The Janissary Theory
On Earth during the 14th Century the Ottoman Sultan Murad I created the Janissaries, an elite fighting force that was loyal only to the Sultan. The Janissary corps was composed of youths who had been taken by force from their villages on the Ottoman frontier and trained and indoctrinated to be fanatically brave warriors. Although they were essentially a slave army (they could not grow beards because beards were only allowed on free men) the Janissaries became an important and trusted part of the Empire and by the 19th century they were an important political force.
One Klingon Discrepancy theory postulates that TOS Klingons were the result of a similar policy of forced conscription by the Klingon Empire. Sometime after First Contact with Humans in 2151 and before Kirk's interaction with Klingons in the 23rd Century, the Klingon Empire began a policy of raiding human settlements and taking captured children back to Q'onoS. These prisoners would be brainwashed until they had no memory of their former lives and they would be raised as Klingon warriors. For all extents and purposes these beings would be Klingon and it would make no sense for Kirk to call them human or to treat them as human.
Sometime between stardate 5943.7 ("All Our Yesterdays") and 7412.6 ("Star Trek: The Motion Picture") the Klingons abandoned this practice for some internal political reason. Perhaps these smooth-headed Klingon Janissaries were, like their counterparts in the Ottoman Empire, becoming too powerful to be tolerated by the Emperor. Whatever the reason, by the 24th century, Klingons viewed the practice of enslaving and brainwashing humans to be shameful and they did not like to talk about it. Kor and other TOS Klingons who were (at least according to this theory) once human, underwent cosmetic surgery to complete their transformation into Klingons.
The Quick-Evolution Theory
Recent research suggests that Evolution does not occur gradually over time - as previously supposed - but in fits and starts through random genetic mutation.
As we have seen, in the 22nd century, during the time of Enterprise, Klingons look much as we are accustomed to seeing them - with complete head ridges. However, in the 23rd century, they appear to no longer have these ridges. One theory states that toward the end of the 22nd century, a genetic mutation swept the Klingon race - the entire race took one huge evolutionary leap forward. We know from Worf's de-evolution ("Genesis") that Klingons evolved from creatures with far more ridges. Thus, it is logical to conclude that evolution was phasing these ridges out.
As this generation of "ridge-less" Klingons aged, the Klingon Empire's power began to falter, and the Empire itself was in a state of limbo. The Klingon people began to feel that this decline was caused - at least partially - by their new appearance. They believed that as well as looking less fierce and intimidating without their ridges, the fact that they seemed to have lost some of their physical strength gave their adversaries the impression that the Klingons, as a people, were now weaker. There was a great outcry to the scientific community to find a way to counteract this mutation. The scientists were quick to respond, and soon found a way to de-activate this new gene.
Most Klingons chose to undergo the procedure, to - as they saw it - restore honor to their people. However, not all underwent the procedure. Instead, they chose to let this characteristic breed itself out. This is why we see various levels of ridges among the Klingon people. To this day the Klingon people view this mutation as a disgrace, and avoid the subject whenever it is raised in conversation.
The Bioengineered Agent Theory
After the Klingons first contact with humanity, concern and suspicion arose in the Empire.
Humans were already receiving assistance from the Vulcans and with the territorial Romulans in a nearby star system, the Empire needed to "enhance" its warriors to prepare for these new threats. Using this as justification, the Klingon High Council introduced and approved widespread use of a Bioengineered Agent (using a synergy of DNA from various species and modified to adapt to Klingon physiology) which enhanced physical and mental reflexes. Many of the initial tests and pilot programs of the Bio-agent were done on Klingons who were most likely to contact other species, especially those serving in the Empire's spacefaring ships, as well as young male and female Klingons who were preparing to become warriors.
At first, the psycho-pharmacological effects proved to be quite effective at enhancing ability but after time, a curious side effect began to emerge - the Klingon forehead ridges began to disappear, as well as overall muscle mass. The more the substance was used, the greater the side effects. It seemed that the Bio-agent was starting to alter Klingon DNA.
