Dragonsdawn is a 1988 science fiction novel by Anne McCaffrey that serves as a prequel to the Dragonriders of Pern series. It chronicles the original colonization of the planet Pern by humans and their struggle for survival against the deadly, sky-falling Thread. The story details how the colonists genetically engineer the native fire lizards into the large, telepathic dragons that become Pern's protectors. 
Plot summary
Colonization: Over 6,000 colonists travel to Pern to establish a new, low-tech agricultural society. 
The Threat: The colony is threatened by "Thread," a deadly, silver, spore-like substance that falls from the sky and consumes all organic matter.