The Science Times this week had a great scifi story opening. The article, "Human DNA, the Ultimate Spot for Secret Messages (Are Some There Now?)", based on the idea of scientists encoding "E=mc^2" in the DNA of a bacterium and a year of New York Times' on the genes of a cockroach, imagined that our own DNA could contain messages implanted billions of years ago by an ancient race that seeded our own planet with life.
What would it say? Most likely, something like "Hi, we're out here," on the assumption it wouldn't be found until a race had reached a certain level of intellectual development. The obvious problem is that if you don't know what someone's encoding system is, you could probably interpret the text of Hamlet being encoded into the 750 megabytes contained in human DNA.
Another issue is the question of whether a certain level of intellectual development reflects an equal level of emotional development. This is one of the fantastic themes that I'm seeing Alistair Reynolds address in "Pushing Ice." Right now a small colony of humans, surrounded by a number of other more advanced alien races, is having knowledge held back from them because the humans are behaving like squabbling children unable to stop fighting over issues that happened years ago.
Still, I really want to do something with the DNA encoding thing. Imagine the combination of disbelief and ridicule a scientist who discovered the message would have to overcome. And imagine how it would change humanity if she finally convinced them.