sapiens, gross physical divergence from baseline humans included a slight but distinct extension of the mandible, and of course of the slight elongation of canine teeth, the classic "fangs" of the predatory grip-and-tear feeding mode (although this wasn't quite as pronounced as the popular mythology would have you believe). Vampires also tended to be taller and longer-limbed than humans. All of this was in service of an increasingly predatory lifestyle.
Vampire skin possessed a ghastly white pallor. This discoloration however, was not the result of anemia; in fact their blood volume was actually seven percent higher than humans. Instead their blood volume was redistributed away from the peripheral tissues and sequestered deep in the core.
They had capillary beds formed in the body core. All humans have these, the digestive system is highly vascularised to facilitate nutrient transport from the intestine into the bloodstream, but the capillary meshes in a vampire's core were far more extensive than anything seen in humans.
Tissue levels of adenosine triphosphate were elevated; ATP is the chemical battery that powers the cell, and this explains the abnormal strength and stamina that vampires displayed.
The gastrointestinal 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.
The central nervous system is where the difference between vampires and humans really showed up. The corpus callosum was twenty percent larger in vampires than in humans. This enabled high-speed broadband communication between hemispheres. Interneuron/glial density, cortical folding and lamination were far above those in a human, particularly in the visual cortex. Their motor neuron nerve axons were almost twice as thick as conventional humans. This gave vampires faster signal transmission and faster reflexes than humans. A vampire could literally snatch a speck out of your eye before you had time to blink.
A vampires's amygdala and visual cortex—essentially, the pattern-matching wetware at the back of the head—were seven and thirteen percent larger than what is found in humans, respectively. Synaptic interconnections between the anterior cingulate gyrus and the rest of the brain were much lower than normal, almost as if the core of the brain were being isolated from the neocortex.
They also possessed some very unusual wiring in the retina. Human eyes contain whole arrays of specialized receptor cells; some fire only when they see light and shadow in conjunction, some fire only when they see horizontal lines-horizons and so on. In a vampire's case, the receptors that responded to horizontal lines were crosswired with those that responded to vertical ones. When both sets of receptors fired simultaneously in a very specific way-that is, when intersecting right angles occupied more than thirty degrees of visual arc-positive feedback generated a neuroelectrical overload in the visual cortex. This what caused the famous adverse reaction to crosses, which consisted of violent convulsions which strongly resembled grand mal seizures. The reaction is commonly called the "crucifix glitch" by scientists.
Within the vampire eye there was an increased reflectivity of the retina, reminiscent of the tapetum lucidum found in the eyes of cats and other nocturnal predators. Vampires also had quadrochromatic vision; while we humans have only three types of cones in our eye vampires had four, the fourth being tuned to near-infrared.
The reason vampires did not resorted to non-human prey and evolved such radical adaptations to accommodate human meat into their diet is because they received something from humans that they couldn't from other species. What they received that was so essential to their diet was PCDH-Y, a protein responsible for certain aspects of central nervous system development. Vampires possessed a genetic defect that resulted in the inability to synthesize the protein. Since this protein occurs only in other hominids, human prey was an essential component of the vampire diet.
Technically vampires were closer to what might be called obligate cannibals, eating human flesh rather than simply drinking the blood. However, given that the only thing they really needed from humans was a certain type of protein, it's theoreticaly possible that a blood diet could meet that need, although they had to drink a large amount of blood. 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 vampires could feed on other species to meet most of their dietary needs. They were much more intelligent than humans, intelligent enough to figure out the virtues of resource conservation.
The vampire mindEdit
Since 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 pattern-matching and analytical skills far in excess of the human norm. Vampires were omnisavants; their groove extended to pretty much every logical and pattern-matching dimension known to man, and more besides. They were insanely smart by human standards. Vampires could even do something that's neurologically impossible for us, they could hold simultaneous multiple worldviews. This allowed them to instantly grasp things that to us seem plagued by contradictions, to see things we have to work out step-by-step. Vampires had to be smarter than people, because they hunted people for a living. Lions are smarter than gazelles for pretty much the same reason.
By the same token, something else vampires had to be is clinically sociopathic. Among our own kind, a lack of conscience, of empathy for one's fellow human beings, is considered a pathology. Among vampires, though, sociopathy is an essential survival trait. If you felt empathy for your prey, you'd starve to death. Natural selection would have weeded "moral" vampires out of the gene pool relatively fast. On the biological level, the sociopathy was caused by the poor connection of the anterior cingulate gyrus, as it is the site of moral reasoning.
The undead stateEdit
A major prey-related problem that vampires faced was the predator-prey ratio. In most every case where one species eats another, the prey species is at least an order of magnitude more numerous than the predator, and breeds faster. The reasons for this are obvious: the transfer of food energy between trophic levels is very inefficient. Cows have to eat ten kilograms of grass to make one kilogram of cow; it takes ten kilograms of cow to make one kilogram of human; and of course, it takes ten kilograms of human to make one kilogram of vampire. So at any given level, the level below you must out-produces you by at least ten to one, or you'll exterminate your own food supply.
Since the metabolic and reproductive rates of vampires were pretty much the same as humans, what they did was cut back on their activity levels. The bodies of vampires contained high concentrations of leuenkephalin, an opioid peptide found in animals like bears and squirrels, and is involved in hibernation. Vampires conserved energy-and their food supply-by extended periods of hibernation. Suspended animation is not uncommon even among higher animals like birds and mammals.
