The Arduino Uno is the result of a relatively short but active evolution. The original production boards date back only to 2005, so not a lot of time has elapsed.
Several prototype versions of the Arduino I/O Board floated around before the familiar form factor emerged. These boards were designed to be easy to build from scratch. The goal was to get boards into the hands of students, artists, and designers as quickly as possible.
The first Arduino board with the canonical form factor as you see it today was simply called the Arduino. See Figure 1-12.