The haggard scruff of facial hair dominates the man's appearance like a cancerous growth or a strange new continent of anguish and misery. The creeping mass makes the eyes recede; and as the luminous orbs so linked with our intellects and individuality diminish, so too does the man. He becomes lost as his own body inexorably marches him towards something primordial, dark, and inhuman.
The trope of the beleaguered and long-suffering hero gradually being covered by a beard of a week's vintage or more is not an unfamiliar hallmark of literature. However, Battlestar Galactica takes this device and accelerates it to speeds faster-than-light, perhaps owing to Ronald Moore's own hirsute appearance. [Please note that this might contain some spoilers.]
Consistent with the beard's placement in our literary tradition, unkempt facial hair coincides with the suffering and hardship of the series' characters. Directly before breaking Cally Henderson's jaw, Galen Tyrol is writing around on the floor, caught in a nightmare, with the stubble of many days adorning his own jaw. After the internment during which he lost his eye, Saul Tigh emerges with the kind of beard that one would expect on a man confined for so long.
However, the accretion of this proteinous harvest is not a mere incidental to the characters' torment. In many ways it parallels the specific types of challenges that our heroes and anti-heroes must overcome, and in some cases, the series' protagonists channel the power of their facial hair in order to triumph. During the occupation of New Caprica, William Adama grows a prominent moustachio - but this is no mere affectation. The lifespan of this natural accoutrement is limited to that time in which he had to drive himself and the men and women remaining aboard Galactica to greater lengths than ever before. Once their oppressed comrades were rescued from the false paradise of the Cylon-occupied planet, the moustache disappeared.
Furthermore, Gaius Baltar's beard and accompanying coiffure grow similarly out of control in the time leading up to his trial. However, this is not the result of laziness or despair. Even during the occupation of New Caprica, Baltar remained relatively clean-shaven. Instead, the shagginess of the condemned man was a calculated move on Baltar's part as he worked to cultivate the image of the revolutionary martyr. The beard was the personal counterpart to the covert composition of his notorious manifesto, My Triumphs, My Mistakes.
The manner in which Baltar's beard was removed is even more telling of the importance of facial hair to Battlestar Galactica. After being adopted by his own cult of personality, Baltar goes to Galactica's head to shave, accompanied by one of his new supporters. After ridding himself of his voluminous whiskers, Baltar and his companion are attacked by men who are incensed by the acquittal of the former president. One of the attackers holds a straight razor to Baltar's throat. However, Baltar does not struggle or try to escape but instead encourages the assassin to take his life, for the first time truly repudiating the arrogance and hubris that had been the cause of so much human suffering in the past.
Razors seem to be of especial importance to the cosmology of Battlestar Galactica. The title of the supplemental movie that chronicled the events on the Pegasus before it met up with the Galactica was "Razor." This was not a mere naming convention either, as Admiral Helena Cain placed a great deal of importance on the concept of the razor: she carried one with her at all times and demanded that her subordinates mirror the qualities of that instrument. Helena Cain herself was single-minded, uncompromising, and unrelenting.
Cain's reliance on the symbol of the razor seems to be at odds with how the shaving instrument intersects with the role of facial hair in regards to the crewmembers of Galactica. However, this is logical because Cain's draconic command style was very much at odds with Adama's more humane leadership. Helena Cain did not permit anything or anyone to interfere the mission. Adama was more tolerant of the human emotions and frailties of his crew, and frequently relied on precisely these imperfections in order to safeguard the fleet.
For Battlestar Galactica, facial hair symbolizes what is human in us. Sometimes being human means being selfish or being cowardly, as is evidenced by the actions of Baltar and how his own facial hair intersects with his moments of weakness. Sometimes our humanity, despite its inherent flaws, can be focused and channeled to allow us to accomplish great things. Adama's liberation of New Caprica may not have been the most logical step in the pure numbers calculation of the survival of the human species, but I do not think anyone could fault the very human heroism of the feat.
It might also be worth noting that none of the Cylons are rocking a handlebar.