I don't think its either. I think it really is a response to some great catastrophe that struck him 100 million years before. Perhaps it's even the only way of keeping himself alive - a sort of way of evading death. But something he himself devised, and is un-natural. So, neither bioweapon or a thing of natural evolution, but rather a result of that thing which is considered most heinous in the Chrono world - un-natural evolution, forced change. Something hit him, and hit him hard, I think. Like I said, that's how the words on Terra Tower strike me. As such... it's not Lavos the Bio-weapon, nor Lavos the Parasite, but Lavos the Dark Lord. The words upon Terra Tower seem to point to it strongly. And I think Hadriel will back me up on this one.