I was unclear if the Engineer who drinks the black liquid and disintegrates at the beginning of Prometheus seeded life or just human life -- was there any indication of when in time that part happened?
I made the same mistake of presuming it happened on earth. But you never know. Prometheus writer shares his views here and clarifies the same.
1-3 billion years ago, give or take. From the initial flyover, we see low level photosynthesis going on, but no trees, no flowers, and no animals. Life on Earth, an evolutionary timeline.
If the Engineers were considering seeding the planet with animal DNA, this would be a good time to get into the game, providing their black mutagen and themselves as the source DNA to build toward the development of multi-cellular life. Since Humanity shared a match of a significant series of DNA structures, it can be assumed, their mutagen was designed to ensure the eventual recreation of structures similar, and I suspect hopefully better versions of the Engineers.
It has also been hinted that the planet we are looking at in the flyover is not necessarily Earth but one of many planets the seed-bearing Engineers may have seeded in a similar manner as they are believed to have done on Earth.