How did T. rex evolve small forearms?

From Zach Weinersmith's Saturday Morning Breakfast Cereal comes a cautionary tale about technology and evolution.

This week, Weinersmith takes a look at what implications a hands-free device like Google Glass might have on the process of descent with modification, while (inadvertently?) responding to this panel, posted last week over at XKCD:

How did T. rex evolve small forearms?

Who knew T. rex used to be such a brawny-armed beastie?

Visit SMBC for the hover text.