Concerned that these side effects were changing the Klingon genotype permanently, the Klingon High Council decided to restrict usage of the Bio-agent. Within a short time, many of the Klingons that stopped taking the Bio-agent began to slowly revert to their original physiology. Nonetheless, it was apparent that the Bio-agent remained dormant in the Klingon physiology. In some cases, the Bio-agent was known to have been passed to Klingon progeny, however with no apparent ill effects.
After ending usage of the Bio-agent throughout the Empire, traces of it became more difficult to detect in the Klingon bloodstream, however records show that some species have been able to "smell" the Bio-agent, which typically causes an adverse reaction in the observer - most notably the cute and inoffensive Tribble species.
The Surgical Alteration Theory
Early Klingon contact with humans, members of the Federation and already allied with the secretive Vulcans, represented a perceived serious security threat to the Klingon Empire. As a result, Klingon agents were recruited who were then surgically altered to look more human should a situation dictate that they would have to perform their duties virtually undercover.
Humans, Klingons felt, could not be trusted. They were not honorable creatures. Therefore, in order to ascertain their motivations and strategies, a network of Klingon/Humans was established in the Alpha Quadrant. Some of these Klingons, Kor, Koloth and Kang to be specific, were early subjects to undergo the surgical alteration. Although their potential missions -- as they acted in virtual "sleeper cells" -- never came to be covert, they nevertheless came into contact with humans. Their look and demeanor was carefully logged and helped establish the Federation's Klingon database.
Later in the 23rd and moving into the 24th century, Starfleet contact with these Klingons and others further established a reversion to their original form for the warriors Kor, Koloth and Kang.
The Hybrid Theory
The Klingons are by nature imperialistic and have a thirst for conquest, and like most conquerors, part of their spoils of victory is to partake of the women of the conquered. One such world annexed by the Klingons was inhabited by a species who looked more like humans than Klingons (TOS is rife with examples of such species).
For perhaps a century or more while that world was ruled by the Empire, cross-breeding between the two species was commonplace. What resulted was a generation of hybrids who had a physical appearance more similar to the natives of that world, but with personalities more like their Klingon fathers. This hybrid race was smart and ambitious and lusted for power, but unlike purer Klingons they had very little sense of honor. They used their cunning to manipulate and backstab their way into the higher ranks of the Empire, into control of the High Council itself.
By the mid-23rd century the hybrids seized absolute power and, declaring themselves by law to be the only "true" Klingons, stripped the other Houses of their land and influence, and appropriated their fleets of starships. They then set out to continue expanding the Empire under their dishonorable rule.
It took several years for the "pure" Klingons to regroup and unify to take back the High Council, which they did in a very messy upheaval during the 2270's. It could have been seen as just another civil war in a society rife with internal conflicts, but the purebred Klingons were so ashamed that they allowed themselves to be supplanted by a sub-race that they desired to wipe all traces of that period from history. They proceeded to systematically execute that entire sub-race -- a bloodbath of unprecedented proportions that even the Klingons refuse to speak of ever again.
However, absolute revisionism of history is impossible in a space faring culture, and there were certain individuals of the hybrid race who achieved great things on behalf of the Empire and established a reputation with allies and adversaries, reputations that could benefit the new regime. So these individuals were allowed to live and continue fighting for the Empire, on the condition that they be genetically modified to appear as pure Klingons. Thus Kang, Kor and Koloth and a small handful of others remained heroes of the Empire as the memory of their origins faded into a wholesale denial that they were ever anything but true Klingon warriors.
The Cosmetic Fashion Theory
In the early 23rd century, Klingon cosmetologists (and there were relatively few) devised a method to dramatically alter the appearance of their fellow warriors. This alteration smoothed out the typically bumpy Klingon forehead and reshaped the fierce Klingon dentition into a softer, less threatening smile (actually more like a grimace, but certainly less feral). This more human-like appearance was all the rage in certain quarters of the Empire, and the fad quickly spread. Practitioners of this revised visage shied away from traditional Klingon apparel as well, instead preferring simpler, less ornamented attire.
