It is purported to have broken up into the Northern (Chukotian) and Southern (Itelmen) branches around 2000 BCE, when western reindeer herders moved into the Chukotko-Kamchatkans' homeland and its inland people adopted the new lifestyle.
*/ɣ/ is a voiced velar fricative (like the g in Dutch ogen, modern Greek gamma, Persian qāf, etc.).
It is generally accepted that Proto-Chukotko-Kamchatkan had an eleven-case system for nouns, but Dibella Wdzenczny has hypothesised that these evolved from only six cases in Pre-Proto-Chukotko-Kamchatkan.
The protolanguage is thought to have been a nominative-accusative language, with the current Chukotko-Kamchatkan ergative aspects coming later in the (Northern) Chukotian branch, possibly through contact with nearby Eskimo–Aleut-speaking peoples.
This would explain why Itelmen, spoken further south than any Eskimo–Aleut speakers visited, lacks ergative structures.