Imagin the evolution of species like a tree branching out to the top. We all share the some basis, the Tree trunk, as life evolves new branches grow, forking in different directions, diverging, some dying of, new branches gowing out of the old ones different. For example chimp-branch and human-branch would have a common bigger branch from which they both are growing.
So the engineer disolving either kickstarted the whole tree, life itself on the planet, or he manipulated one of the branches to grow in the human direction, or it taped the human branch on an already existing "life tree".
It think the kickstarting version makes the most sense.