"Where are we?" asks the Doctor, stepping out of the TARDIS. "Where did you take me, River?" He raises a brow as he looks around at the desolate planet that he is standing on. "Wherever it is, it doesn't look that interesting...how about we go."
"Wait! Don't tell me you don't recognize it, Sweetie." She steps out of the blue and white double doors behind him. She blocks him, back and forth, from getting back on the ship. "It's the Earth...before life appeared on it."
"So, why are we here? Best to just let things take their course. Nothing to do here, except interfere...and with the delicate balance of things that start out here, we best be going..." He stops his back and forth struggle of trying to get past River. "Now." River smiles.
"Do you know ho life on Earth began?"
"Yes, Earth..." He indicates their surroundings, by gesturing wildly with his arms. "stops being just amino acids in a primordial soup, four-hundred-million years ago, and starts producing functioning cells when a Jagaroth...Do you know a Jagaroth...? I ran into one in my fourth generation. His name was Scaroth. Caught him trying to steal the Mona Lisa. His ship exploding is what starts the whole thing." And then to himself. "I wonder if I will run into myself...again."
"Yes, Sweetie, but what brought the amino acids here in the first place?"
"Well, some globs of DNA from an unknown source somehow make their way to the Earth's surface...in MANY places. DNA that is both strong enough to survive the Earth's harsh environment at this time and adaptable enough to become every living species on the planet..." River puts her finger on the Doctor's mouth to make him stop talking. She then pecks him on the mouth. "Tell me. Why are WE here?"
"We are here to start the process." She pushes past him and closes the doors behind him in a quick maneuver before pinning him to the TARDIS.
The Doctor begins to speak matter-of-fact-ly: "Like I said, the first bits of DNA are no more than globs...we can't possibly have..." River reaches her hand down THERE and a light bulb goes off as he realizes what she intends for them there, without actually saying it. "Oh..."
"And the coins drops." And before the Doctor can object..."Let's go make life."