Why did the Engineer at the start of the movie drink the dark liquid?
From the Wikipedia article for Prometheus*:
Above a waterfall, a humanoid alien consumes a dark liquid, and a hovering spacecraft departs. After drinking the liquid the alien starts to disintegrate. The bodily remains fall into the waterfall below and the alien’s DNA triggers a biogenetic chain reaction.
* Note that since this question was posted, the Wikipedia article has been changed and no longer contains the above excerpt. This can be found in the page history for the Prometheus article.