carbon + silicon
We are a carbon-based life form. Though silicon is abundant in our world, it is not incorporated into us or other “organic” systems. One reason for this is based on the atomic structure – in short, Si can’t do pi bonds (double bonds), which are critical for diversity in molecular bonding. But enzymes can be modified to do things they don’t normally do. The most recent case in point, Frances Arnold’s group at Caltech engineers an enzyme to make C-Si sigma (single) bonds. While not a new form of life, an interesting result and points toward further world.