The babookaris evolved sapience and built a civilization.
They then caused a mass extinction which wiped themselves out along with most other mammals, either through pollution or through global war, thus explaining why mammals are all but extinct in the next installment. Alternatively, babookaris never developed civilization.
They never evolved bipedalism, and together with a lack of megafauna carcasses to exploit, they never had an incentive to create stone tools. Humans didn't leave the planet because of the coming ice age
First installment is set only 5 million years in the future, but seems to have several orders extinct or almost extinct, including cats, dogs and bears, ungulates, primates, whales, birds of prey, etc. More than the effect of glacial and interglacial succession, this seems to have followed a planetary wide mass extinction likely caused by humans, and that this was the reason or was related to the reason they left the planet (planetary wide ecological collapse and/or total war). The network took this proposal from the scientists and bowdlerized it into humans spontaneously leaving the planet.- Confirmed, actually. In the original British broadcast, it is stated that humans are extinct, along with most other large animals of today. The edited American version changed it to them leaving the planet in order to be less disturbing to viewers.
The land squid eventually evolve into the Inklings in Splatoon
Not much to explain, really. The Animal Planet special ends with no humans left on Earth and Squid moving onto land and filling the same niche as primates, with the implication that they will one day achieve sapience. Splatoon features squid kids on a post-Humanity Earth.