If life in Dark Souls originated from the Abyss like the intro possibly implies, perhaps creatures like Manus and the Four Kings, as well as the Humanity blobs in Oolacile, are what happens when life stays down in the Abyss for too long.
That would also explain why the Pygmy was "so easily forgotten", he/she never came out of the Dark like the other lords, then became Manus because of the Humanity and its effects.
Manus's Humanity "went wild" which is likely an effect of the Abyss, explaining also the Humanity blobs that float around down there, possibly others who tried to enter the Abyss and could not escape, or Humanity is sentient and grows when left in the Abyss.
Destroying Humanity (i.e. kindling a Bonfire) makes light and fire, the opposite of the Abyss. Gwyn, as you know is the most powerful Lord and Lord of Sunlight, again with the light. He wanted the Age of Fire to keep going, possibly because he was afraid of change, but also possibly because he was afraid/knew of what the Abyss was capable of should the light die out.
Kaathe tries to make the player into the Dark Lord, the opposite of Gwyn's intentions (Kaathe is also an enemy of Gwyn based on this, and the fact that he opposes Frampt, a friend of Gwyn) killing off the fire and light, and in the DLC Chester and Gough both mention the Oolacile citizens being fooled by a "toothy and dark serpent" into unleashing the Abyss. By fooling Oolacile into unleashing the Abyss and Manus, had the player and Artorias not stopped it the world may have become dark before the Age of Fire was even close to ending. In the present, Kaathe has decided it would be easier to just extinguish the fire directly, instead of opening up the Abyss, since Manus is dead.
Just some heavy speculation but I thought it might interest you guys. There are probably holes in this that I haven't noticed. What do you all think?