Okay, so I have to do a 15 minute senior speech in front of my school help!!!?

Other than the fact i'm scared out of my mind, I need to find a theme. The speech is supposed to be an autobiography and how we have changed theses past four years. Well I really want the theme to be sciences related, biology would be best, but I have no ideas. I need to compare it to life.. any suggestions?!?! please, I have a couple of weeks but I need to get it started..