I was really taken aback when Lux started saying "buh buh buh" recently. It shouldn't have been surprising—I think it's pretty much normal for six months—but something about those particular syllables seemed suddenly grown up, in a way that none of the previous half-year of sounds had stood out.
The sound "b" is plosive, and of course Wikipedia identifies it properly as a voiced bilablial plosive: made by both lips, sounded by the vocal chords, and resulting from a released stop in airflow. And no, you don't find this sound anywhere else in the animal kingdom.
It's interesting that six months of shrieks and wails and cries and howls and gurgles and giggles and coos had not until now produced a sound which was uniquely human. Suddenly, with the introduction of this particular consonant, Lux's burblings become personal: quite literally, she's a person now—from a speech perspective.
Here's Lux on day 203: