Airs Monday, February 13 at 8:00 p.m. In this hour, Radiolab asks what is natural in a world where biology and engineering intersect. Biotechnology is making it easier and easier to create new forms of life, but what are the consequences when humans play with life? We travel back to the first billion years of life on Earth, take a look at how modern engineers tinker with living things, and meet a woman who could have been two people. Guests:Brian Baynes, George Church, Nigel Goldenfeld, Karen Keegan, Laurel Kendall, Steven Payne, Reshma Shetty, Lee Silver, Steve Strogatz and Lynne Uhl.