This look and lifestyle was popularized in the broadcast holo-programs of the time. The names of these programs are not directly translatable from the Klingon language, but they involved comic situations revolving around mating rituals, vocational dilemmas and family squabbles settled with Bat'Leths.
By the early 24th century the number of individuals practicing this alternative lifestyle had dwindled dramatically, perhaps due in part to their seeming indifference to procreation, resulting in an unsurprisingly small number of offspring.
Never ones to tolerate even minor differences among their people, the majority of Klingons did not appreciate the contributions this group had made to Klingon arts and culture. When the last of these unique Klingons found his reward in Sto Vo Kor, the Klingon High Command hoped the memories of this divergent group would be erased from the collective consciousness.
The Q Theories
Early in the 23rd Century, the Klingon Empire was beset by members of the Q Continuum (or some other equally powerful race) who conducted, basically, social experiments upon the Klingons on an incredible scale. One such experiment these Q conducted began with the rising tensions between the Klingon Empire and the rapidly expanding upstart United Federation of Planets.
As the Klingons trumpeted their superiority over the physically inferior and comparatively docile humans, the Q decided to teach the Klingon race a lesson in humility by transforming them all into near replicas of humans; the proud forehead ridges no longer part of their anatomy. This humiliating transformation also exacerbated Klingon tensions with the Federation.
In order not to upset the balance of power in the quadrant, the Q made the Klingon transformation retroactive, effectively altering the databanks and memories of any race who had ever encountered the Klingons.
When finished with the experiment (or perhaps bored with it), the Q eventually returned the Klingons to their original state, as well as the rest of the quadrant. Unfortunately, the Klingons themselves did not forget, and upon their reversion were especially anti-Federation in their policies and practices.
Because of the lingering resentment over being shamed for decades, Klingon historians who noted these events were subsequently ostracized and eventually deemed heretics and were purged in all sorts of unnecessarily brutal ways. As part of these purges, all public records and references to this time were destroyed, and although the events are sometimes whispered about, Klingons are loath to speak of, much less remember, this terrible time.
Another Q Theory postulates that when the Q Continuum first began observing the Klingons, they found the fierce warrior race to be overly proud - arrogant, barbaric and ceremonious to a fault. Seeking to humble the Klingons, one particularly playful member of the Continuum decided to play an "innocent" prank on them. This Q used his powers to cause the Klingons of a certain region of Qo'noS to lose all of their head ridges, making them significantly less fierce in appearance; in essence looking like generic humanoids, and were suddenly not particularly menacing.
As this Q did not make his presence known to the Klingons, the sudden loss of ridges caused much confusion and strife among the Klingon people, and the "ridgeless" Klingons were left to ponder whether they had experienced some sort of spontaneous genetic mutation. After several generations of contention between the various factions of Klingons, the Q finally decided that the novelty had worn off, and returned all Klingons to their natural, ridged-forehead state.
The Under-Resourced Creator Theory
This theory states that the universe is a creation of one or more beings, and that our universe exists solely for their entertainment. Being almost but not quite all-powerful, the Creator at one time envisioned Klingons as they exist today, but at that time was unable, with the limited resources at hand, to fully realize the original vision for them.
However, as time moved forward and the universe grew, the Creator was provided with resources vastly superior to the ones originally available, and thus Klingons were recreated in the original vision. Because every being in this galaxy is controlled by the Creator, few beings ever questioned the discrepancy and those that did were met with evasive answers, all at the behest of the Creator. Being a very busy Creator, there were always more pressing issues at hand than explaining the discrepancy, and thus the issue has remained a mystery to this day.
While someday the truth about this odd discrepancy in Klingon appearance may be known, it's entirely possible that the truth has been lost over the course of history.
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