Sentinels are a bipedal primate mammal species related to humans by biological and behavioral similarities. Biologically, they are classified as Homo sentus (sentus being from Latin "sent[tire]" feel + "tus" suffix denoting the verb form), and belong to the family of great apes, along with humans, chimpanzees, bonobos, gorillas, and orangutans. Sentinels are outwardly indistinguishable from humans, however there is a significant physiological difference. Compared to humans, Sentinels possess extraordinary sensory capabilities.
Until recently, knowledge of the existence of Sentinels had become lost. Only with modern technology would we even be able to tell humans from Sentinels. As a result, many people considered not sane due to chemical imbalances or even unknown causes, may actually be Sentinels. In fact, the genetic differences in the species may also account for adverse reactions to medicines, chemicals, or situations that would not affect humans the same way. Since humans are the more numerous, a decidedly human prejudice exists with regards to the properties and usage of such medicines or chemicals. Medical knowledge, too, assumes that all patients are human, and treats everyone the same way only until a problem develops with that treatment, at which time, the treatment is modified accordingly - but still without considering the possibility that a patient may not even be human.
Like humans, Sentinels also evolved from the bipedal arboreals, but then went a slightly different route. There is plenty of fossil evidence which proves that several species of hominids existed, some of them side-by-side. Not all of those "sister species" died out. One of them evolved into Sentinels. Even though humans and Sentinels are different species that diverged many millions of years ago, they still look the same on the outside, and still are of the genus Homo. Sentinels came from other hominid branches that we have not yet traced in unbroken line to their ultimate beginnings.
It is theorized that Sentinels came from a bipedal arboreal that did not develop pair-bonding, but instead remained solitary, we may have the following developments in behavior. If this is true, then the solitary nature of Sentinels co-existed (and still does) with their territorial nature. Each creature would have staked a claim to a specific region, and would have restricted it's hunting to this area. That being the case, there would have been no competition for food among themselves, but there probably would have been between the Sentinels' arboreal ancestors, and humans'. This, combined with the low birth rate and lack of mutual protection inherent in multi-member groups, might have been what led to the beginnings of the hyper-acute senses Sentinels are known for. These senses would have been useful in finding food, and in detecting danger. It would also account for why Sentinels tend to be stronger, faster, and able to resist disease and heal from wounds better than most humans. All of these things would have combined to raise a Sentinel's chances of survival.
Because both kinds of creatures overlapped in territory, and because the ancestors of humans were gregarious beings, they may have not only recognized, but also decided to take advantage of, these heightened senses. Quite possibly, somewhere along the line, a bargain may have been struck between humans and Sentinels, wherein the Sentinels provided services such as location of game animals and early warning of danger, in exchange for protection and shelter. The fact that Sentinels are both solitary and territorial probably would not have proven a problem, as humans tend to band together in groups that are more or less tightly-knit while at the same time separating themselves one group from another. Thus, one Sentinel would have agreed to provide the above-mentioned services to one group or "tribe" in one area, while another Sentinel would have allied with a group in another region, and "never the twain shall meet". The result was that everyone's needs would have been satisfied.
At this point, it is speculated that Sentinels probably show as much diversity among their species as humans do. Some Sentinels would have shown one or more senses to be stronger than others, for example, and some Sentinels may be generally more powerful, and able to sense things from greater distances or with better precision than others. Therefore, stronger Sentinels were almost certainly capable of holding a larger territory, with a greater population of Humans living therein. It is possible that one Sentinel may have been strong enough to attempt to enlarge his or her territory by forcibly taking territory from another. Given both the solitary nature and the territoriality of Sentinels, the result may well have been a fight to the death, with the winner both annexing the territory, and agreeing to provide services for the Humans within that new area. This would have been especially true if Sentinels had by this time developed the strong protective instinct we see today. In fact, this instinct may actually be part of the instinct to protect the young, Humans being weaker than Sentinels, and, therefore, the Sentinels may equate this weakness with the vulnerability seen in children.
There is also the possibility that, in addition to the Sentinels' actions when annexing new territory and agreeing to protect all tribes within that area, humans may have reacted as much to the presence of the stronger Sentinel as to their own gregariousness, by joining together the different groups into one larger one, thus making it easier for the Sentinel to provide the agreed-upon services. This, along with a growing population that spread out to occupy available territory, therefore reducing the amount of territory available, may actually have been the beginnings of the human predilection for living in larger and larger groups. These larger groups led to villages, then towns, and so on, all protected by any Sentinel strong enough to do so. It may even, though no study has been done, result in continued evolution by Sentinels into stronger and stronger individuals; this evolution taking place through the time-tested "survival of the fittest" method: the stronger the Sentinel, the better the chance of survival and, therefore, procreation, resulting in the passing on of this ever-increasing strength.
The Victorian era explorer Sir Richard Burton was the first person to study Sentinels. Burton first discovered Sentinels among certain tribes of South America. He studied them for many years and wrote a monograph called "The Sentinels of Paraguay." He described a Sentinel as "a person, in pre-civilized cultures, who patrolled the border, with senses sharper than normal humans, honed by time spent in isolation." For more than a hundred years his work was widely regarded as a flight of fantasy. It was only in the 1990s that Sentinels were noted in the modern world.
The existence of Sentinels was met with some disquiet; their enhanced senses seemed to pose a possible threat to those not so gifted in areas as mundane as athletic competition and as sensitive as privacy rights. Further controversy arose when it was discovered by geneticists that Sentinels are a separate species. A gradual and uneasy public acceptance of Sentinels turned to wild enthusiasm during the September 11, 2001 attacks. Many Sentinels participated in the rescue operations. Enhanced senses found victims in the rubble of the World Trade Center and The Pentagon. Today Sentinels are encouraged to find work in military and paramilitary organizations such as police and search and rescue; arenas where enhanced senses could serve the public.
The existence of two different species with the Homo genus may account for what we now assume is simply an enormous variation in "human norms." This wide variety is rarely seen in any other "natural" species. In other words, species that have not been interfered with the way humans have interfered with, for example, dogs, or cats, or horses, by deliberately creating different breeds for different uses. For example, there is little variation from member to member among zebras - they are all equines of similar height, build, and stripe pattern. They do not, for instance, vary by as much as two feet in height, nor do any of them boast spots instead of stripes. Even foxes, which have dozens of different subspecies worldwide, do not vary that much from member to member among a single subspecies. Perhaps, as with most species of animals, the variation from member to member within the human species is not so broad as we believe. What exists instead may be the variation from species to species between humans and Sentinels.
Inter-species pairing is not unknown. There is, however, no possibility of offspring. This, combined with the fact that, because, until recently, we didn't even know humans and Sentinels were different species, has probably resulted in many inadvertent inter-species pairings which have not resulted in the birth of children. It is also very possible, therefore, that many such couples who are infertile, and for which no clear reason for this infertility can be found except the vague "genetic incompatibility", may unknowingly be a Sentinel/Human couple.