Why did the Engineer at the start of the movie drink the dark liquid?

Question

Why did the Engineer at the start of the movie drink the dark liquid?

From Wikipedia:

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.

Answer 1

As we can see after he drinks the liquid, his DNA seems to desintegrate into the water. But from the particules of his DNA, life is created. It is showed by the particules turning step by step into cellular life.

So my hypothesis is that his suicide was necessary to give birth to a new species: Us.

A friend of mine told me that he might have missed his flight (we can see a ship leaving), but I don't think that it is a good reason to get suicidal.

Answer 2

The humanoid wanted to break down the biological make-up of him, and then his infected DNA which eventually combined with Earth's water gives rise to all living beings on earth, including humans.

According to IMDB,

Why did the humanoid die at the beginning of the movie?

It can be assumed that the Humanoid or "Space Jockey" at the beginning of the movie was standing on Earth, and the liquid that he drank was probably the same matter that David discovers (when he says "big things have small beginnings"). Supposedly this liquid dematerialises the Space Jockey's DNA which eventually combined with Earth's water gives rise to all living beings on earth, including humans. Why the Space Jockeys couldn't have extracted DNA from stem cells or even just manufactured it, remains a mystery. One theory is that that along with the mythology behind the movie title, the space jockey standing on Earth stole the technology from the biological weapons seen on LV-223, to create new life instead of destruction. This lone dissident having no means to add the space jockey/human genome to the liquid compound had to make do with what he had; his own body. Concept art of the film showed an extended opening scene involving two Space Jockeys. An elder SJ gives a younger looking SJ the cup of black liquid in a ritualistic manner. So the opening scene is part of a ritual to begin the creation of organic life on any given planet. The scene is rumored to have been shot so it may end up on the home release as behind the scenes or deleted footage.