Silicon Valley startup incubator Y Combinator announced Monday that Geoff Ralston would be named president, replacing former president and chairman Sam Altman. Ralston has been at Y Combinator since 2011 and has known cofounder Paul Graham for 20 years. He most recently led Startup School, Y Combinator's free online class for potential founders. Ralston says that part of the draw to early-stage investing is "seeing Uber before it's Uber," and watching founder teams build an idea into a mutli-billion-dollar business. Y Combinator has helped mentor companies like Airbnb, Dropbox, and Stripe.