Vampires were able to shut themselves down for decades, dessicating down to this biologically dormant condition and entering what's commonly known as an "undead state." This worked in three ways: firstly, it drastically reduced their energetic needs, redressing the original imbalance between prey production and predator consumption. Secondly, it gave the prey population time to recover in the event that it had been severely hammered by predation, and let the vampires wait out food shortages. And thirdly, it's possible that these extended leaves-of-absence might have given humans time forget that they were prey. By the Pleistocene era, humans were intelligent enough to pass information from generation to generation, but were also intelligent enough for skepticism. Stories of "nightstalking demons" were likely taken as mere tales of fantasy by senile elders. Primitive cultures were likely to get careless after a few decades with no vampires on the horizon.
At any rate, scientists believe that this is where the blood-pooling strategy started; part of being "undead" involved sequestering blood around the vital organs and letting the peripheral tissues starve, much the way seals and whales triage their oxygen supply when cut off from the air. This likely proved so effective that over time, it became a normal state of affairs even among active vampires; the ghastly white pallor of vampires was actually a strategy for increasing their gas mileage. When lactate levels in the surface tissues got too high—or when vampires were feeding—blood was redirected to the skin and the complexion flushed.
As the classic mythology would have it, vampires reproduced 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.
Many scientists have theorized that the genes responsible for vampirism are widely spread amongst the human population, simply dormant in most people. They lie within junk DNA, which are ancient genes that haven't expressed themselves in thousands of years. These scientists believe that in certain cases, some of these genes do express spontaneously; that psychopathy, autism and certain types of schizophrenia arise partly from the partial expression of these genes in a broken and rudimentary form. That sociopaths and savants show us one or two bits of the vampire subspecies.
Judging by nuclear introns and mitochondrial satellites, scientists believe that vampires split off the human lineage something less than 90 thousand years ago, and persisted (albeit in small numbers) into the beginning of historical times. Their genesis has been traced 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 mentioned they play a critical role in brain and central nervous system development. 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. A headwater mutation had such a huge impact on so many aspects of CNS development that suddenly there was far more variation for natural selection to work on, and so vampires arose relatively quickly.
Despite common misconception, natural selection does not optimize anything. It's actually something along the lines of "survival of the least inadequate." It doesn't matter if a given adaptation is the best possible solution. All that matters is rather 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. The biggest were the broken pathway which forced them to eat other hominids and the defect referred to as the so-called crucifix glitch. It is this glitch that doomed them from the moment humans developed Euclidean architecture. Vampires would have been barred from approaching any human dwellings that featured quartered windows, supporting crossbeams, and so on. And this weakness was likely discovered by the people of early civilization rather soon. The cross is not an exclusively Christian icon: it has been used as a religious symbol back into prehistoric times, and vampires were apparently the reason.
The crucifix glitch, such a lethal trait remained in the vampire population and was not weeded out by natural selection very soon because the trait wasn't lethal at first. An aversion to crosses is not a disadvantage in a world where crosses don't exist and there aren't many right angles in nature. Neutrally selective traits can become fixed in small populations through a simple process called "genetic drift". In this case the trait wasn't even neutral: 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 human prey discovered geometry and Euclidean architecture.
The crucifix glitch spelled the end of the vampire lineage. Suddenly denied access to its prey, the entire subspecies went extinct shortly after the dawn of recorded history around 2,000 years B.C. Although they obviously persisted long enough to embed themselves in our cultural mythology.
The crucifix glitch is likely the source of the myth that vampires can't enter someone's house uninvited. It would be more accurate to say that vampires couldn't come into a house unless they kept their eyes closed; and since that would have made them extremely vulnerable to attack, they usually could only enter a house when the house's inhabitants didn't wish them ill.
Vampires had very sensitive night vision and their pupils didn't react as quickly as humans to changes in light intensity; they could be easily snow blinded. Although it couldn'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 likely presented a real problem to the subspecies.
It's possible that vampires themselves spread the myth of garlic repelling them, to engender a false sense of security among their prey. 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. But it's worth remembering that these creatures were both faster and more intelligent than humans, and their superlative pattern-matching skills would have given 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, for example, the flushing of some startled animal caught in its path, and a primitive human might think that some kind of shape-shift had occurred.
Some people have suggested that vampires had a positive impact to its ecosystem by helping control our ancestor's population, but ever since its extinction, our numbers had multiplied, causing us to negatively destroy lots of habitats, so there are plans on resurrecting vampires by cloning. Some scientists have confirmed they have successfully sequenced the vampire's dna.
Despite some criticism, scientists had successfully cloned and brought 3 vampire children back through cloning in 2008 using the modern human as a surrogate mother, with no side effects during the cloning process, unlike the failed Pyrenean Ibex experiment in 2003, so they are in a healthy condition and they are currently living in England Zoo, where people are successfully breeding vampires, as for the first three vampires that were brought back, they are now 8 years old and they are expected to live for more than thousands years, longer than a modern human, and longer than any other vertebrate, so they are the longest-living animals on earth. The members of England zoo are hoping to reintroduce vampires into their former homeland, where they could help control the population of humans of Europe and Western Asia.