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.