He took a sample of 567 languages spoken around the world, and compared the locations and altitudes of those that either contained or ignored ejective consonants.
Though he did include some ejective phonemes, voiceless consonants produced with a contracted glottis, like ch'ah and p'ah, similar to those found in real-world languages like Mayan and Chechen.
Everett suggests that the sounds are more popular at altitude because lower air pressure may make it easier to produce the burst of air that is a key characteristic of ejective consonants.
There are many dozens of observed consonants, from the clicks of some African languages to the " ejectives" (which make use of air pressure built up in the mouth) of Caucasian ones.