The Movie: Shaw discovers that the DNA from the weird Engineer head is a 100% match with human DNA.
Real Life: Humans don't even have 100% DNA matches with other humans! Unless you have an identical twin, there is no other person in the universe with your exact genetic instructions. That's kind of the point of evolution: the most successful genes are passed on, but variety is essential because it makes the species as a whole more adaptable. Plus, even if the Engineers were identified simply as being the same species as humans, why do they look so different? Also, how is Shaw randomly a fully trained geneticist? I guess she is just very devoted to scattering her dumb-dumbs across many scientific disciplines.